Wei Wang, Assistant Professor at the University of Wisconsin-Madison, will present a talk, “Autonomy in Marine Robotics: Design, Control, and Coordination in Complex Environments.”

Abstract: Marine robots have undergone significant growth, driven by recent advances in artificial intelligence, sensing technologies, and decision-making systems. As demands for ocean exploration, exploitation, and conservation continue to rise, there is an increasing necessity for advanced autonomy in marine robots at both individual and group levels, as well as for frequent and safe deployment of marine robots at an affordable cost. However, the current marine robots available still fall short in meeting the demands of real-world applications, facing challenges such as robust and safe control in complex environments, self localization and mapping of their environment, and efficient sensing and coordination within a group. My long-term research goal is to enable fully autonomous robotic operation in challenging marine environments through the development of novel robotic platforms and advanced algorithms. In this talk, I will present my work in three areas: (1) learning-based control and navigation of 2D surface vehicles in urban waterways; (2) the design and coordination of multi-robot systems operating on the water surface; and (3) the development of biologically inspired robots and sensing strategies to address challenges in 3D underwater robotics and robotic swarms. I will conclude with a discussion of open research questions in these domains.

Bio: Dr. Wei Wang is an Assistant Professor in the Department of Mechanical Engineering at the University of Wisconsin–Madison since 2024. Prior to this appointment, he was a postdoctoral researcher and later a Research Scientist at the Computer Science and Artificial Intelligence Laboratory (CSAIL) at the Massachusetts Institute of Technology, where he worked from 2016 onward. He received his Ph.D. in Mechanical Engineering from Peking University in 2016 and his B.E. in Electrical Engineering from the University of Electronic Science and Technology of China in 2010. Dr. Wang has published extensively in leading robotics journals and conferences, including Science Robotics, IEEE Transactions on Robotics (T-RO), Soft Robotics, Robotics and Automation Letters (RA-L), Bioinspiration & Biomimetics (B&B), ICRA, IROS, and CDC. His work has also been featured in major international media outlets such as Reuters, NBC, CNBC, and MIT News.

Raffaele Ferrari, Professor of Oceanography at Massachusetts Institute of Technology, will present a talk, “The fluid dynamics of the ocean abyss.”

Abstract: The abyssal ocean is stably stratified, with density increasing with depth. This layering is sustained by cold waters sinking at high latitudes, filling the bottom of ocean basins, and by heat diffusion from the surface. Unlike molecular diffusion, which is slow, turbulent mixing greatly accelerates the downward transfer of heat. Although clear evidence exists for cold water sinking from the surface to the abyss around Antarctica and in the North Atlantic, evidence for turbulent mixing is much more elusive. Recent theoretical work by our group suggested that most of this mixing occurs in thin boundary layers along the ocean’s sloping seafloor. Recently, we conducted a field experiment in the Atlantic to document the fluid dynamics of turbulent mixing along the seafloor. I will discuss the fluid dynamics of this along seafloor turbulent mixing and its implications for global ocean stratification, focusing on dynamics from centimeters to thousands of kilometers.

Bio: Raffaele Ferrari is a Professor of Oceanography in the MIT Earth, Atmospheric, and Planetary Science Department. After earning a BS and MS in physics from the Università di Torino in 1994, Ferrari pursued PhD studies in oceanography at the Scripps Institution of Oceanography (2000). Before arriving at MIT, he was a postdoctoral scholar at the Woods Hole Oceanographic Institution. Ferrari served as the chair of the EAPS Program in Atmospheres, Oceans, and Climate from 2012 to 2022 and is currently co-director of the MIT Lorenz Center. His research focuses on turbulence in the ocean and atmosphere, the impact of ocean turbulence on marine biology and the carbon cycle, and the ocean atmosphere-land interactions that shape past, present, and future climates.

Irmgard Bischofberger, Associate Professor of Mechanical Engineering at MIT, will present a talk, “Fractured Flows: Suspensions Pushed Too Far.”

Abstract: Particle suspensions can fracture into intricate patterns as they are pushed out of equilibrium. We probe the fracture and relaxation characteristics of dense aqueous cornstarch suspensions that exhibit discontinuous shear-thickening behavior. Air injection into three-dimensional bulk suspensions can lead to smooth bubbles that rise upwards under the action of buoyancy or to sharp fractures that remain attached to the injection nozzle. We link the shape and the relaxation dynamics of the air cavity to the suspension rheology. We discuss how the bubbles exhibit distinct bursting patterns as they reach the air interface, and how the bursting characteristics might reveal information about the rheology of thin suspension films. In a second example, we report the crack dynamics and morphology occurring as drops of aqueous nanoparticle suspensions evaporate on a glass surface and leave behind a solid particle deposit. We show that in the final stage of drying, the stresses in the deposit can be released in two distinct ways: by bending out of plane or by forming a second generation of cracks.

Bio: Irmgard Bischofberger is an experimentalist working in the fields of fluid dynamics and soft condensed matter. She obtained her Ph.D. degree in Physics from the University of Fribourg and has been a postdoctoral fellow in the Physics Department at the University of Chicago. She is an associate professor in the Department of Mechanical Engineering at MIT. Her research interests include the spontaneous pattern formation from fluid instabilities and drying processes and nonequilibrium phenomena in soft gels. Irmgard is passionate about communicating science to a diverse audience and has longstanding ‘Science and Arts’ collaborations with artists and Musicians.

Hosted by: Prof. Tom Powers

Morgane Houssasis, Research Scientist at Clark University, will present a talk, “Sediment creep and groundwater flow co-dynamics: toward better prediction of failure and long-term dynamics in landscapes.”

Abstract: Most of the ground we walk on falls into the category of soft matter. As it appears poroelastic while one walks up a hill for a few hours (100-104 seconds), over geological times (>103 years, or >1010 sec), our granular environment keeps experiencing large deformations recorded in sedimentary strata. The intermediate range of time scales [104 - 1010 sec] encompasses the lifetime of all our infrastructures and human risk management, and is particularly challenging to study in the field, in the lab, and computationally. Indeed, over this time range of observation, the environment dynamics alternates between very slow, if any, creep type deformation and short periods of sudden erosion or failure, such as during a historical flood or a landslide.

In this seminar, I will present recent experimental results that connect our understanding of very slow (creep) and very fast (failure or flowing) deformation  of the ground while perturbed by a groundwater flow, such as following a rainstorm.  Over these experiments, the creep deformation of a saturated sediment layer was tracked as its initial inclination angle and the intensity of vertical porous flow were varied. Such experiments allow for a careful investigation of the transition from creep to failure in any granular material subjected to porous flows and in a configuration that is relevant to natural systems. The results show that, even for sediment layers of very low inclination, weak porous flows have a systematic influence on sediment creep dynamics. A new parameter, combining mean stresses acting on the grains and the perturbation intensity, is introduced to produce a unique curve of all the deformation observations and enlightens the fundamental mechanisms at play. In the end, I will discuss how these results can be upscaled to model earth systems such as steep hillslopes experiencing rainstorms and natural dams experiencing flooding.

Bio: Morgane Houssais is a Research Scientist at Clark University with a broad interest in sediment transport. After a PhD in Geophysics at the Institut de Physique du Globe de Paris in France, she did postdocs at the University of Pennsylvania and City College of New York. Her expertise and collaborations lay at the interface between the Physics of complex fluids and materials and Earth Surface Processes and Geomorphology. She is currently founded by NSF to study sediment creep with a particular interest for natural dams failure.

Hosted by: Prof. Tom Powers

Center for Fluid Mechanics Seminar Series

Chase Gabbard, Hope Street Postdoctoral Fellow at Brown University, will present a talk, “Drop-Substrate Interactions: Sliding, Bouncing, and Vanishing in Thin Air.”

Abstract: Our ability to harness liquid drops has shaped modern life–cleansing the air we breathe, regulating heat and moisture across engineered systems, and enabling precision technologies such as inkjet printing. Despite their everyday familiarity, drops reveal rich interfacial physics in the way they move, deform, and vanish upon contact with surrounding surfaces. In this talk, I will explore these subtle interactions across three canonical systems. I begin with drops sliding between pairs of fibers, where geometric tuning enables control over their shape and speed, and can even suppress the capillary instability from which they emerge. I then move to Newtonian drops impacting rigid, non-wetting substrates, combining experiments and reduced-order modeling to reveal distinct deformation dynamics and energy recovery in the low-inertia limit. Shifting from solid to liquid substrates, I conclude by examining how the intervening gas layer governs drop–bath impacts, identifying regimes where drops bounce, coalesce, or transiently float, and comparing these results with model predictions that account for nanoscale gas-kinetic and disjoining pressure effects in the evolving gas layer. Together, these findings deepen our understanding of drop–substrate interactions and provide physical insights that bridge fundamental interfacial physics with the design of advanced environmental and industrial technologies.

Bio: Chase Gabbard is a Hope Street Postdoctoral Fellow in the School of Engineering at Brown University, where he works in the Harris Lab. His research lies at the intersection of fluid mechanics and soft matter physics, with a focus on interfacial phenomena, multiphase systems, and complex fluids. Currently, he investigates droplet rebound, air-layer effects in liquid–liquid interactions, particle interface interactions, the formation and stability of liquid sheets, and the water entry of solid spheres. He earned his Ph.D. in Mechanical Engineering from Clemson University, where he developed an experimental program to study capillary phenomena in flow down fibers, revealing a rich diversity of flow patterns and strategies to harness or suppress capillary instabilities. His overarching goal is to translate fundamental insights from carefully controlled tabletop experiments into practical advances in industrial and environmental technologies.

A joint seminar between the Center for Fluid Mechanics and the Lefschetz Center for Dynamical Systems

Nicholas Pizzo, Assistant Professor of Physical Oceanography, University of Rhode Island, will present a talk, “Exact solutions to Euler’s equations and harmonic maps.”

Abstract: Exact solutions to Euler’s equations are presented in Lagrangian coordinates. These solutions arise due to a particle relabeling invariance, a subset of which, associated with particle label rotations, are shown to transform time independent solutions to time dependent solutions by these infinitesimal canonical transformations. The associated compatibility conditions of these maps restrict the label dependence to be harmonic maps from cartesian label space to these two dimensional surfaces, directly connecting the rotational relabelling symmetry with harmonic maps. Using a Lax pair approach, harmonic maps from the plane to the sphere are associated with a negative sinh Laplace equation. Solutions to this equation, and a Backlund transformation to derive an infinite number of new solutions, are presented and their associated vorticity contours are found numerically. The implications of these results are then discussed.

Bio: Nick Pizzo is an Assistant Professor of Physical Oceanography at the Graduate School of Oceanography at the University of Rhode Island. He has undergraduate degrees in math and physics from UCSB and received his PhD at the Scripps Institution of Oceanography at UCSD, where he was advised by Ken Melville. 

Hosted by: Dan Harris, Associate Professor of Engineering

Center for Fluid Mechanics Seminar Series

Keaton Burns, Principal Research Scientist in the Dept. of Mathematics at MIT, will present a talk, “A new near-surface model for the solar dynamo.”

Abstract: We will present a new dynamical model of the onset of the solar dynamo cycle. No prior background in solar physics will be required: we will begin with a pedagogical overview of key theory and observations of solar magnetism, discuss the fluid dynamics behind our proposed instability mechanism, and conclude with numerical results using the spectral Dedalus solver.
The solar dynamo displays two striking patterns: sunspot emergence appears around 30° latitude and drifts toward the equator over an 11-year period, and zonal flows called “torsional oscillations” track this migration and likely share a common physical origin. Traditional models assume the cycle begins with instabilities deep within the Sun, but corresponding simulations have struggled to produce solar-like behavior. Seismological observations instead show that torsional oscillations are confined to just the outer 5–10% of the Sun, within the “near-surface shear layer.” There, inwardly increasing differential rotation coupled with a poloidal magnetic field suggests the presence of the “magnetorotational instability” (MRI), a magnetohydrodynamic instability well established in accretion-disk theory. These insights motivate the question: could the solar dynamo arise as a purely near surface instability?
Here we will present strong evidence that it can. Simple analytical estimates show that the near-surface MRI reproduces the observed spatial and temporal scales of torsional oscillations and the amplitudes of the inferred magnetic fields. High precision numerical simulations confirm these results and reproduce hemispheric current-helicity laws and other hallmarks of the solar cycle. A dynamo driven by a near-surface process challenges the traditional deep-seated paradigm and opens new prospects for predicting solar magnetic activity and its effects on space weather. 

Bio: Keaton is a Principal Research Scientist in the Department of Mathematics at MIT. He completed his PhD in Physics at MIT and was a postdoc at the Simon Foundation’s Flatiron Institute. Keaton’s work focuses on the development of high-order numerical methods, their implementation in open-source software, and their application to problems in astrophysical, geophysical, and biological fluid dynamics. He is the lead developer of Dedalus, an open-source framework for solving PDEs using global spectral methods.

Hosted by: Tom Powers, Professor of Engineering, Professor of Physics

Center for Fluid Mechanics Seminar Series

Orr Anvi, Fulbright Postdoctoral Scholar in Engineering, Brown University, will present a talk, “From Primary Atomization to Impact: Dynamics of Viscoelastic Droplets.”

Abstract: Aerial firefighting and agricultural spraying rely on the efficient atomization and deposition of liquids released under aerodynamic loading. The released fluids, e.g., fire retardants and pesticides, are non-Newtonian, viscoelastic fluids; while being used in many applications, our understanding of the atomization and subsequent ground impact dynamics of such fluids remains underdeveloped. We investigate two related problems central to these operations.First, we examine the spreading of viscoelastic droplets upon impact. We show that elastic stresses can substantially limit spreading when the elastic relaxation and deformation timescales are comparable; a modified energy-balance model incorporating elastic contributions captures these deviations from Newtonian behavior. The second effort concerns the fragmentation of liquid jets released into a crossflow. We focus on the regime of intermediary Weber numbers, where the transition between capillary- and shear-dominated breakup is not well characterized. We aim to reveal the rheology effects on the mixed breakup regime, where the droplet size distribution resultant from the primary atomization process depends on the balance between aerodynamic drag, viscosity, and surface tension. Together, these studies provide a unified framework to analyze the effect of viscoelasticity on liquid fragmentation and droplet impact, establishing a physical basis for the possible engineering of ‘smarter’ materials for improved aerial firefighting and spraying applications. 

Bio: Orr Avni is a Postdoctoral Fulbright Scholar in the School of Engineering at Brown University. His research focuses on the fluid mechanics of multiphase and phase-changing systems. Before joining Brown, he was a Research Associate at the Technion, where he also completed his Ph.D. in Aerospace Engineering, studying the thermodynamic and hydrodynamic behavior of liquids under negative pressures. His current work investigates the fluid dynamics of aerial firefighting and agricultural spraying. 

Hosted by Prof. Roberto Zenit

Center for Fluid Mechanics Seminar Series

Patrick Musgrave, Assistant Professor of Mechanical and Aerospace Engineering, University of Florida, will present a talk, “Adapting to the Flow: Intelligent Structures that Turn Flexibility into Function.”

Abstract: Flexible structures are often governed by nonlinear dynamics, and when submerged in fluids they must contend with complex fluid-structure interactions. However, what if these nonlinearities and fluid-structure interactions are not a challenge? Instead, what if these complex dynamics could become a benefit? This talk will present a vision for Intelligent Structures that can think and respond to their fluid environment by turning their flexibility into function. On flight vehicles, responding to unsteady aerodynamics such as vortices and shocks is a significant challenge due to the high sensor density and large computational cost required to sense and process the flow. This talk will present our research into Mechanical Neural Networks which harness the nonlinear dynamics of vibrating structures to  sense and process environmental information with minimal digital computation. On underwater vehicles, achieving high maneuverability and stability in the coastal environment is a challenge due to the punishing waves and currents. This talk will present our ongoing work to realize Soft Robotic Swimmers which harness a flexible body with distributed muscles to achieve the efficiency, maneuverability, and adaptability of biological swimmers.

Bio: Patrick Musgrave is an Assistant Professor in the Mechanical and Aerospace Engineering Department at the University of Florida. He received his PhD in Mechanical Engineering from Virginia Tech, and prior to joining UF prior he was a research scientist at the US Naval Research Laboratory. He leads the Fluids and Adaptive Structures (FASt) Lab with the vision to realize intelligent structures that can think and respond to their fluid environment. His research aims to embody actuation, sensing, processing and control into the flexible structures to realize Intelligent Structures such as Soft Robotic Swimmers, Mechanical Neural Networks, and Ocean Wave Energy Harvesters. Dr. Musgrave’s work has been supported by AFOSR, NSF, and DOE, and he has been recognized with the AFOSR Young Investigator award and the NSF CAREER award.

Hosted by: Kenny Breuer

Center for Fluid Mechanics Seminar Series

Matteo Bucci, Associate Professor of Nuclear Science and Engineering at MIT, will present a talk, “High resolution investigations of nucleate boiling, from single bubbles to boiling crisis, from cryogenic fluids to high pressure water.”

Abstract: In every field of science, the possibility of discovering and understanding new phenomena or testing new hypotheses is strongly related to and limited by the capability of observation. Here, we will discuss recent advances in experimental boiling heat transfer research made possible by unique experimental facilities and non-intrusive high-resolution optical diagnostics. We will analyze the capabilities and limitations of these techniques in supporting the understanding of fundamental two-phase heat transfer problems, with a focus on extreme boiling conditions such as the boiling of water at high pressure and temperature, close to nuclear reactor conditions, the boiling of dielectric fluids for electronic cooling applications, or the boiling of cryogenic fluids relevant to space propulsion and energy storage. The use of these diagnostics has been instrumental in providing answers to long-standing fundamental questions on the fluid dynamics and heat transfer nature of these processes.

Bio: Matteo Bucci is the Esther and Harold E. Edgerton Associate Professor of Nuclear Science and Engineering at MIT. His research group studies two-phase heat transfer mechanisms in nuclear reactors and space systems, develops high-resolution non-intrusive diagnostics and surface engineering techniques to enhance two-phase heat transfer, and creates machine learning tools to accelerate data analysis and conduct autonomous heat transfer experiments. He has won several awards for his research and teaching, including the MIT Ruth and Joel Spira Award for Excellence in Teaching (2020), ANS/PAI Outstanding Faculty Award (2018 and 2023), the UIT-Fluent Award (2006), the European Nuclear Education Network Award (2010), and the 2012 ANS Thermal-Hydraulics Division Award. Matteo is the founding editor and deputy Editor-in-Chief of AI Thermal Fluids. He also serves as Editor of Applied Thermal Engineering, is the founder and coordinator of the NSF Thermal Transport Café and works as a consultant for the nuclear industry.

Host: Prof. Roberto Zenit

Center for Fluid Mechanics Seminar Series

Zhao Pan, Associate Professor in the Department of Mechanical and Mechatronics Engineering at the University of Waterloo, will present a talk, “From Urinals in Bathrooms to Diagnostics in Fluid Mechanics Lab: Engineering with Simple Mathematics.”

Abstract: This seminar will present two distinct research projects that demonstrate how simple mathematics can provide elegant solutions to engineering problems. In the first part, we solve isogonal curve problem to design splashless urinals, which are a staple of public spaces yet their designs have remained essentially stagnant for over a century. Our experiments confirm that the new designs reduce splashback to 1.4% of that produced by a standard commercial urinal. Broad adoption of these designs could significantly conserve human resources and water, delivering large-scale benefits for sustainability, hygiene, and accessibility. In the second part, we turn to the well-posedness analysis of elliptic equations to resolve a longstanding misconception in flow diagnostics: that the omnidirectional integration method (ODI) invented by Liu and Katz (2006) for pressure field reconstruction from image velocimetry is fundamentally different and superior to the pressure Poisson equation (PPE) based methods. We show that ODI and PPE are equivalent, clarifying two decades of misunderstanding in the field, while unveiling their shared limitations and inspiring potential improvement. 

Bio: Zhao Pan is an Associate Professor of Mechanical and Mechatronics Engineering at the University of Waterloo, Canada. He earned his Ph.D. from Brigham Young University in 2016 and was a postdoctoral fellow at the Florida Center for Advanced Aero-Propulsion and Utah State University before joining Waterloo in 2019. His interdisciplinary research spans flow diagnostics, uncertainty quantification, data assimilation, and the physics of droplets, bubbles, and granular flows. 

Hosted by: Dan Harris, Associate Professor of Engineering

Center for Fluid Mechanics Seminar Series

Amaresh Sahu, Assistant Professor in the Department of Chemical Engineering, University of Texas at Austin, will present a talk, “Osmotic forces modify lipid membrane fluctuations.”

Abstract: In hydrodynamic descriptions of lipid bilayers, the membrane is often approximated as being impermeable to the surrounding, solute-containing fluid. However, biological and in vitro lipid membranes are influenced by their permeability and the resultant osmotic forces—whose effects remain poorly understood. Here, we study the dynamics of a fluctuating, planar lipid membrane that is ideally selective: fluid can pass through it, while the electrically-neutral solutes cannot. We find that the canonical membrane relaxation mode, in which internal membrane forces are balanced by fluid drag, no longer exists over all wavenumbers. Rather, this mode only exists when it is slower than solute diffusion—corresponding to a finite range of wavenumbers. The well-known equipartition result, regarding the size of membrane undulations due to thermal perturbations, is consequently limited in its validity to the aforementioned range. Moreover, this range shrinks as the membrane surface tension is increased, and above a critical tension the membrane mode vanishes. Our findings are relevant when interpreting experimental measurements of membrane fluctuations, especially in vesicles at moderate to high tensions.

Bio: Amaresh Sahu is an Assistant Professor in the Chemical Engineering Department at the University of Texas at Austin. His theoretical research group broadly focuses on understanding various soft and living matter systems over a wide range of length and time scales. Prior to UT, Prof. Sahu was a postdoctoral fellow at Weill Cornell Medicine, where he worked with Prof. Emre Aksay to develop theories of electrically excitable membranes. During his PhD, he worked with Prof. Kranthi Mandadapu at UC Berkeley to develop theories and numerical methods describing biological membranes. His academic career began at Princeton University, where he worked with Prof. Howard Stone as a member of the chemical engineering department.

Hosted by: Tom Powers, Professor of Engineering and Professor of Physics

Center for Fluid Mechanics Seminar Series

Gareth McKinley, Professor of Teaching Innovation, Department of Mechanical Engineering at MIT, will present a talk, “Extensional Rheology and the Spatio-temporal Signatures of Elasto-Inertial Turbulence in Complex Fluids.” 

Abstract: The addition of small amounts of polymers to a Newtonian solvent makes a fluid viscoelastic and can lead to significant modifications of high-speed turbulent flows. The interaction of viscoelasticity and inertia even in a very dilute polymer solution results in the emergence of unique inertio-elastic instabilities that are still far from being understood, as well as significant modifications to frictional drag. The nonlinear evolution of these instabilities engenders a state of turbulence with significantly different spatiotemporal features compared to Newtonian turbulence, now termed elastoinertial turbulence (EIT). We systematically explore EIT by studying the dynamics of low-speed submerged jets of dilute aqueous polymer solutions injected through a nozzle into tanks of quiescent water or polymer solution. A key kinematic feature of such flows is the presence of extensional kinematics and streamwise stretching. Extensional flows of complex fluids are prevalent in many industrial applications such as spraying, atomization (sneezing), and microfluidic-based drop deposition. In this talk we use the distinctive kinematic features of such flows to understand the mechanics of how, and why, elastoinertial turbulence is different to classical turbulence. 

Bio: Gareth H. McKinley FRS is the School of Engineering Professor of Teaching Innovation and former Associate Head and Interim Head of the Department of Mechanical Engineering at MIT. His research interests include extensional rheometry, microfluidic rheometry and non-Newtonian fluid dynamics. He is a co founder of Cambridge Polymer Group and a member of the Scientific Advisory Boards of Rheosense Inc. and ActNano Inc. He is the author of over 380 technical publications and has won the Publication Award of the Society of Rheology twice (2007; 2022) as well as the 2021 Walters Award from J. Non-Newtonian Fluid Mechanics. He was awarded the Bingham Medal of The Society of Rheology in 2013, the Gold Medal from the British Society of Rheology in 2014 and the G.I.Taylor Medal from the Society for Engineering Science (SES) in 2022. In 2019 he was elected to the National Academy of Engineering and also inducted as a Fellow of the Royal Society of London. He holds honorary professorships at the University of Swansea (Wales) and Monash University (Australia) and in 2023 he was awarded an honorary degree from the Katholike University of Leuven (KU/Leuven). He is a Corresponding Member of the Australian Academy of Sciences (AAS), and a Foreign Fellow of the Indian National Academy of Engineering.

Hosted by: Mauro Rodriguez, Assistant Professor

Center for Fluid Mechanics Seminar Series

Paul Milewski, Professor, Department of Mathematics at Pennsylvania State University, will present a talk, “Resonance of surface water waves in cylindrical containers.”

Abstract: Nonlinear waves sloshing in a container of rectangular cross-section can behave very differently than those with different cross sections. Nonlinear resonance is a mechanism by which energy is continuously exchanged between a small number of wave modes and is common to many nonlinear dispersive wave systems. In the context of free-surface gravity waves such as ocean surface waves, nonlinear resonances have been studied extensively over the past 60 years, almost always on domains that are large (or infinite) compared to the characteristic wavelength. In this case, the dispersion relation dictates that only quartic (4-wave) resonances can occur. In contrast, nonlinear resonances in confined three-dimensional geometries have received relatively little attention, where, perhaps surprisingly, stronger 3-wave resonances can occur. We will present the results characterizing the configuration and dynamics of resonant triads in cylindrical basins of arbitrary cross sections, demonstrating that these triads are ubiquitous, with (the commonly studied) rectangular cross section being an exception where they do not occur.

Bio: Paul Milewski is a Professor and current Department Head in the Department of Mathematics at Penn State. He received his B.Sc. and M.Sc. in Aerospace Engineering at Boston University and his Ph.D. (1993) from Mathematics at M.I.T. Prior to joining Mathematics at Penn State in 2023, he had positions in the Departments of Mathematical Sciences at the University of Bath (UK, 2011-2023), and in Mathematics at Wisconsin-Madison (1995-2011) and Stanford. He has held visiting positions, among others at IMPA (Brazil) and ENS (France). He was the recipient of a Royal Society Wolfson Fellowship and a Sloan Fellowship. His research is in applied and computational mathematics, mainly in nonlinear waves in fluids, but also with interests across mathematical modeling of physical and biological systems, and data science.

Hosted by: Dan M. Harris, Assoc. Professor of Engineering, Brown University

Center for Fluid Mechanics Seminar Series

Olivia Pomerenk, Hope Street Postdoctoral Fellow at Brown University, will present a talk, “Quasi-steady modeling predicts the dynamics of free-falling and flapping plates.”

Abstract: The flight of a thin wing or plate is an archetypal problem in flow structure interactions at intermediate Reynolds numbers. Free-falling plates display an impressive variety of steady and unsteady motions that are familiar from fluttering leaves, tumbling seeds and gliding paper planes, while flapping wings or foils are emblematic of bird flight and fish swimming. This talk will show that the key behaviors of both passive and flapping flight may be captured by a quasi-steady nonlinear aerodynamic model that predicts forces from plate kinematics without needing to solve for the flows. Regarding passive flight, we show that a nonlinear model successfully reproduces previously documented unsteady states such as fluttering and tumbling while also predicting new types of motions, and a linear analysis accurately accounts for the stability of steady states such as gliding and diving. Regarding flapping flight, simulations reproduce the well-known transition for increasing Reynolds number from a stationary state to a propulsive state, where the latter is characterized by a Strouhal number that is conserved across broad ranges of parameters. These findings extend the phenomena of unsteady locomotion that can be explained by quasi-steady modeling, and they broaden the conditions and parameter ranges over which such models are applicable.

Bio: Olivia Pomerenk is a Hope Street Postdoctoral Research Fellow in the Center for Fluid Mechanics at Brown University, where she is affiliated primarily with the Breuer Lab. Her research focuses on the development of reduced-order mathematical models and/or computational simulations of problems inspired by physical and biological phenomena. Such problems broadly involve themes of propulsion, flow-structure interactions, actuated mechanics and materials, and (more recently) collective behavior and vortex dynamics. Olivia obtained her Ph.D. in mathematics from the Courant Institute of Mathematical Sciences at NYU and her B.S. in applied and computational mathematics from Caltech.

Hosted by: Prof. Kenny Breuer

Center for Fluid Mechanics Seminar Series

Shaung Zhou, Assistant Professor of Phyics at the University of Massachuetts, Amherst, will present a talk, “How a micro-corkscrew swims without external torque.”

Abstract: Axoneme-based flagella, the primary motility organelles of eukaryotic microorganisms and sperm cells, typically generate propulsion through planar bending waves. By tracking the three-dimensional trajectories of fluorescent marker particles, we find that unicellular parasite Trypanosoma brucei propagates a rapid right-handed helical wave along the flagellum to generate thrust like a corkscrew. The cell body, laterally attached along the flagellum, counter-rotates at a lower frequency due to reactive torque, and traces out a left-handed helical path as the cell swims forward. The observed flower-like tracer trajectories result from the superposition of these coupled motions under torque-free constraints. Simulations using the regularized Stokeslets method reproduce the observed dynamics and reveal an optimal body bending angle that enhances forward motion with reduced rotation. These findings uncover a distinct mode of eukaryotic flagellar motility and rebuild the foundation for understanding T. brucei navigation in vivo and inspire new designs of biomimicking actuators.

Bio: Shuang Zhou is currently an Assistant Professor at the Physics Department of University of Massachusetts, Amherst. His primary research interests include the properties of novel lyotropic liquid crystal materials, active matter, and the dynamics of microorganisms. He received his B.S. degree in Applied Physics from Xi’an Jiaotong University (China) and holds a Ph.D. in Chemical Physics from Kent State University (Ohio, USA). After his Ph.D., he did postdoc research at Harvard University (MA, USA), working on hydrogels. He has received several awards, including the NSF CAREER Award and the Glenn H Brown Prize from the International Liquid Crystal Society.

Hosted by Jay Tang, Professor of Physics, Professor of Engineering, Brown University

The Art Fluids Engineering exhibit will take place on May 5. Students will show their final projects. Some examples are wind-reacting kinetic sculpture, chocolate gravity current sculpture, suminagashi vortex streets visualization, and many more. 

Center for Fluid Mechanics Seminar Series

Snezhana Abarzhi, Professor and Chair of Applied Mathematics at the University of Western Australia, and Guest Professor at California Institute of Technology, will present a talk, “Fluid Instabilities and Interfacial Mixing.”

Abstract: Rayleigh-Taylor instabilities and Rayleigh-Taylor interfacial mixing control a broad range of processes in nature and technology. Examples include the evolution of supernova remnants, the formation of Solar flares, the pollutant dispersion in the atmosphere, the atomization of liquid jets, the materials transformation under impact, the multi-phase micro-fluidics, and the inertial confinement fusion. Rayleigh-Taylor unstable flows are driven by accelerations and shocks, have sharply and rapidly changing fields, and are anisotropic, heterogeneous, and statistically unsteady. At macroscopic and microscopic scales, their properties depart from canonical scenarios; yet, they can exhibit self organization and order. We discover a special self-similarity class in Rayleigh- Taylor mixing with variable accelerations, by exploring its symmetries, scaling laws, spectral shapes, correlations and fluctuations. We find that Rayleigh-Taylor mixing can vary from super-ballistics to sub-diffusion depending on the acceleration and retain memory of the deterministic conditions for any acceleration. We explain long-standing puzzles in Rayleigh-Taylor experiments at high Reynolds numbers, and discuss perspectives, unexplored before, for understanding and controlling fluid instabilities and interfacial mixing in nature and technology. 

Bio: Snezhana Abarzhi is theoretical physicist and applied mathematician specializing in the dynamics of fluids, plasmas, materials, and their applications in nature and technology. Her key results are the mechanism of interface stabilization, the special self-similarity class in the interfacial mixing, and the fundamentals of Rayleigh-Taylor instabilities. Her key contributions to the community are the program ‘Turbulent Mixing and Beyond’ and the editorial work. Her achievements are recognized nationally and internationally (by, e.g., National Science Foundation and National Academy of Sciences in the USA, Japan Society for Promotion of Science in Japan, Alexander von Humboldt Foundation in Germany). She is Fellow of the American Physical Society, elected for ‘for deep and abiding work on the Rayleigh-Taylor and related instabilities, and for sustained leadership in that community’. She serves the Committee on Scientific Publications of the American Physical Society.

Hosted by: Martin Maxey, Professor of Applied Mathematics and Engineering

Center for Fluid Mechanics Seminar Series

Peyman Givi, University of Pittsburgh, will present a talk, “Quantum-Ready and Quantum-Inspired Computing for Modeling and Simulation of Turbulence and Combustion.”

Abstract: Within the past decade, significant progress has been made in using quantum computing (QC) for solving classical problems. In this talk, an overview is made of the ways by which QC has shown promise in turbulence and combustion research. This is via both quantum-ready and quantum-inspired algorithms. The former deals with problems that either have the potential to benefit from quantum speed-up on universal gate-based digital computers, or those that can be solved on quantum simulators. The latter deals with new classical algorithms that have emerged from many-body quantum physics. Some recent results will be presented in which QC has proven effective for Reynolds-averaged simulation (RANS), large eddy simulation (LES) and direct numerical simulation (DNS) of turbulent flows under both non-reactive and reactive conditions.

Bio: Dr. Peyman Givi is Distinguished Professor and James T. MacLeod Chair of Mechanical Engineering and Petroleum Engineering at the University of Pittsburgh. He received Ph.D. from the Carnegie Mellon University (PA), and BE from the Youngstown State University (OH). Peyman serves on the Editorial Boards of the AIAA Journal, Combustion Theory and Modelling, and Journal of Applied Fluid Mechanics.

Hosted by: George Karniadakis, Professor of Applied Mathematics and  Engineering

Center for Fluid Mechanics Seminar Series

Meera Ramaswamy, Princeton University, will present a talk, “Bridging Scales in Soft and Living Matter: From microscale structure and interaction to emergent macroscale function.”

Abstract: Microscale interactions often govern macroscale properties in soft and living matter. In this talk, I will highlight two examples where I have used experimental tools to understand and tune this micro- macro coupling. First, I will discuss shear thickening suspensions, where interparticle interactions drive a dramatic increase in viscosity with increasing stress. I will show how thickening can be fully described by a universal function, enabling suspension agnostic ways to tune the viscosity. Next, I will discuss the growth of multispecies bacterial colonies in three dimensions, mimicking natural environments like the soil or human gut. By experimentally tracking the growth of two E. coli strains, I will show that initially well-mixed strains segregate into distinct microcolonies, with the size and shape of each microcolony determined by the initial cell density and colony width. These finding identify the key control parameters needed to engineer colonies with specific spatial organization.

Bio: Meera Ramaswamy is a Princeton Center for Complex Materials (PCCM) postdoctoral fellow, working in the lab of Professor Sujit Datta in the Department of Chemical Engineering. Her research focuses on the motility and growth of bacteria in complex environments. She earned her PhD in Physics from Cornell University, studying shear thickening suspensions. Her research connects microscale interactions to emergent properties, studying soft and living systems near a mechanical phase transition.

Hosted by: Roberto Zenit, Royce Family Professor of Teaching Excellence in Engineering

Center for Fluid Mechanics Seminar Series

Saurabh Nath, MIT, will present a talk, “Of the Surface of Things: Ice | Drops | Bubbles | Metastability.”

Abstract: Place a drop of water on an icy surface – it crystallizes at the point of contact, and a slow ice front steadily grows upwards. Place a bubble instead, and a burst of ice crystals erupts across its interface, far from where it touches the surface. Place a collection of dew drops – the drops now start ‘talking’ to each other, freezing in succession, breaking the stochasticity of the nucleation process. The emergent percolation patterns reveal distinctly different frozen morphologies, shaped by the wettability of the surface. The ordinary freezing of drops and bubbles hides a fascinating array of rich emergent phenomena, far from the ordinary. In the language of experiments and scaling laws, I will explore in this talk the nature and origins of these intriguing behaviors – a story often hidden in the metastability of liquid water and the character of surfaces. Drawing on examples from my research, I will show how a surface, far from being merely a boundary, often defines and dictates the very nature of the system it bounds and the phenomena that follow. I will conclude by reflecting on how the same surfaces that shape drops and bubbles also leave their imprint on the patterns of our climate—from the formation of clouds to the withering of plants—drawing on the poem of Wallace Stevens, whose words echo in the title of my talk: Of the Surface of Things.

Bio: Saurabh Nath is an experimentalist and a Marie Skłodowska-Curie Fellow working at the intersection of ice physics, soft matter, and hydrodynamics. He completed his PhD with David Quéré at ESPCI Paris, where he studied droplet motility on infused solids – ambiguous hemi-solid materials with liquid-like properties. He then joined MIT, where he works with Kripa Varanasi on ice percolation and soft lubrication, and with John Bush on the curious behavior of ants walking (or rather slipping) on pitcher plants. Saurabh’s achievements have been recognized with several awards from the APS, ACS, and ERC, and has also led to multiple patents in deicing, defoaming, and friction-reducing technologies (now deployed in watches by Richemont). Saurabh’s philosophy of science is to design simple experiments that distill the essence of complex natural phenomena, seeking general laws and universality across scales—that can also inform design principles for creating new classes of materials and technologies that dramatically enhance performance in energy, water, agriculture, and transportation.

Hosted by: Roberto Zenit, Royce Family Professor of Teaching Excellence in Engineering

Center for Fluid Mechanics Seminar Series

Xiaojue Zhu, MPI Goettingen, Germany, will present a talk, “Pushing computational limits: From multiphase flow to AI assisted turbulence control.”

Abstract: Understanding and controlling turbulence remains one of the grand challenges in fluid dynamics, requiring both physical insight and extreme-scale computations. In this talk, I will present our recent developments in high-performance computational fluid dynamics, particularly through the open-source code AFiD-GPU, designed for exascale computing—a computational paradigm that enables simulations at over 1018 operations per second. Exascale capabilities allow us to resolve turbulent flows with unprecedented accuracy, bridging the gap between theoretical modeling and real-world applications. Building on this framework, we have extended AFiD-GPU to simulate multiphase flows, capturing complex phenomena such as wave breaking dynamics, which are critical for coastal engineering and ocean-atmosphere interaction. Additionally, we explore fluid-structure interactions, including bio-inspired flapping propulsion and the effects of flapping motions near the water-air interface, relevant for both biological locomotion and naval engineering. Beyond advancing simulations, we leverage deep reinforcement learning (DRL) to develop adaptive control strategies for turbulence. Our results demonstrate that DRL-based control can significantly enhance heat transfer and momentum transport, surpassing traditional control methods. Furthermore, we show that the complex strategies learned by DRL can be distilled into simplified models, making them interpretable and applicable to new flow conditions. These advancements highlight how extreme-scale computing and AI-driven approaches are transforming our ability to analyze and control flows in engineering and nature.

Bio: Dr. Xiaojue Zhu is an independent Max Planck Research Group Leader in Göttingen. He earned his Ph.D. in 2018 from the Physics of Fluids group at the University of Twente and later worked as a postdoctoral scholar in Applied Mathematics at Harvard University from 2019 to 2021. Dr. Zhu has been recognized with several awards, including the Fellowship of the Daimler and Benz Foundation for Junior Professors, the David Crighton Fellowship from the University of Cambridge, and the NWO Physics Thesis Award for the best physics dissertation in the Netherlands. His group has hosted four Humboldt Fellows to date. His research focuses on computational fluid dynamics, with an emphasis on turbulence and multiphase flows.

Hosted by: Roberto Zenit, Royce Family Professor of Teaching Excellence in Engineering

Center for Fluid Mechanics Seminar Series

Giovanni Bordinga, Harvard University, will present a talk, “Automating the discovery of nonlinear architected materials.”

Abstract: Architected materials leverage structure rather than composition to unlock unprecedented capabilities, ranging from enhanced material properties to machine-like functions such as locomotion, sensing, and computation. However, despite advances in material modeling and computational tools, a key challenge remains: how to systematically encode nonlinear dynamic responses into material architectures while enabling structures to seamlessly switch between multiple functionalities. We tackle this problem by developing differentiable reduced-order models for architected materials to systematically and efficiently discover nonlinear dynamic responses. This approach integrates two essential ingredients: (i) reduced-order models, which retain the key physics of nonlinear architected materials while being significantly more efficient than full continuum models, and (ii) differentiability of the full nonlinear dynamics, allowing efficient navigation of the vast design space to automatically discover optimal architectures. Together, these elements provide a scalable and data-efficient framework for designing materials with tailored nonlinear behaviors. Applying this framework to flexible mechanical metamaterials, we demonstrate how simple materials can acquire advanced capabilities—manipulating the flow of mechanical energy, executing complex motion conversion with minimal actuation, computing their own deformed configuration, and serving as architected “skins” for fully analog robotic control. Additionally, we show how to achieve on-the-fly reprogrammability of the encoded task, allowing a single material architecture to seamlessly switch between functionalities. These examples illustrate how embedding intelligence directly into material architectures reduces the need for external actuation and computation, opening new possibilities for adaptive and efficient engineering systems. Beyond flexible mechanical metamaterials, this framework extends to the inverse design of shape-morphing kirigami structures, nonlinear metamaterial cloaks, and liquid crystal elastomer lattices. These models will be central to advancing nonlinear architected materials, paving the way for a new generation of adaptive, multifunctional, and intelligent material systems with applications in robotics, structural mechanics, and beyond.

Bio: Giovanni Bordiga is a Postdoctoral Fellow in the Bertoldi Group at Harvard University’s School of Engineering and Applied Sciences. His research integrates nonlinear material platforms with computational design to advance architected materials for physically intelligent systems. He develops differentiable reduced-order models that bridge material modeling and computational discovery, enabling the systematic design of architected materials with tailored nonlinear dynamic behaviors. He obtained his PhD in Solids and Structural Mechanics from the University of Trento (Italy), where he developed homogenization techniques and analytical modeling of wave propagation to investigate and control the onset of shear bands and flutter instabilities in beam lattice materials. His research goal is to automate the discovery of nonlinear architected materials, unlocking the potential for reprogrammable, multifunctional systems that embed computation, sensing, and actuation, paving the way for the next generation of autonomous, adaptive material systems. Outside of research, he loves hiking and playing beach volleyball.

Hosted by: Roberto Zenit, Royce Family Professor of Teaching Excellence in Engineering

Center for Fluid Mechanics Seminar Series

Andrew Akerson, Reality Labs Research, Meta Inc., will present a talk, “Optimal Design for Emerging Material Technologies.”

Abstract: The rapid development of new responsive and structural materials, along with significant advances in synthesis techniques that may incorporate multiple materials in complex architectures, provides an opportunity to design functional devices and structures of unprecedented performance. These include soft-robotic actuators, wearable haptic devices, and mechanical protection. However, realizing the potential of these emerging technologies requires a robust, systematic design approach. In this talk, we explore optimization frameworks that enable us to tackle full-scale engineering design. We elucidate the challenges of mathematical regularization, high-fidelity modeling, manufacturability, and material characterization and explore strategies to overcome them through three pressing design problems. First, we investigate the design of 3D-printed soft responsive actuators. We incorporate complex, non-linear behavior to concurrently design the structure, the microstructure, and the manufacturing pathway in a robust design framework. We demonstrate the method by producing responsive lifting actuators of unprecedented capabilities, greatly outperforming traditional designs. Next, we investigate the design of structures for impact resistance. We develop an optimization framework incorporating a novel computational method for modeling the dynamic evolution of plasticity and damage. The design algorithm navigates the unintuitive trade-offs between strength and toughness to design spall-resistant structures undergoing impact loading. These design methodologies are rooted in continuum modeling, and thus require calibration of constitutive laws to accurately capture often-unexplored material behaviors. Our final study borrows ideas from optimal design to cast data-driven characterization as a PDE-constrained optimization problem, exploiting modern data-rich measurement techniques to calibrate models over complex stress-strain trajectories. We demonstrate the method by characterizing visco-plastic behavior in both quasi-static and dynamic settings. We conclude the talk by highlighting the remaining challenges and open problems that will allow optimal design to leverage the full capabilities of emerging material technologies.

Bio: Andrew Akerson is a Research Scientist at Reality Labs Research, Meta Inc. He received his PhD in Mechanical Engineering from the California Institute of Technology (Caltech) in 2023. Before arriving at Caltech, he received his Bachelors and Masters from the University of Minnesota in Aerospace Engineering and Mechanics. His research is broadly in the field of theoretical and computational mechanics, with an emphasis on developing applications-focused optimal design methods for highly complex scenarios.

Disclaimer: Andrew Akerson is speaking as an individual, not on behalf of Meta Inc.

Hosted by: Roberto Zenit, Royce Family Professor of Teaching Excellence in Engineering

Center for Fluid Mechanics Seminar Series

Anna Tarakanova, University of Connecticut, will present a talk, “From Molecules to Mechanics: Computational Insights into Molecular and Multiscale Determinants in Health and Disease.”

Abstract: The rapid development of high-performance computing has enabled unprecedented advances in the mechanistic understanding of molecular biomaterials. We direct the power of our “computational microscope” to investigate complex biological molecular systems such as the extracellular matrix, to understand how and when these systems function in health and break down in disease. We also develop computational methods for the multiscale and data-driven characterization and design of novel functional biomaterials from the nanoscale, for medical and engineering applications, drawing inspiration from natural design principles. I will present recent work that combines multiscale modeling and big data frameworks to probe and predict behavior of complex biomolecular systems. 

Bio: Dr. Anna Tarakanova is an Assistant Professor in the School of Mechanical, Aerospace, and Manufacturing Engineering and in the Department of Biomedical Engineering at the University of Connecticut. Her research focuses on advancing molecular, multiscale and data-driven modeling methods to study the structure, function and mechanics of complex nanoscale biological materials. These tools provide the foundation to ask essential questions about the behavior of tissues such as arterial elastic tissue and bone, and to repurpose molecules for new functions like improved immunogenicity, thermal stability or resilience in aging. In particular, her work aims to characterize extracellular matrix proteins including elastin, collagen and fibrillin, and their role in the context of aging and human disease. She received her BS from Cornell University, and her MS and PhD from the Massachusetts Institute of Technology. Dr. Tarakanova leads a multi-disciplinary research program funded by NIH and NSF, among other sources, and is a recipient of the NSF Career Award, the Eshelby Mechanics Award for Young Faculty, the University of Connecticut Excellence in Research and Creativity Early Career Award, the InCHIP Junior Faculty Research Excellence Award, and the Mara H. Wasburn Early Engineering Educator Award.

Hosted by: Roberto Zenit, Royce Family Professor of Teaching Excellence in Engineering

Center for Fluid Mechanics Seminar Series

Peko Hosoi, MIT, will present a talk, “Filtration and fundamental fluid mechanics inspired by the manta ray .”

Abstract: Heating, ventilation, and air conditioning (HVAC) systems account for about 20% of energy consumption in the US of which at least 7% is consumed by fans. In addition, approximately 4% of US energy is consumed by the production, treatment, and distribution of water. A key component of the efficiency in all of these systems is the performance of filters in which reducing resistance can result in significant energy savings. In this talk, we will explore novel strategies for filtration inspired by the manta ray which has evolved a system for filtering zooplankton that appears to be unlike any filtration mechanism previously observed in biological or industrial settings. Rather than adopting a sieve strategy, the manta deploys microstructures which are hypothesized to instigate eddies that push particles away from the filtration pores, resisting clogging, and enabling the filtration of particles much smaller than the pore size. In this talk we will examine a toy “leaky pipe” problem that mimics various features of the filtration strategies employed by manta rays.

Bio: Anette “Peko” Hosoi is the Neil and Jane Pappalardo Professor of Mechanical Engineering, Professor of Mathematics, and a core faculty member of the Institute for Data, Systems and Society at MIT. Her research contributions lie at the junction of fluid dynamics, biomechanics, and bio-inspired design. More recently, she has turned her attention to problems that lie at the intersection of biomechanics, applied mathematics, and sports. She previously served as the Associate Dean of the MIT School of Engineering. Prof. Hosoi has received numerous awards including the American Physical Society Stanley Corrsin Award, the SIAM I. E. Block Community Lectureship, and the Jacob P. Den Hartog Distinguished Educator Award. She is a Fellow of the American Physical Society, a Radcliffe Institute Fellow, and a MacVicar Faculty Fellow.

Hosted by: Kenneth Breuer, Professor of Engineering

Center for Fluid Mechanics Seminar Series

Nick Pizzo, University of Rhode Island, will present a talk, “Exact planetary waves and jet streams.”

Abstract: We investigate exact nonlinear waves on surfaces locally approximating the rotating sphere for two-dimensional inviscid incompressible flow. Our first system corresponds to a beta-plane approximation at the equator and the second to a gamma approximation, with the latter describing flow near the poles. We find exact wave solutions in the Lagrangian reference frame that cannot be written down in closed form in the Eulerian reference frame. The wave particle trajectories, contours of potential vorticity and Lagrangian mean velocity take relatively simple forms. The mean flow arises due to potential vorticity conservation on fluid particles. In the gamma approximation a class of waves are found which, based on analogous solutions on the plane, we call Ptolemaic vortex waves. These waves, which we can describe in highly nonlinear scenarios due to the exact nature of the solutions, exist on polar jet streams. Several illustrative solutions are used as initial conditions in the fully spherical rotating Navier-Stokes equations, where integration is performed via the numerical scheme presented in Salmon and Pizzo (2023). The potential vorticity contours found from these numerical experiments vary between stable permanent progressive form and fully turbulent flows generated by wave breaking.

Bio: Nick Pizzo is an Assistant Professor of Physical Oceanography at the Graduate School of Oceanography at the University of Rhode Island. He has undergraduate degrees in math and physics from UCSB and received his PhD at the Scripps Institution of Oceanography at UCSD, where he was advised by Ken Melville.

Hosted by: Daniel Harris, Associate Professor of Engineering

Center for Fluid Mechanics Seminar Series

Theresa Ann Saxton-Fox, University of Illinois at Urbana-Champaign, will present a talk, “Turbulent boundary layers in spatially and temporally varying pressure gradients.”

Abstract: Turbulent boundary layers coat aero- and hydrodynamic vehicles. The “separation’’ of a turbulent boundary layer from the vehicle surface during maneuvers dramatically changes the forces experienced by the vehicle. The accurate prediction of these forces and the implementation of control is challenging due to the complexity of turbulent physics. In this talk, we focus on the physics of turbulent boundary layers undergoing temporal and spatial accelerations that can lead to separation. The experiments are carried out using a variable-positioned ceiling in a wind tunnel and are measured using time-resolved, planar particle image velocimetry. Statistical results are shown for both statically held and dynamic pressure gradients. History effects (prior pressure gradients imposed upstream of the position of interest) and dynamic effects (a dependence on the rate of pressure gradient imposition) are both observed. The formation of an internal layer due to the spatially varying pressure gradients is identified for stronger but not weaker static pressure gradient variations, yielding distinct downstream boundary layer evolution. The growth rate and vortex activity of the internal layer are reported. Dynamic pressure gradients in the present configuration are observed to lessen the influence of the internal layer.

Bio: Theresa Saxton-Fox is an Assistant Professor of Aerospace Engineering at the University of Illinois at Urbana-Champaign. She received her Masters and PhD from Caltech and did her postdoctoral research at Princeton University, prior to starting at the University of Illinois in January 2019. Her work focuses on wall-bounded turbulent flows with particular interests in nonlinear interactions, global unsteadiness, and curvature effects. She was awarded the Young Investigator Program award from the Office of Naval Research in 2021, the College Award for Leadership or Institutional Impact in Diversity, Equity, and Inclusion from the Grainger College of Engineering at UIUC in 2023, and the NSF CAREER award in 2023.

Hosted by: Roberto Zenit, Royce Family Professor of Teaching Excellence in Engineering

Center for Fluid Mechanics Seminar Series

Olivier Desjardins, Cornell University, will present a talk, “Making a computational splash: high-fidelity multi-scale computational modeling of spray formation.”

Abstract: Liquid sprays surround us and play important roles in our daily lives, in obvious ways (taking a shower), in more subtle ways (drinking instant coffee produced via spray drying or driving a car powered via liquid fuel injection), or in more profound ways (with sea sprays playing a key part in ocean-atmosphere transfers impacting climate change). As such, we need to understand sprays better and for that we need to be able to predict how they form.

Unfortunately, computational prediction of turbulent multiphase flows presents enormous challenges. This is especially true whenever break-up and topology changes happen, such as in spray formation, in part because of the wide range of length and time scales involved in the spray break-up process.

In this work, we propose to tackle the challenge of high-fidelity modeling of liquid atomization with a novel multi-scale framework. New developments to the geometric volume of fluid method are presented that enable the tracking of sub-grid scale interfacial features. By reconstructing the liquid-gas interface with multiple planar surfaces or with paraboloid surfaces, we show that the small-scale ligaments and sheets that abound in spray formation can be represented accurately independently of mesh resolution while preserving exact conservation, good computational efficiency, and easy integration with finite-volume-based flow solvers.

A consequence of such strategies is that lack of mesh resolution no longer induces topology change, which then need to be reintroduced explicitly using physics-informed models. We discuss various flavors of such models in the context of the break-up of thin liquid films, which are common features in aerodynamic liquid atomization, and show that this approach can predict droplet size distribution accurately even with limited resolution.

Bio: Olivier Desjardins is a Professor in the Sibley School of Mechanical and Aerospace Engineering at Cornell University. He joined the Sibley School faculty in 2011 after three years as an Assistant Professor at the University of Colorado Boulder. He received a Master of Science in Aeronautics and Astronautics from ENSAE (Supaero) in Toulouse, France, and a Master of Science in Mechanical Engineering from Stanford University in 2004. He graduated from Stanford University in June 2008 with a Ph.D. in Mechanical Engineering. Olivier’s research interests are in the general area of multiphase turbulence. His group develops innovative numerical methods and new physical models with specific focus on liquid-gas flows and particle-laden flows. In 2014, he received an NSF CAREER award to work on turbulence modeling around liquid-gas interfaces. In 2016, he was presented with the Junior Award from the International Conference on Multiphase Flow. Currently, Olivier serves as associate editor for Atomization and Sprays, board member of the International Conference on Multiphase Flow, and board member of the Institute for Liquid Atomization and Spray Systems.

Hosted by: Mauro Rodriguez, Assistant Professor of Engineering

Center for Fluid Mechanics Seminar Series

Raul Bayoan-Cal, Portland State University, will present a talk, “Tiny wind turbines in an immense ocean: Flow, power and motion dynamics.”

Abstract: Wind and water tunnel experiments of turbulent wakes in a scaled floating wind farm are performed. Scaling of a floating wind farm with a scaling ratio of 1:400 is made possible by relaxing geometric scaling of the turbine platform system, such that the dynamic response can be correctly matched, and to allow for relaxing Froude scaling such that the Reynolds number can be kept large enough. The response and performance of a single turbine scaled model are characterized for different wind and wave conditions. Subsequently, a wind farm experiment is performed with twelve floating turbine models, organized in four rows and three columns.

Bio: Raúl Bayoán Cal is the Daimler Professor in the Department of Mechanical and Materials Engineering at Portland State University; a faculty member since 2010. He received his Ph.D. in Mechanical Engineering from Rensselaer Polytechnic Institute in 2006. During 2006 to 2009, he was a postdoctoral fellow at Johns Hopkins University. His area of research is focused on understanding fluid flow phenomena as it relates to physical systems such as turbulence with emphasis placed on physics related of wall-bounded, free-shear and multi-phase flows as well as wind/solar energy, and capillary microfluidics with interests in both terrestrial and space applications.

Hosted by: Roberto Zenit, Royce Family Professor of Teaching Excellence in Engineering

Center for Fluid Mechanics Seminar Series

James Bird, Boston University, will present a talk, “From microplastic aerosolization to spreading stains, seeking understanding through model simplicity.”

Abstract: Predictive modeling is a cornerstone of mechanics research, including when addressing problems involving drops and bubbles. Although sophisticated modeling techniques have value, the precision that they offer can come at the cost of clarity. Here a case is made for the utility of simple mechanistic models based on our lab’s research in interfacial fluid dynamics. The talk highlights two areas of research within our group: the enrichment of microplastics and pathogens from bubble-bursting aerosols and the stain size and cooling associated with drops wicking within thin, porous structures like fabric. By combining measurements and models over enormous scales, our aim is to develop a fundamental understanding that can be generalized to provide insight into a diverse set of applications.

Bio: James Bird is an Associate Professor in the Department of Mechanical Engineering at Boston University. He received his B.S. from Brown University and his Ph.D. from Harvard University, after which he completed an NSF postdoctoral fellowship at MIT. His research focuses on interfacial fluid dynamics with an emphasis on the dynamics of drops and bubbles. He is the recipient of a Fulbright Fellowship (2003), an NSF CAREER award (2014), and an ONR YIP award (2016), and his work has been featured in popular press outlets including the New York Times, BBC, and PBS Nova.

Hosted by: Kenneth Breuer, Professor of Engineering, Director of the Center for Fluid Mechanics

Center for Fluid Mechanics Seminar Series

Safa Jamali, Northeastern University, will present a talk, “Quantifying and characterizing the mesoscale in particulate systems: A network science approach.”

Abstract: Attractive colloidal particles in a simple fluid, depending on their packing fraction and interactions can exhibit a wide range of exotic rheological behavior. For instance, they can assemble into space spanning networks with mechanical properties of a viscoelastic solid, aka colloidal gels. Over the past couple of decades and owing to a tremendous advance in our experimental and computational capabilities, we have built an understanding of the complex  dynamics that give rise to such physical and rheological behavior: rather than particle-scale micromechanics, it is the collective dynamics of the colloids at a coarser scale that control the macroscopic/bulk properties of a particulate system. Whether it’s a force network that carries the highest stresses in a shear thickening suspension, or a porous network of particles that gives a gel its elasticity, it is a “network” referring to the collective particle dynamic/behavior that is responsible for the physical characteristics of a system. Thus, understanding the physics of this particulate network is the key to controlling and designing particulate systems with desirable properties. I will discuss how borrowing well-established concepts from network science can help us interrogate and characterize these particulate networks and build a coarse-grained description of the system. These mesoscale structures, identified through community detection techniques that are commonly used in social or economic networks, provide a new understanding of physics and rheology in attractive colloidal gels. Finally, I will discuss some of the unexplored avenues and potential directions that these new techniques can make an impact in.

Bio: Safa is an Associate Professor of Mechanical and Industrial Engineering at Northeastern University, in Boston, USA where he has been since 2017. Prior to joining Northeastern, he received his PhD from Case Western Reserve  University’s Macromolecular Science department, followed by a period of postdoctoral training at MIT’s Chemical Engineering, Mechanical Engineering and Energy Initiative. His research group’s activities are currently focused on developing and using a series of data-driven and computational techniques to study physics and  rheology of complex fluids. Science-based data-driven methods and machine-learning platforms for rheological applications have been a major thrust of his efforts in recent years, in addition to using computational and network analytic tools to better understand the physics of particulate systems.

Hosted by: Daniel Harris, Associate Professor of Engineering

Center for Fluid Mechanics Seminar Series

Howard A. Stone, Princeton University, will present a talk, “Intersections of classical and modern ideas in fluid mechanics: Self-similarity, time derivatives for complex fluids, and biological condensates.”

Abstract: In this talk I sketch some recent themes* from my research group, starting with a brief survey of some of the fluid mechanics problems that we have been investigating in recent years. Second, traditional similarity solutions in course work and research typically involve nonlinear equations with two independent variables. I will illustrate an unusual case involving three independent variables, which is inspired by an experiment and rationalized with an analysis of the appropriate thin film equation. Third, time derivatives are common in fluid mechanics and I will discuss their use in complex fluids, using as a guide the idea of a line element in fluid mechanics. Finally, if there is time, I will discuss the formation of the spindle in a dividing cell, which is a fundamental aspect of molecular biology, and show how some fluid mechanics questions appear. *The research described was performed by many people in my research group, as well as some external collaborations.

Bio: Professor Howard Stone received the B.S. degree in Chemical Engineering from UC Davis in 1982 and the PhD in Chemical Engineering from Caltech in 1988. After a postdoctoral fellowship at the University of  Cambridge, in 1989 he joined the faculty of the School of Engineering and Applied Sciences at Harvard University. In July 2009 Howard moved to Princeton University where he is Donald R. Dixon ’69 and Elizabeth W. Dixon Professor in Mechanical and Aerospace Engineering. Howard’s research interests are in fluid dynamics, broadly interpreted. He is a Fellow of the American Physical Society and is past Chair of the Division of Fluid Dynamics. He was the first recipient of the G.K. Batchelor Prize in Fluid Dynamics (2008). He has been elected to the National Academy of Engineering (2009), the American Academy of Arts and Sciences (2011), the National Academy of Sciences (2014), the Royal Society (United Kingdom) as a Foreign Member (2022), and the American Philosophical Society
(2022).

Hosted by: Mauro Rodriguez, Assistant Professor of Engineering

Center for Fluid Mechanics Seminar Series

Karen Daniels, NC State University, will present a talk, “Rigidity, Failure, and Flow in Dense Granular Materials.”

Abstract: Granular materials lie at the boundary of rigidity and flow, and even when the grains flow their dynamics range from slow creep to continuous flow to intermittent failure. If we focus on the contacts between particles, we observe that the patterns of force transmission exhibit spatial correlations often known as a force network. However, these networks are not homogeneous at some larger scale: instead they map out both rigid and floppy subregions of the material. This behavior complicates the utility of taking system-wide averages, and makes continuum modeling challenging to achieve. In my talk, I will show how experiments using photoelastic materials provide a means to address these challenges, and what frameworks can be used to explain the observations and predict the failure and flow of granular materials.

Bio: Karen Daniels is a Distinguished Professor of Physics at NC State University. Her lab at NC State investigates a number of problems in the deformation and failure of materials, from fluid flows, to piles of sand, to fracturing gels. When not working with her students on experiments in the lab, she likes to spend time in the outdoors, which has led her to contemplate the implications of her research for geological and ecological systems. She is a Fellow of the American Physical Society and AAAS, and during 2023-24 she was a Fulbright-Nehru Fellow at the Indian Institute of Science in Bangalore, India.

Hosted by: Kenneth Breuer, Professor of Engineering, Director of the Center for Fluid Mechanics

Center for Fluid Mechanics Seminar Series

Laura Villafane, University of Illinois Urbana Champaign, will present a talk, “Cluster and inertial particle dynamics across flow regimes, from fully developed
turbulence to impinging jets.”

Abstract: Small heavy particles, in non-negligible amounts, are commonly present in environmental flows and engineering processes, on Earth and beyond. In this talk we will discuss two ongoing experimental efforts on particle laden flows, on very distinct flow configurations and regimes but both challenged by imaging particles in high concentration regions, and motivated by understanding parametric dependances on the driving flow physics. The first  part will focus on preferential concentration of small inertial particles by turbulence in fully developed flows and the dynamics of regions of high local concentration, i.e. particle clusters. Interest in cluster temporal statistics is rooted on modulation of the underlying turbulence as well as on other particle mediated phenomena such as heat transfer, reaction rates or transmission through a dynamic inhomogeneous media. We will present a new four-stories-long vertical particle-laden channel flow facility and discuss recent results from high-speed planar imaging on particle cluster size, local concentration conditioned velocities, and lifetime statistics, for varying Stokes number – ratio of particle to dissipative time scale. Point-particle DNS results are also used to understand the effect of 3D to 2D data reduction. The second part of the talk will deal with plume granular surface interactions, with focus on measurements of the dense particle cloud formed as an underexpanded jet erodes a bed of about 100 um particles. The dense cloud of highly inertial particles poses major risks to Lunar and planetary exploration missions and challenges visibility, both from lander instruments and in ground facilities. A novel mm-wave interferometric technique enables time varying spatial ejecta concentration measurements, which combined with other diagnostics is being used to reconstruct 4D ejecta evolution and estimate erosion rates for the first time in an optically opaque particle-laden flow.

Bio: Laura Villafañe is an Assistant Professor in the Department of Aerospace Engineering at the University of Illinois Urbana-Champaign. Her research explores a wide range of fluid dynamic problems, with particular interest on turbulent and particle-laden flows, and on the development of data analysis tools and non-intrusive diagnostics, including non-conventional flow diagnostics such as Magnetic Resonance Imaging. She graduated on Aerospace Engineering at the Polytechnic University of Madrid, completed her Ph.D at the von Karman Institute for Fluid Dynamics, Belgium,and was a Postdoctoral Fellow and Research Engineer at the Center for Turbulence Research at Stanford University prior to joining the faculty at UIUC. She is the recipient of three NASA Early-Stage-Innovation Awards for plume-surface interaction and  parachute fluid and structural mechanics, and of NASA FINNEST and NSTGRO awards together with her students. Laura was elected Young Observer to the US National Committee for Theoretical and Applied Mechanics in
2024.

Hosted by: Mauro Rodriguez, Assistant Professor of Engineering

Center for Fluid Mechanics Seminar Series

Maciej Lisicki, University of Warsaw, will present a talk, “Tales of tails: Elastohydrodynamics of microscale locomotion.”

Abstract: A look into the microworld reveals a plethora of swimming microorganisms, which display a rich variety of shapes and swimming gaits. Despite this diversity, the physics of microscale imposes universal limitations on their propulsion strategies. In my talk, I will review the basic properties of Stokes flows and their consequences on swimming. Next, I will show an artificial system of microscale oil droplets that have the ability to swim due to a surface phase transition driven by environmental temperature fluctuations. I will demonstrate how a coarse-grained elastohydrodynamic model can be successfully employed to quantitatively describe the motion of droplets. I will also show a couple of other examples where a simplified elastohydrodynamic model proves useful for the prediction of diffusive properties.

Bio: Maciej Lisicki is a professor at the Faculty of Physics, University of Warsaw, currently on sabbatical as Fulbright Scholar at the University of Pennsylvania. He works in the field of soft matter physics and biological fluid dynamics. He is also interested in the physics of everything that flows around us. After his PhD in colloidal science at the University of Warsaw, Maciej spent 3.5 years as a postdoctoral fellow at the University of Cambridge, researching how bacteria swim and induce microscale flows in their surroundings. Their propulsion mechanisms inspire artificial designs and foster the development of the field of active matter systems.

As a side activity, he recently co-authored a review paper on kitchen flows and leads a team researching the physics of brewing an ideal espresso. Maciej is also keen on sharing his experience with the general public through various outreach activities. Apart from science, Maciej greatly enjoys the outdoors, hiking, skiing, cycling, and chasing life’s little pleasures.

The 99th Quarterly New England Complex Fluids Workshop encourages worldwide collaboration among researchers from industry and academe studying Soft Condensed Matter. These workshops offer opportunities for discussion and exchange of ideas.

New England Complex Fluids’ goal is to encourage collaboration among  researchers from industry and academe in the New England area studying Soft Condensed Matter. They hold one day workshops four times a year which offer the opportunity for discussion and exchange of ideas. An additional objective of these meetings is to help the career development of students and postdocs by introducing them to the local academic and industrial research community.

Center for Fluid Mechanics Seminar Series

Joanna B. Dahl, Assistant Professor, University of Massachusetts, Boston, will present a talk, “Measuring the Mechanical Behavior of Small, Squishy Bio-Things Using Microfluidics.”

Abstract: Understanding the mechanical behavior of microscale biological bodies such as cells and vesicles are important for fundamental cell biology research and for disease diagnostics and therapeutics in clinical settings. Microfluidic devices are ideally suited for studying small, soft objects due to their well-defined laminar flows, transparent material for direct observation, and high-throughput capabilities. With accompanying mechanical modeling, we can perform detailed mechanical analysis of biological soft bodies trapped at the stagnation point or passing through the extensional flow region. the presentation focuses on our current projects performing miniaturized creep tests on biomimetic hydrogel microparticles, exploring how the stiffnesses of large extracellular vesicles from cancer cells vary with lipid-altering mutations, and investigating the continuum of cell spheroid biomechanical behavior with spheroid size.

Bio: Joanna B. Dahl, Ph.D., earned a B.S. in mechanical engineering from the University of Illinois at Urbana-Champaign in 2007. She earned a Ph.D. in mechanical engineering from the University of California, Berkeley in 2013 under the supervision of Prof. David B. Bogy. During the pursuit of her doctorate, Dr. Dahl simulated the mechanics at the critical head-disk interface of a hard disk drive and provided product-level predictions of the performance for the emerging heat-assisted magnetic recording technology. From 2013-2015, Dr. Dahl served as an NSF postdoctoral research fellow under the mentorship of Profs. Susan J. Muller (chemical engineering) and Sanjay Kumar (bioengineering) at the University of California, Berkeley. There she developed a microfluidic platform to quantitatively study the mechanics of microscale soft bodies. In the fall of 2016, Dr. Dahl joined the new engineering Department at the University of Massachusetts Boston as an Assistant Professor. Her research program is in experimental biomechanics. Projects include measuring the stiffnesses of extracellular vesicles and cells with applications for cancer biology, developing viscoelastic measurement techniques using, and investigating synthetic vesicle biophysics.

Alvaro Romero-Calvo, Assistant Professor, Georgia Tech, will present a talk, “Touchless Magnetohydrodynamic Actuation for Spacecraft Fluid Management.” 

Abstract: The space community is undergoing an accelerated transformation boosted by the commercialization of the sub-orbital and orbital environments and the ambition to make humans a multi-planetary species. Upcoming space architectures depend critically on fluid management systems to position and gauge propellants, electrolyze water for oxygen production, transport heat, or feed biological samples, among many others. However, the state-of-the-art in low gravity fluid control relies on methods that are highly sensitive to contamination, hard to design and operate, or unacceptably unreliable for human missions. To address these issues, a series of touchless electromagnetic fluid control mechanisms are currently being developed to actuate a new generation of low gravity devices. The first, magnetic polarization, provides an inhomogeneous mid range force that enables bubble steering and collection without moving parts. This effect has been demonstrated in Proton Exchange Membrane (PEM) water electrolysis and is being considered in the design of compact bubble traps for space biology experiments. The second, the magnetohydrodynamic (MHD) Lorentz force, exploits the interaction between magnetic and electric current fields to drive liquid movement in microgravity. The approach has been successfully adopted in developing high-power liquid metal heat transfer systems and alkaline water electrolysis cells for Earth and microgravity conditions. This seminar introduces the field of low-gravity magnetohydrodynamics (LG-MHD) with a focus on space engineering applications. Experiments carried out at ZARM’s drop tower, Zero Gravity Corp.’s parabolic plane, and Blue Origin’s New Shepard suborbital rocket are discussed in the context of multiple technology development efforts. The resulting space systems feature no moving parts and a limited number of failure modes, offering promising solutions to the fluid management challenges of the Artemis Era.

Bio: Dr. Álvaro Romero-Calvo is an Assistant Professor at the Daniel Guggenheim School of Aerospace Engineering at the Georgia Institute of Technology, where he leads the Low-Gravity Science and Technology Laboratory. His work leverages electromagnetic and acoustic actuators to address longstanding engineering problems in low-gravity fluid mechanics and dust dynamics. He has flown several payloads at multiple microgravity facilities to support the development of oxygen generation, dust mitigation, and propellant positioning systems. He serves as a governing board member of ASGSR, vice-chair of the COSPAR Commission G: Material and Fluid Sciences in Space Conditions, and member of the AIAA Microgravity and Space Processes Technical Committee.

Host: Roberto Zenit, Royce Family Professor of Teaching Excellence in Engineering

Center for Fluid Mechanics Seminar Series

Christina Harvey, Asst. Professor in Mechanical and Aerospace Engineering a the University of California at Davis, will present a talk, “Morphing for flight stability and control: Lessons from birds.”

Abstract: Uncrewed aerial vehicles (UAVs) struggle to maneuver in cluttered or unpredictable environments. In contrast, birds regularly accomplish an impressive array of in-flight transitions, from maneuvering through cities, evading predators or gliding in gusty conditions. Birds rapidly adapt and maneuver in these variable flight conditions by actively or passively adjusting their wing or tail shape in flight, known as morphing. In this talk, I will discuss how avian wing morphing is associated with their enhanced maneuverability and adaptability. Furthermore, I will highlight how a fundamental understanding of biological principles improves our ability to design UAVs with expanded operational capabilities and advances us towards intelligent UAV concepts.

Bio: Dr. Christina Harvey is an Assistant Professor in Mechanical and Aerospace Engineering at UC Davis, where she leads the Biologically Informed Research and Design (BIRD) lab. Her group studies how, when, and why flying animals adjust to their environment with the goal of improving the maneuverability and adaptability of future uncrewed aerial vehicles (UAVs) as well as advancing our fundamental understanding of biological flight. She is a 2023 Packard Fellow and 2021 Amelia Earhart Fellow. She holds a PhD in Aerospace Engineering from the University of Michigan, a M.Sc. in Zoology from the University of British Columbia and a B.Eng. in Mechanical Engineering from McGill University.

Host: Kenny Breuer, Professor of Engineering 

Tali Khain, Ph.D. candidate at the University of Chicago, will present a talk, “Chiral fluids across scales: from Stokes flow to turbulence.”

Abstract: Systems composed of spinning particles or driven by a magnetic field break mirror symmetry at the microscopic level. These chiral fluids can be described by adding additional co-called “odd” viscosities, which do not dissipate energy, in the Navier-Stokes equations. Using a combination of analytical and numerical methods, we show how these odd viscosity coefficients modify how fluids flow across a range of Reynolds numbers. In the Stokes regime, the rotational flow induced by the odd viscosity affects the flow past a sphere and the sedimentation of a cloud of particles. The interplay between particle geometry and fluid chirality gives rise to counterintuitive phenomena such as the motion of particles transverse to applied forces and spinning of achiral particles under the force of gravity. At the opposite end of the Reynolds number spectrum, the non-dissipative nature of odd viscosity disrupts the energy cascade that naturally occurs in full developed turbulent flows, leading to the formation of patterns with a tunable wavelength. 

Bio: Tali Khain is a fifth-year physics Ph.D. student at the University of Chicago, where she works with Vincenzo Vitelli. Her current research develops a theoretical fluid mechanics framework to capture the effect of broken symmetries in soft matter systems. At the 2024 APS March Meeting, she won the DSOFT Emerging Soft Matter Excellence Award for her research on chiral fluids. Tali received a B.S. in Physics and Mathematics from the University of Michigan in 2019. As an undergraduate researcher, she worked primarily on solar system dynamics and the effect of the hypothesized Planet Nine on the outer reaches of the solar system. For this work, she received the 2019 American Physical Society Apker Award.

Host: Professor Kenny Breuer, Director of the Center for Fluid Mechanics

Yahya Modarres-Sadeghi, Professor of Mechanical Engineering at the University of Massachusetts at Amherst, will present a talk, “In the wake of a rotating cylinder.”

Abstract: When a cylinder placed in flow is forced to rotate, the frequency and strength of vortices that are shed in its wake deviate from those for a fixed cylinder in uniform flow. Depending on the incoming flow velocity, the cylinder rotation rate and the waveform of the imposed rotation, the vortices in the wake could be deflected to one side, shed at difference frequencies, or completely suppressed. This then introduces an opportunity to induce a desired motion on a structure that is placed in the wake of such a cylinder by controlling the rotation of the cylinder. The structure of course must be free to move or flexible, so that it can react to the forces that are inserted on it from the shed vortices. Here, we consider the response of a rigid body (a free hydrofoil or a hydrofoil attached to a double pendulum) and a flexible body (a flexible uniform sheet or a flexible sheet with a kirigami pattern) to the wake of a cylinder forced to rotate. We will see cases where a walking gait trajectory is imposed on a passive walking device (i.e., a double pendulum), cases where oscillations of a flexible sheet are suppressed, and cases where rotation has induced vortices in the within the subcritical range.

Bio: Yahya Modarres-Sadeghi is a Professor of Mechanical Engineering at the University of Massachusetts, Amherst, a Fellow of the Radcliffe Institute for Advanced Study at Harvard University, and a Research Affiliate at the Massachusetts Institute of Technology (MIT). He received his Ph.D. from McGill University and spent three years at MIT as a postdoctoral associate before joining the University of Massachusetts in 2009. His research focuses on Fluid-Structure Interactions (FSI) and Nonlinear Dynamics. He uses experimental, theoretical, and numerical tools to understand different FSI phenomena both from a fundamental point of view and with applications in several fields including wind energy, bioinspired robotics, and biomedical science. He is an Associate Editor for the Journal of Fluids and Structures, and has recently published a book titled Introduction to Fluid-Structure Interactions. 

Host: Kenny Breuer, Professor of Engineering and Director of the Center for Fluid Mechanics

Center for Fluid Mechanics seminar

Mona Kanso, Postdoctoral Associate at MIT, will present a talk, “Recent Advances in Polymeric Flows: Exploring Dynamic Coronavirus Shapes and Lipid Nanoparticle Manufacturing.”

Abstract: One good way to explain the elasticity of a polymeric liquid, is by just considering the orientations of its macromolecules. We can thus delve into the intricate dynamics of coronaviruses and thus their viscoelasticity, and into the interplay of lipid nanoparticles (LNPs) with their mRNA payloads. For this we use general rigid bead-rod (called rotarance) theory. Usually idealized as spiked spherical capsids, coronaviruses exhibit a histological reality of elliptical cross-sections, called pleomorphism. The analysis begins by elucidating the impact of macromolecular architecture on the elasticity of polymeric liquids, employing rotarance theory. Using rotarance theory, the rotational diffusivity of spiked spherical coronavirus suspensions is calculated, for capsids oblate, prolate, and neither. Further, energy minimization is applied to model the spreading of identically charged spikes over oblate or prolate capsids. We explore the profound impact of coronavirus ellipticity on its rotational diffusivity, the transport property crucial to cellular attachment. In addition, advancing recent studies in polymer viscoelasticity, the innovative dimension is the application of macromolecular models of LNPs, particularly in the context of mRNA delivery. Known for their versatility in delivering genetic material like mRNA, LNPs present a fascinating avenue for understanding and manipulating their transport properties in the context of gene therapy and vaccine development. We further discuss the potential implications of LNPs in modulating these transport properties for therapeutic purposes. This multidimensional modeling approach deepens our understanding of coronavirus architecture dynamics and reveals promising avenues presented by mRNA-LNPs in the sustainable manufacturing realms of gene therapy and vaccine development.

Bio: Dr. Mona Kanso is a Postdoctoral Associate in the Chemical Engineering Department at Massachusetts Institute of Technology in Boston. She earned her Ph.D. in Chemical Engineering at Queen’s University, Canada where she held a Vanier Canada Research Scholarship. Born in Beirut, Lebanon, Kanso earned her Bachelor’s in Chemical Engineering from the American University of Beirut. She then earned her Master’s degree in Chemical Engineering from Queen’s, under Professor Jeffrey Giacomin’s supervision, her thesis titled “Polymeric Liquid Behavior in Oscillatory Shear Flow”. She has developed a way to design polymer molecules and calculate their complex viscosities from first principles. Dr. Kanso defended her PhD thesis in December 2022 on “Coronavirus Hydrodynamics” at Queen’s University.

Hosted by: Mauro Rodriguez , Assistant Professor of Engineering

Center for Fluid Mechanics Seminar Series

Saverio Spagnolie, Professor of Mathematics at Wisconsin-Madison, will present a talk, “Active matter in complex fluids.”

Abstract: The survival of many microorganisms depends on their ability to navigate through complex biological environments. Complex fluid phenomena like viscoelasticity and anisotropy can have unexpected and important consequences for motility. We will discuss analytical and numerical insights into swimming through model viscoelastic and anisotropic fluids, either by one active swimmer or by a suspension of swimmers. The theoretical framework also speaks to non motile active suspensions that arise in other biological and synthetic settings. We will observe the outsized role played by the presence of nearby boundaries, and we will propose a classification which seeks to place a multitude of systems based on the relative size and timescale of active bodies and surrounding obstacles, broadly interpreted.

Bio: Saverio Spagnolie is a Professor of Mathematics at the University of Wisconsin-Madison, with a courtesy appointment in Chemical and Biological Engineering. He received his PhD in Mathematics from the Courant Institute in 2008, and then held postdoctoral positions in the Mechanical/Aerospace Engineering department at UC San Diego and in the School of Engineering at Brown University before joining the faculty in Madison.

His research interests include the dynamics of soft and active matter in viscous and complex biological environments, which he studies using numerical simulation, classical methods of applied mathematics, and tabletop experiments. He is a UW Madison Vilas Associate, and is the director of the Madison Applied Mathematics Lab. 

Center for Fluid Mechanics Seminar Series

Ken Kamrin, Professor of Mechanical Engineering and  Applied Mathematics at MIT, will present a talk, “Simulating solids like fluids: A fully Eulerian approach to fluid-structure interaction.”

Abstract: Fluids and solids tend to be addressed using distinct computational approaches. Solid deformation is most commonly simulated with Lagrangian finite element methods, whereas fluid flow is amenable to Eulerian-frame approaches such as finite difference and finite volume methods. Problems that mix fluid and solid behaviors simultaneously present interesting numerical challenges. This is true when fluids and solids occupy different regions of space i.e. fluid-structure interaction (FSI) or in cases where materials behave like a solid but can undergo enormous levels of plastic flow more common of fluids i.e. granular materials and yield stress fluids. Here we focus on FSI, and discuss the development of a method called the Reference Map Technique, which allows us to simulate deformable solids on a fixed Eulerian grid. The key is to store and update the reference map field on the grid, which tracks the inverse motion. Using this technique to represent the solid phase, we can solve FSI problems on a single fixed grid using fast update procedures very similar to those used in two-phase Navier-Stokes fluid simulations. Various solid constitutive behaviors can be used, including nonlinear elasticity and plasticity. Systems of many submerged and interacting solids can be simulated, and, by activating the solids internally, we can simulate systems of soft active media. Incompressibility and/or rigidity constraints can also be applied by adopting Eulerian projection approaches commonly used in CFD. The addition of the reference map field to the grid also presents certain benefits when computing level-set interface advection, including a procedure to guarantee mass conservation. 

Bio: Ken Kamrin received a BS in Engineering Physics with a minor in Mathematics at UC Berkeley in 2003, and a PhD in Applied Mathematics at MIT in 2008. Kamrin was an NSF Postdoctoral Research Fellow at Harvard University in the School of Engineering and Applied Sciences before joining the Mechanical Engineering faculty at MIT in 2011, where he was appointed the Class of 1956 Career Development Chair. He is currently a professor of Mechanical Engineering and Applied Mathematics at MIT. Kamrin’s research focuses on constitutive modeling and computational mechanics for large deformation processes, with interests spanning elastic and plastic solid modeling, granular mechanics, amorphous solid mechanics, and fluid-structure interaction. Kamrin has been awarded fellowships from the Hertz foundation, US Defense Department, and National Science Foundation. Kamrin’s honors include the 2010 Nicholas Metropolis Award from APS, the NSF CAREER Award in 2013, the 2015 Eshelby Mechanics Award for Young Faculty, the 2016 ASME Journal of Applied Mechanics Award, and the 2022 MacVicar Faculty Fellowship from MIT. He is co-author of the recent undergraduate textbook Introduction to Mechanics of Solid Materials (Oxford).

Host: Mauro Rodgriguez; Assistant Professor, School of Engineering

Center for Fluid Mechanics Seminar Series

Peter Lewin-Jones, Ph.D. student in Applied Mathematics at the University of Warwick, UK, will present a talk via Zoom, “Impacts of liquid drops: When do gas microfilms prevent merging?” 

Abstract: Collisions and impacts of drops are critical to numerous processes, including raindrop formation, inkjet printing, food manufacturing and spray cooling. For drop-drop collisions, increasing the relative speed leads to multiple transitions: from merging to bouncing and then back to merging - transitions which were recently discovered to be sensitive to the drops’ radii as well as the ambient gas pressure. The outcome of a drop impacting a liquid bath is even more complex: for a fixed speed, the result can go from merging to bouncing to merging and back to bouncing with increasing bath depth.

To provide new insight into the physical mechanisms involved and as an important predictive tool, we have developed a novel, open-source computational model for both drop-drop and drop-bath events, using the finite element package oomph-lib. This uses a lubrication framework for the gas film and incorporates fully, for the first time, the crucial micro- and nano scale influences of gas kinetic effects and disjoining pressure.

Our simulations show strong agreement with experiments for the transitions between merging and bouncing, but can also go beyond these regimes to make new experimentally-verifiable predictions. We will show how our model enables us to explore the parameter space and discover the regimes of contact (that are inaccessible to experiments). Finally, we will overview potential extensions to the computational model, including impacts in Leidenfrost conditions and post-contact dynamics.

Center for Fluid Mechanics Seminar Series

Alexander Gehrke, Postdoctoral Researcher at  Brown University, will present a talk, “From Inspiration to Application: Advancing Unsteady Aerodynamics Through Biomimetic Design.”

Abstract: Unsteady aerodynamic systems offer potential for improved performance, particularly in energy harvester designs or low Reynolds number flight. Actively or passively controlled wings can achieve higher wing loading and adapt to time-varying flow conditions, such as increasing turbulence or strong gusts. Despite their potential advantages, these systems are seldom integrated into practical applications due to the challenges associated with their design and control. The interaction of large-scale flow structures and complex wing dynamics can lead to strong force peaks, unstable flight dynamics, and eventual system failure. Where our engineered unsteady systems still struggle, nature has already figured it out. Natural fliers and swimmers control their movement with ease, using complex wing and fin dynamics for efficient propulsion and maneuverability. Additionally, many natural fliers and plant life use flexible and lightweight structures for passive control of unsteady flow and load alleviation in gusty conditions. In this research, data-driven learning is applied to the kinematics of insects, birds, and bats in hovering flight using an experimental engineering model. The study demonstrates that precise control of the leading-edge vortex growth is crucial for achieving high wing loading and power-efficient hovering flight. While the large scale vortex can dramatically increase lift, it can also lead to high unsteady loads and increased power expenditure. Then, the use of porous and flexible materials for passive flow control is proposed. Inspired by natural structures like the seeds of the dandelion, bristled wings, and bird feathers, we conduct experiments using poro-elastic membranes for drag mitigation. These membranes deform under fluid loading, leading to pore expansion and increased flow through the membrane. The poro-elasticity leads to the stabilization of the unsteady wake, and a reduction in load fluctuations experienced by the membrane. This research highlights the potential of biomimetic design and passive flow control strategies in advancing complex, unsteady aerodynamic systems.

Bio: Alexander Gehrke started as a postdoctoral research associate in the School of Engineering at Brown University in February 2023. He is an experimental fluid dynamicist with a focus on unsteady, bio-inspired aerodynamics, and fluid structure interaction. Alex studied mechanical engineering at the University of Kassel in Germany. He received his PhD in mechanical engineering from the École Polytechnique Fédérale de Lausanne (EPFL) in Switzerland in 2023 for his work on the effects of kinematics, flexibility, and planform on adaptive, bio-inspired propulsors.

Arnaud Lazarus, Visiting Scholar in Materials Science and Mechanical Engineering at Harvard University, will present a talk, “Exploiting periodicity in the dynamic stability of systems with time-varying properties.”

Abstract: Dynamic stability is the ability of a system’s configurational state to overcome a disturbance over time. A common way to passively control this dynamical feature is to periodically modulate the properties of the system in time. This is what is done to trigger the swings of the Butafumeiro at the Santiago de Compostela Cathedral, to dynamically stabilize charged particles in mass spectrometers or to sustain exotic quantum states in Floquet engineering. In this presentation, we show that new dynamical phenomena emerge when the periodic variations of the system’s properties are large and occur on similar time scales than the natural ones. In this regime, it becomes straightforward to break the record of superharmonic orders observed in the parametric resonances of an oscillator, to optimally trap a magnetic dipole on its unstable equilibrium and to generate a complex nonlinear mechanical response from a single two-state input. All the shown examples are rationalized through a fundamental 1 degree-of freedom model and some desktop-scale experiments but the dynamical concepts being universal, it should offer new functionalities across scales and engineering domains.

Bio: Arnaud Lazarus received a PhD degree in mechanical engineering from Ecole Polytechnique, Palaiseau, France, in 2008. After two post-doctoral years in Paris, he joined the Massachusetts Institute of Technology as an associate researcher from 2010 to 2013. In 2013, he became an associate professor at Sorbonne Université, Paris, France, doing his research at Institut Jean le Rond ∂’Alembert. Since 2023, he is an invited professor in Katia Bertoldi’s group within the School of Engineering and Applied Sciences at Harvard. His current interests include the stability of dynamical systems and the mechanical behavior of slender elastic structures.

Hosted by: Dan Harris, Assistant Professor of Engineering & David Hennan, Associate professor of Solid Mechanics

Center for Fluid Mechanics

Minki Kim, Postdoctoral Researcher, Brown University, will present a talk, “Multiphase Flows of Sustainability: Applications in Arctic Ocean Observation and Cultivated Meat Production.”

Abstract: Multiphase flows, ubiquitous in various sustainability applications, play a crucial role in climate projection, environmental conservation, and food processing optimization to address pressing global challenges. I will discuss two applications of multiphase flow systems: Arctic Ocean observation and cultivated meat production. The first introduces the framework for quantifying the Arctic Ocean eddies using small pieces of sea ice. In the Arctic, small-tomoderate- scale ocean eddies (1–100 km) play a critical role in the freshwater budget and heat transport. Quantifying the properties of eddies, however, poses significant challenges due to sparse in-situ measurements and the limited resolution of satellite altimetry. Here, we outline our methodology for inferring ocean eddy characteristics by leveraging observations of sea ice over days to weeks, ranging in size from 4 to 75 km, providing valuable inputs for better climate projections. In the second part of the talk, I will introduce a simulation toolkit designed for food engineering, with a focus on its application in cultivated meat production. In this industry,rocking bioreactors have emerged as a promising solution for large-scale cell cultivation, offering advantages such as disposability, cost-effectiveness, and scalability. To ensure proficient cell growth, it is essential to optimize mixing and oxygen transfer while minimizing shear stresses. We explore the detailed flow features and their influence on the degree of mixing, oxygen transfer coefficient, and shear stress in the bioreactor under various operating conditions. Our findings are intended to provide guidelines for designing and operating bioreactors in the cultivated meat industry.

Bio: Dr. Minki Kim is a postdoctoral research associate at Brown University in the School of Engineering. He obtained his Ph.D. from the University of Michigan–Ann Arbor in 2022 in Mechanical Engineering and Scientific Computing where he studied energy transport in the growth and collapse of cavitation bubbles after he received his B.S. from Seoul Nation University in South Korea. His research aims to tackle sustainability challenges by leveraging his expertise in computational fluid dynamics. 

Center for Fluid Mechanics Seminar

Ellen Buckley, Brown University, will present a talk, “Remote Sensing of Arctic Sea Ice.”

Abstract: Arctic sea ice, once a thick, healthy, extensive barrier between the atmosphere and ocean, is consistently retreating to extents lower than the long term average. Although models tend to agree that sea ice is diminishing quickly, there is a wide spread of predictions of when we will reach an ice-free summer Arctic. To best predict the future extent and thickness of Arctic sea ice, we must refine our understanding of processes represented in models. This seminar focuses on the work to extract detailed information from remotely sensed observations to study ocean and sea ice properties and processes at both large and small scales. We examine high-resolution satellite and airborne imagery and altimetry to understand the evolution of the ice cover through spring and summer. As the Arctic warms and the sea ice continues to disintegrate and become less reliable and stable for in situ measurements, remote sensing will become increasingly important for monitoring the polar regions, an indicator of global climate change. 

Bio: Dr. Ellen Buckley is a postdoc at Brown University in the School of Engineering. Her research aims to understand the ongoing changes in the polar oceans by synthesizing information from remote sensing observations. She was previously a graduate student member of the NASA Ice Cloud and land Elevation Satellite -2 (ICESat-2) and the NOAA Ocean Remote Sensing Sea Ice and Polar Dynamics Science Team. Ellen received her PhD in Atmospheric and Oceanic Sciences at the University of Maryland in 2022 for studying melt ponds on Arctic sea ice from airborne and spaceborne imagery and altimetric data.

Host: Assistant Professor Mauro Rodriguez

Center for Fluid Mechanics Seminar

Karen Mulleners, Associate Professor at the Swiss Federal Institute of Technology Lausanne, will present a talk, “Getting Smarter Overnight with Automated Experiments of Unsteady Vortex-dominated Flows.”

Abstract: Typical unsteady vortex-dominated flows like those involved in bio inspired propulsion, unsteady airfoil separation, and vortex-induced vibrations can be prohibitively expensive to simulate and impossible to measure comprehensively. They are inherently non-linear, often involve moving boundaries, high-dimensional parameter spaces, and multiscale flow structures. The classical way to get around these challenges has been to reduce the experimental complexity by using canonical motions (e.g. ramp-up or sinusoidally pitching motions) or simplified unsteady inflow conditions (e.g. one-minuscosine or trapezoidal gust shapes). In our lab, we design automated experiments that can run continuously and autonomously such that we can explore and exploit higher dimensional parameter spaces that cover more realistic and technically relevant unsteady conditions compared to what is traditionally feasible when conducting supervised canonical motion experiments. This approach give us the ability to derive more robust and generalizable models and control solutions while still discovering rare and extreme events. Our recent experiments allowed us to uncover flapping wing kinematics that maximize lift and efficiency, to optimize blade pitching kinematics that improve the power production of vertical axis wind turbines, and to gain insight into the influence of morphology on the forces of thin flexible objects. 

Bio: Karen Mulleners started as an assistant professor in the Institute of Mechanical Engineering in the School of Engineering at EPFL in 2016 and was recently promoted to associate professor. She is the head of the Unsteady Flow Diagnostics Laboratory (UNFoLD). She is an experimental fluid dynamicist who focuses on unfolding the origin and development of unsteady flow separation and vortex formation. Karen studied physics in Belgium (Hasselt University, previously Limburgs Universitair Centrum) and the Netherlands (TU Eindhoven). She received her PhD in mechanical engineering from the Leibniz Universität Hannover in Germany in 2010 for her work on dynamic stall on pitching airfoils that she conducted as a member of the German aerospace centre (DLR) in Göttingen.

Host: Professor Kenny Breuer 

Center for Fluid Mechanics Seminar

Megan Leftwich, Professor, GWU, will present a talk, “Locomotion and transitions of a sea lion inspired amphibious system: Biologic to Robotic.”

Abstract: California Sea Lions are highly maneuverable swimmers, capable of generating high thrust and agile turns. Their main propulsive surfaces, the foreflippers, feature multiple degrees of freedom, allowing their use for thrust production (through a downward, sweeping motion referred to as a “clap”), turning, stability and station holding (underwater “hovering”). Additionally, their flexible bodies and amphibious nature make them an excellent platform to inspire the next generation of unmanned underwater vehicles. Through multiple interdisciplinary studies of the fundamental systems of sea lion locomotion, we have designed and built a swimming robotic platform to further both our scientific and robotic goals. These studies include: geometric and kinetic studies of both forward swimming and maneuvering, Fore- and hindflipper studies, several robotic platforms to investigate the fluid mechanics and control of their thrust production, in situ measurements of swimming, and many more. These have all informed the robotic platform, SEAMOUR, that is capable of sea lion-inspired swimming. 

Bio: Dr. Megan C. Leftwich is a Professor in the Department of Mechanical and Aerospace Engineering at The George Washington University. She holds a Ph.D. in Mechanical and Aerospace Engineering from Princeton University and a B.S.E. degree from Duke University. Prior to joining GW, she was the Agnew National Security Postdoctoral Fellow at Los Alamos National Lab from 2010 to 2012. Her current research interests include the fluid dynamics of rotating airfoils, high performance jetting for aquatic locomotion, unsteady activation for undulatory propulsion, and the fluid dynamics of human birth. Prof. Leftwich has a deep interest in diversity in technical fields and STEM education from the first year through the Ph.D., and currently serves as the Director of the Center for Women in Engineering. Professor Leftwich is an Office of Naval Research 2017 Young Investigator Award Recipient. Additionally, she is the winner of the 2019 Early Career Researcher Award at George Washington University, the 2018 SEAS Dean’s Faculty Recognition Award, the 2017 SEAS Outstanding Young Researcher Award and the 2016 SEAS Outstanding Young Teacher Award. Her work on unsteady propulsion has been profiled in over 20 popular media venues including: Wired, CNN’s Great Big Story, the Smithsonian Magazine and the New York Times.

Host: Professor Kenny Breuer, Professor, School of Engineering

Center for Fluid Mechanics Seminar

Amir Pahlavan, Assistant Professor of Mechanical Engineering and Materials Science, Yale University, will present a talk, “Gradient sensing across complex flow landscapes.”

Abstract: To navigate their environment, bacteria follow chemical cues in search for nutrients or new territories, a process known as chemotaxis. Colloids, emulsions, and macromolecules also respond to their chemical environment, moving up/down the gradients in a process known as diffusiophoresis. While our understanding of chemotactic/diffusiophoretic migration of bacteria and colloids in idealized environments, and in the absence of flow has significantly improved over the last decade, it is still unclear whether these processes could play an important role in more complex environments with spatiotemporal gradients in solute gradients and flows. In this talk, we will explore how flow, disorder, and mixing conspire to impact the phoretic migration of colloids on macroscopic length scales.

Host: Professor Dan Harris

Center for Fluid Mechanics Seminar

Ehssan Nazockdast, Assistant Professor, UNC, will present a talk: “Hydrodynamics of a single filament in curved viscous films and membranes.”

Abstract: Dynamic organization of the cytoskeletal filaments and rodlike proteins in the cell membrane and other biological interfaces occurs in many cellular processes, including membrane transport, and morphogenesis. Previous modeling studies have considered the dynamics of a single rod on fluid planar membranes. We extend these studies to the more physiologically relevant case of a single filament moving in a spherical membrane. First, we assume that the membrane is surrounded by 3D Newtonian fluids on the interior and the exterior. We use slender-body theory to compute the translational drag of the filament’s drag along its axis and in perpendicular direction, and its rotational drag as function of membrane viscosity, surrounding 3D fluid viscosity, membrane radius and the filament’s length. We find that the boundedness of spherical geometry gives rise to flow confinement effects that increase in strength with increasing the ratio of the filament’s length to membrane radius (L/R). These confinement flows result only in a mild increase in filament’s drag along its axis and its rotational drag. In contrast, we find that the drag in the perpendicular direction increases sharply with the filament’s length, when L/R > 1. Next, we extend these calculations to supported membranes.
We assume the filament is embedded in the outer leaflet of the bilayer, as a model for monotopic proteins. The inner leaflet is supported by a rigid spherical boundary, and the two leaflets are coupled by a frictional force. We discuss the scaling behavior of translational and rotational drag coefficients at different asymptotic limits and compare these results against those of a freely suspended membrane. Finally, we outline a theoretical framework that allows computing the linear viscoelastic properties of a freely suspended and a supported spherical membrane as a function of thermal energy and the time-dependent amplitudes of the
semi-flexible filament’s transverse fluctuations at different wavelengths. We show that the confined spherical geometry and the presence of rigid boundaries in the supported membrane give rise to unique dynamical features in the relaxation behavior of the fluctuating filament.

Center for Fluid Mechanics Seminar

Nick Derr, Mathematics Instructor, MIT, will present a talk, “Reciprocal swimming with finite inertia.”

In Stokes flow, Purcell’s scallop theorem forbids objects with time-reversible (reciprocal) swimming strokes from moving. In the presence of inertia, this restriction is eased and reciprocally deforming bodies can swim. A number of recent works have investigated dimer models that swim reciprocally at intermediate Reynolds numbers Re ≈ 1–1000. These show interesting results (e.g. switches of the swim direction as a function of inertia) but the results vary and seem to be case-specific. Here, we introduce a general model and investigate the behavior of an asymmetric spherical dimer of oscillating length for small-amplitude motion at intermediate Re. Using a combination of numerical and analytical methods we solve the system to obtain the dimer’s swim speed and show that there are two mechanisms that give rise to motion: an effective slip velocity on the boundary and Reynolds stresses in the bulk. Each mechanism is driven by two classes of sphere–sphere interactions, between one sphere’s motion and 1) the oscillating background flow induced by the other’s motion, and 2) the geometric asymmetry stemming from the other’s presence. We can thus unify and explain behaviors observed in other works. Our results show how sensitive, counter-intuitive and rich motility is in the parameter space of finite inertia of particles and fluid.

Center for Fluid Mechanics Seminar

Tony Gao, Associate Professor, Michigan State University, will present a talk, “Towards multiscale simulation and modeling of soft active matter.”

Abstract: Active matter, a class of materials composed of self-driven microparticles, exhibit probably the wealthiest yet most exotic non-equilibrium behaviors (e.g., density fluctuation, ordering transition). They provide new means of energy conversion from local fuel to mechanical work, and have inspired novel designs of active materials. In this talk, I will review our recent computation and modeling works of various active systems to uncover their underlying multiscale origins of unstable dynamics. First, I will present direct simulations of dense active particle assemblies such as bacterial swarming and motor-driven biopolymer assemblies. Our new particle simulator fully resolves short- (e.g., collisions) and long (e.g., hydrodynamic)-range interactions between complex-shaped particles. The discrete-particle data from large-scale simulations are then be used to construct bottom-up, coarse-grained PDE models for macro-scale modeling. Under this multiscale modeling framework, I will illustrate how the local particle-particle interactions lead to large-scale collective dynamics via a concatenation of hydrodynamic instabilities. Moreover, I will show examples of building “living” soft machines by leveraging activity, coherent structures, and geometric confinements. In addition, I will briefly introduce our ongoing projects regarding robotics, rheology, and biomedicine.

Bio: Dr. Tong (Tony) Gao is an Associate Professor at the Department of Mechanical Engineering and Department of Computational Mathematics, Science, and Engineering at Michigan State University, where he directs the Complex Fluids Group. He obtained his Ph.D. degree in Mechanical Engineering at the University of Pennsylvania in 2012. Then he worked as a research scientist in the Applied Mathematics Lab at the Courant Institute of Mathematical Sciences of New York University. Dr. Gao works in the interdisciplinary areas of soft condensed matter, fluid mechanics, and materials via mathematical modeling and high-performance computing. His expertise lies in constructing advanced computational mechanics models for fluid-solid systems with high complexities and nonlinearity, and developing scalable simulation tools to promote data-driven, physics-informed studies. Dr. Gao received the NSF CAREER award in 2020. The current focused research topics include soft condensed matter, soft robotics, and patient-specific medical models.

Center for Fluid Mechanics Seminar

Xiyue (Sally) Zhang, Assistant Research Scientist at Johns Hopkins University, will present a talk, “Reducing uncertainty in Arctic amplification by enhanced understanding of cloud dynamics and pollutant transport.”

Abstract: The Arctic climate is changing rapidly. For the past four decades, the Arctic has been warming three times faster than the global average. Climate models underestimate the observed Arctic warming trend and show a large inter-model spread of projected Arctic warming. Clouds and their feedbacks remain a key contributor to this uncertainty due to small-scale unresolved processes in climate models. I will demonstrate a novel framework using large eddy simulations (LES) driven by large-scale circulation from an idealized climate model. Cloud-scale turbulence is explicitly resolved in LES instead of parameterized in climate models. I first investigate the seasonal cycle of Arctic clouds in the current climate. Despite the idealized setup, the observed seasonal cycle of Arctic clouds is qualitatively reproduced. This framework highlights the role of large-scale advection of temperature and moisture in driving the maximum cloud liquid amount in summer and early autumn. I will also discuss the idealized climate change response of Arctic clouds and its implication for Arctic cloud feedback. 

In addition to heat and moisture, the atmospheric eddies also transport pollutants from the midlatitude to the Arctic. These pollutants can have a radiative impact on the Arctic climate, and can serve as cloud condensation nuclei. In the second part of my talk, I will focus on the pollutant transport’s response to increased CO2 concentration using a passive tracer of northern midlatitude surface origin simulated by a coupled atmosphere ocean climate model. I will show how a poleward shift of the winter midlatitude jet can drive an increase in passive tracer concentration in the Arctic troposphere, as well as its implications for realistic chemical tracers.

Center for Fluid Mechanics Seminar

Matthieu Labousse, CNRS Researcher, ESPCI Paris, will present a talk, “Soft violation of Bell’s inequality.”

Abstract: Walking drops on Faraday waves are one of the rare examples of non-quantum wave-particle duality. A series of striking experiments with one walking drop has led to behaviors that were thought to be the peculiar to the quantum scale. I will present a recent numerical and experimental investigation involving the coupling of two walking drops. To our great surprise, we found that the statistical behavior of this system shares some non-expected features of collective emission of photons in quantum optics, including superradiance and violation of Bell’s inequality. This result is very intriguing as the quantum counterpart is the signature of non-separable states which in our case, is the result of a collective wave self-organization.

Center for Fluid Mechanics Seminar

Adrian Herrera-Amaya, Postdoctoral Research Associate at Brown University, will present a talk, “Swimming at Intermediate Reynolds Numbers.”

Abstract: The hydrodynamics of swimming at the millimeter-to-centimeter scale often present the challenge of having both viscous and inertial effects playing nontrivial roles. Efforts to understand the hydrodynamics of swimming have mainly focused on the extremes of fully viscous-dominated (Re≪1) or inertia-dominated flow (Re≫1). However, many animals swim in an intermediate regime, where inertia and viscosity are both significant. This talk focuses on ctenophores (comb jellies), as an impactful and generalizable case study of swimming at intermediate Reynolds numbers. Ctenophores swim via the coordinated rowing of numerous highly flexible appendages (ctenes), with Reynolds numbers on the order of 10-100. With a combination of animal experiments, reduced-order analytical modeling, and physical-robotic modeling, this talk shows how the kinematic and geometric variables of beating ctenes vary across Re and how they affect swimming (including force production, speed, and maneuverability). Ctenophores successfully rely on the spatiotemporal asymmetric motion of their appendages to locomote with near-omnidirectional maneuverability at the intermediate Reynolds regime. This combination of animal experiments, analytical modeling, and physical modeling provides a foundation for applications in bio-inspired design. Lastly, I will conclude with current and future research directions on bio-inspired locomotion and propulsion.

John Kolinski, Assistant Professor at École Polytechnique Fédérale de Lausanne, will present a talk, “Make it or break it: contact and cracks at soft interfaces.”

Abstract: Some of the most challenging and pressing challenges in engineering science arise when materials are adjacent to one another – from the bottom of an impacting droplet during impact, to the separating faces of a crack. Here, I will discuss two vignettes on these important topics: first, a calibrated, nano-scale, direct measurement of the intervening air film at the critical juncture of contact formation during droplet impact, and second, the toughening that can emerge from geometric complexity at the tip of a propagating crack. These seemingly disparate systems are deeply connected on a variety of levels, from their sensitivity to defects, to the propagating singularity that defines the mathematical problem of both the propagating contact line and the propagating crack. The talk will conclude with a discussion of perspectives and some puzzles that remain open despite incredible recent progress.

Bio: Dr. Kolinski studied Applied Mathematics (Sc.M.) and Applied Physics (Ph.D.) at Harvard University, completing a PhD under the supervision of L. Mahadevan and Shmuel Rubinstein on the role of air in droplet impact. John did his post-doc at the Hebrew University of Jerusalem in Israel supported by the Fulbright post-doctoral fellowship. At HUJI, he worked on interfacial instabilities in soft matter in the labs of Eran Sharon and Jay Fineberg. Since 2017, he is a tenure-track assistant professor in the Institute of Mechanical Engineering at EPFL, where his group studies propagating singularities in the form of cracks and contact lines.

Host: Professor Mauro Rodriguez

Dr. Nak-seung Patrick Hyun, Assistant Professor, ECE Department at Purdue University will present a talk, “Autonomous Control of Extreme Behaviors in Bio-Inspired Robotics.”

Abstract: Highly agile and extreme behaviors of many biological systems offer examples for future research directions to target similar mobility in bio-inspired robots understanding of the complex dynamics and subsequent design of a robust and adaptive control framework. Examples of extreme behaviors in biological systems are the fast oscillation-driven maneuvers of bees flapping their wings around 200 Hz and the rapid impulsive striking of mantis shrimp releasing their stored potential energy within milliseconds. The challenges for control of robots with similar extreme behaviors lie in the highly nonlinear dynamics operating over multiple timescales. Specifically, one has to account for fast dynamics (extreme motions) and slow dynamics (time-averaged motion or slower drift in the system), and the time-varying actuation model in the high-frequency regime (fast-dynamics) vs the low-frequency regime (slow dynamics).

This talk will address the control theoretic aspects of dealing with such challenges in bio-inspired robots based on first principles in mathematical system theory. The first part of this talk will address the recent progress on controlling the Harvard Robobee, an insect scale flapping-wing vehicle that flaps its wings around 150Hz. In addition, the recent findings in the nonlinear modeling of the dynamic principles of mantis shrimp strike will be covered, which allows the striking speed to reach 27 m/s within a few milliseconds. The second part of this talk will address the causality of modeling nonlinear impulsive systems, which utilize a singular impulsive contact force in nonlinear mechanical system modeling. The third part of this talk will introduce the recent work on safe trajectory optimization and multi-agent system control, envisioning the future of swarms of flapping wing vehicles. Lastly, I will conclude this talk with future research on the control autonomy of extreme behaviors in bio-inspired robotics.

Bio: Nak-seung Patrick Hyun is an assistant professor in Electrical and Computer Engineering at Purdue University. His research focuses on the control-theoretic aspects of bio-inspired robots, emphasizing systems with extreme behaviors such as flapping vehicles and impulsive systems. He is interested in the broad range of nonlinear control, including optimization-based control, geometric control, and contraction-based control. His research program provides a cyclic learning cycle between biology, mathematical system theory, and robotics. He was formerly a research associate at the Harvard Microrobotics Laboratory, hosted by Robert J. Wood. He received a Ph.D. in electrical and computer engineering in 2018, an M.S. degree in mathematics in 2013, and an M.S. degree in electrical engineering in 2013 from the Georgia Institute of Technology.

Robyn Javier, STEM Communication Specialist from the California Institute of Technology, will present a talk, “The Art of Successful Job Talks.”

Are you preparing to go on the job market? Applying for faculty positions at colleges or universities? Pursuing a career in industry? Whatever path you’re considering, it’s likely that a job talk will be an important part of the interview process. This high-stakes presentation can be an intimidating task – but you can do it! Come join us for a one-hour workshop covering key techniques for success. You’ll learn: how to present a compelling and cohesive story about your research history, how to adapt and optimize materials you’ve already created for the unique requirements of a job talk, and what search committees are looking for beyond your data.

Robyn will also host 30-minute one-on-one meetings on June 5 and 6 for postdocs and 5th-year+ graduate students. Please email Carolyn_Sherman@brown.edu for the link to sign up. Appointments will be on a first come-first served basis.

Seminar host: Mauro Rodriguez, Jr.

Center for Fluid Mechanics Seminar Series

Nuris Figueroa, Assistant Professor at the University of Colorado, Boulder, will present a talk, “Bacteria navigate anisotropic media using a flagellar Tug-of-Oars.”

Abstract: In natural environments, bacteria seldom traverse through simple Newtonian fluids. Instead, they typically navigate through complex and anisotropic surroundings,such as the highly dynamic viscoelastic and anisotropic mucus gel lining our organs, the birefringent domains of extracellular matrix and bacterial cellulose-based materials, and biofilms growing on solid surfaces where swimming is inefficient. In these settings, bacterial movement is primarily dictated by hydrodynamic interactions with the surrounding anisotropic fluid and confinement, resulting in bacteria aligning with the direction of decreased viscosity rather than the well-documented Run-and-Tumble random walk. Our recent findings indicate that bacteria may exploit the anisotropy of the fluids to enhance their motility. The peritrichous Bacillus subtilis adopt a polar configuration to navigate the biocompatible liquid crystal DSCG using two opposing bundles working against each other in a flagellar Tug-of-Oars. The numbers of flagella in each bundle are dynamic, and their rearrangement is contingent on the buckling of individual flagella during motor reversal. The Frank constant for DSCG significantly shifts the critical compression for the Euler buckling of the filaments, preventing flagellar rearrangements at high liquid crystal concentrations and hindering almost all swimming reversals. However, in this regime, we identified what we think is a new mechanism for bacterial exploration of anisotropic media, where bacteria flip swimming directions by dynamically turning on and off individual entire bundles on opposite sides of the cell body. Our results raise questions on the signal transduction at the level of groups of flagella motors located closely on the cell surface.

Center for Fluid Mechanics Seminar Series

Akhil M. Vijayan, Postdoctoral Research Associate at the University of Illinois, Urbana-Champaign, will present a talk, “Kinetic Modeling of Particulate Mobility Parameters in Compressible Regime.”

Abstract: Over the last thirty years, a significant amount of numerical research has been focused on particle-laden flows that are commonly encountered in nature and industry. This research has aimed to understand the fluid-induced forces acting on particulates and the behavior of particulate systems. While there has been extensive study on particle-laden systems in incompressible gas regimes, the compressible regime has not been explored as thoroughly. Recently, there has been a growing interest in studying particle-laden multiphase systems in compressible gas regimes to better understand phenomena like plume surface interactions during planetary landings and ash cloud formation during volcanic eruptions. This presentation will discuss the latest developments in using the Direct Simulation Monte Carlo (DSMC) approach to analyze the mobility parameters of solid particulates in rarefied and compressible gas flow regimes. By utilizing a gas-kinetic approach, which resolves the momentum transfer between the gas and the particulate surface at a molecular scale, the impact of gas compressibility and rarefaction on the fluid-induced forces can be naturally analyzed without having to model explicit boundary conditions. The presentation will focus specifically on the mobility parameters of individual irregular particulates in compressible gas flows and explore the dynamics of a system of spherical particulates exposed to high-speed flows.

Center for Fluid Mechanics Seminar Series

Randy H. Ewoldt, Alexander Rankin Professor at the University of Illinois, Urbana-Champaign, will present a talk, “Inverse rheology: flipped perspectives for design, inference, and protorheology.”

Abstract: From curious observations to engineering flow objectives, non Newtonian rheological behavior is responsible for a wide range of remarkable phenomena [1]. We naturally ask, “What rheological properties are important?” This poses an inverse question, and the complexity introduces challenges, since there is no universal constitutive equation for non-Newtonian behavior. This talk will consider such inverse questions related to design [1,2] and inference [3]. The approach organizes the complexity with four key phenomena of rheology: viscoelasticity, shear normal stress differences, extensional thickening, and non-constant shear viscosity, broadened to include thixotropic time dependence and yield-stress fluids. The design perspective will be shown with case studies such as direct-write 3D printing, fire suppression, and microfluidic particle focusing to identify continuum-level rheological properties agnostic to the underlying chemistry and material structure. This enables creative design at the material level to explore multiple possible microstructures to achieve the rheological objective [2]. The inference perspective motivates use of non-standard flows to infer rheological properties, which we call do-it-yourself rheometry [3], or protorheology: approximate rheological inference from deformations that are readily accessible to photos and videos. Like a prototype, protorheology may be an approximation, but it is fast, accessible, and gives persuasive insight that enables high throughput characterization.

Center for Fluid Mechanics Seminar Series

Andres Goza, Assistant Professor, University of Illinois at Urbana-Champaign, will present a talk, “Feather-inspired flaps for unsteady aerodynamic flow control: physics and reinforcement learning-based control.”

Abstract: Birds have deployable covert feathers that are hypothesized to aid in aerodynamic flow control. Inspired by this biological control solution, we use high fidelity simulations to explore a simplified configuration in which a flat plate is mounted via a torsional spring on an airfoil. We first characterize what dynamical regimes this system can undergo for various spring and inertia values, explaining some of the underlying fluid-structure interaction mechanism and their implications on performance changes. Then, we discuss the use of a reinforcement learning based control law, in which the controller is allowed to actively tune the hinge stiffness to induce different fluid-structure interaction dynamics. This control paradigm can be viewed as a hybrid active-passive approach, in the sense that the spring stiffness is being actively tuned but the aerodynamic changes are indirectly obtained by the coupled interaction between the flap and the flow. We discuss the benefits obtainable through this control approach, and what changes to the flow field enable them.

Bio: Andres Goza is an Assistant Professor in the Aerospace Engineering Department at the University of Illinois at Urbana-Champaign. He received his PhD from Caltech in 2017 and was a postdoctoral researcher at Princeton University from 2017-2018. His research focuses on the use of computational fluid dynamics and modeling techniques to gain fundamental physical insights into fluid-structure interaction (FSI) systems. Andres’s interest in these systems ranges from harnessing flow-induced vibrations for robust energy harvesting to utilizing FSI for passive control and/or estimation of unsteady aerodynamic flows. His group’s research has been funded through NSF, AFOSR, and Sandia National Labs. He was awarded an NSF Graduate Research Fellowship to perform his doctoral work, and his thesis work led to his selection as a Caltech Everhart Lecturer. He was also a “Teacher Ranked As Excellent” at UIUC in 2020-2021, and was awarded the 2021 “AIAA Teacher of the Year” award by the Aerospace Engineering Department at UIUC.

Center for Fluid Mechanics Seminar Series

Nicole Xu, Visiting Assistant Professor at the University of Colorado, Boulder, will present a talk, “Bioinspired aquatic vehicles for ocean exploration: robotic jellyfish, shark skin surfaces, and fish fins.” 

Abstract: As robotics advances, swimming robots can potentially be used for monitoring the ocean, performing tasks in remote locations, and other practical applications. Looking towards nature for inspiration can address some of the grand challenges of robotics, such as improved dexterity and adaptive abilities in unstructured environments. This work presents examples of bioinspired swimming robots using approaches that combine laboratory and field experiments, theoretical models, and computational fluid dynamics. First, we demonstrate a biohybrid robot that uses a microelectronic system to induce swimming in live jellyfish in the laboratory and ocean. Using entirely synthetic materials, we also address how bioinspired shark skin surfaces and robotic fish fins can improve the performance envelope of vehicles. Future applications include improving swimming speeds, efficiencies, and antifouling properties for enhanced persistence. These examples provide a strong foundation for continued work to design and implement robots for real-world applications.

Center for Fluid Mechanics Seminar Series

Ellen Longmire, Professor of Aerospace Engineering and Mechanics, University of Minnesota, will present a talk: “Motion of Large Particles in Turbulent Boundary Layers.”

Abstract: Our work is motivated by the need to understand and predict turbulent particle-laden flows across a range of environmental and industrial applications. We consider a relatively canonical yet challenging experimental flow designed to be accessible to direct numerical simulation. A spherical particle in a turbulent boundary layer undergoes complicated particlewall and particle-turbulence interactions. Particles with significant diameter are subject to variations in shear and normal forces around their circumference. Wall friction will affect the particle rolling and sliding motions while coherent flow structures can lift the particle away from the wall. To resolve the sphere dynamics in such a flow, 3D tracking experiments are conducted in a water channel facility. The translation and rotation of individual spheres released from rest were tracked over distances of 6δ multiple flow Reynolds numbers and particle-to-fluid density ratios. Simultaneous stereoscopic PIV measurements were acquired in the logarithmic region surrounding the moving spheres. While neutrally buoyant particles typically lift off from the wall upon release, denser particles travel mostly along the wall. The relative contributions of turbulence, wall friction, and mean shear to the resulting particle motions will be discussed for the different cases considered.

Bio: Ellen Longmire is a Professor of Aerospace Engineering & Mechanics and Associate Dean of Academic Affairs for Science & Engineering at the University of Minnesota. She received an A.B. in physics from Princeton University and M.S. and Ph.D. in mechanical engineering from Stanford University. She is a Fellow of the American Physical Society and received the UM Distinguished Women Scholars Award, the McKnight Land-Grant Professorship, and the NSF National Young Investigator Award. She is currently an Editor-in-Chief for Experiments in Fluids. She previously served as Chair of the APS Division of Fluid Dynamics, member of the US National Committee on Theoretical and Applied Mechanics, and Associate Editor for Physics of Fluids. 

Center for Fluid Mechanics Seminar Series

Ebru Demir, Assistant Professor at Lehigh University, will present a talk, “Swimming and Squirming through Biological Fluids.”

Abstract: Bioinspired artificial microswimmers are promising candidates to realize coveted biomedical applications such as targeted drug delivery and minimally invasive surgery. However, swimming at small scales is a challenge onto itself, as the physics of locomotion is different due to the dominance of viscous force over inertial force at small scales. Furthermore, in their biomedical applications, these microswimmers often encounter biological fluids such as blood and mucus, which are complex fluids with non-Newtonian rheological behaviors. A fundamental understanding of how complex rheology of biological fluids affect propulsion performance is therefore crucial in the development of artificial microswimmers for biomedical applications and requires an interdisciplinary approach. In this talk, I will discuss the effects of geometry and fluid rheology (Newtonian and non-Newtonian) on the swimming characteristics at zero Reynolds number limit, and how we approach this problem using numerical simulations combined with analytical methods to elucidate the physical principles underlying the swimming performance. I will also discuss other outstanding issues in the emerging field of microswimmers and plans to tackle these challenges with tools across different disciplines.

Bio: Ebru Demir received both her BS and PhD degrees in Mechatronics Engineering from Sabanci University, Istanbul, Turkey. She continued her research activities as a postdoctoral research fellow in Mechanics Division of Computational Science Research Center in Beijing, China, and in Mechanical Engineering Department at Santa Clara University, California, before joining the Department of Mechanical Engineering and Mechanics Faculty at Lehigh University, Bethlehem, Pennsylvania in 2021. Her research activities lie at the interface of fluids, biomedical engineering, and robotics, with a focus on biomedical applications of artificial microswimmers. She studies locomotion in complex fluids and biological environments for biomedical and environmental applications such as drug delivery, diagnostics, and waste elimination.

Center for Fluid Mechanics Seminar Series

Maria Tatulea-Codrean, Postdoctoral Research Associate, University of Cambridge, will present a talk, “Asymptotic Approaches to Solving the N-flagella Problem of Bacterial Motility.” 

Abstract: Cell motility has been a topic of great interest in the fields of fluid mechanics and biophysics, particularly in the last two decades. Amongst the microswimmers most frequently put under the microscope is the model organism Escherichia coli, a motile bacterium with a decidedly finite number of propellers or “flagella” (by which we mean that N>1 but not >>1). Bacteria with a single flagellum (N=1) can be reasonably well understood using the force-balance arguments originally put forward by Purcell (1976), while swimmers covered in cilia (N>>1) can be approached using the squirmer model first proposed by Lighthill (1952). But the N-flagella problem posed by multi-flagellated bacteria remains unsolved to this day. In this talk, we will present recent theoretical advancements towards modelling the interactions between bacterial flagella—both short-range steric interactions due to the helical geometry of flagellar filaments, and long-range hydrodynamic interactions. Our approach builds on well-established asymptotic theories such as the slender-body theory of hydrodynamics and the method of multiple scales. We will reveal novel applications of these methods to the topic of bacterial motility and investigate whether two rotating bacterial flagella (N=2) can remain tangle-free and synchronize in phase with each other due to steric and hydrodynamic interactions, respectively. In the last part of the talk, we will demonstrate that hydrodynamic interactions within a bundle of filaments (N>2) can have surprising effects on the swimming speed of multi-flagellated bacteria. 

Center for Fluid Mechanics Seminar Series

Tracy Mandel, Assistant Professor from the University of New Hampshire, will present a talk: “What lies beneath: Fluid mechanics through the lens of the coastal free surface.”

Abstract: Flow interactions at coastlines play a major role in global ocean energy and nutrient budgets. However, coastal flows can be expensive, logistically challenging, and even dangerous to study in situ. In this talk, I will cover two problems that connect the water surface and subsurface dynamics in the coastal ocean, using idealized laboratory experiments to understand the physics of these systems. First, I will discuss the first step in remotely characterizing seagrass meadows by studying the overlying water surface. Flow through a seagrass bed can generate large overturning vortex structures, which cause small perturbations in the free surface slope. Using laboratory experiments, we develop a parameterized model to reconstruct within-canopy velocity profiles solely from water surface measurements, suggesting that in some environmental flows, the subsurface hydrodynamics and geometry may be predicted by measuring the water surface behavior alone. Second, I will examine the dynamics of turbulent buoyant plumes, such as those that might emerge at the base of a marine terminating glacier. I will discuss our recent work that teases apart the role of buoyancy in enhancing entrainment in plumes, and preliminary work that quantifies the surface expression of these buoyant plumes.

Bio: Tracy Mandel is an Assistant Professor in Mechanical Engineering and Ocean Engineering at the University of New Hampshire. Her research explores turbulent flow at ocean margins, with a focus on laboratory experiments and coastal ecosystems. Her group works to understand the natural physical processes that occur at ocean margins in order to better protect, restore, and consider these systems in engineering decisions. Tracy received her B.S. from Cornell University and her M.S. and Ph.D. from Stanford University, and was awarded the 2018 Lorenz G. Straub Award for most meritorious doctoral dissertation in hydraulic engineering, ecohydraulics, and related fields.

Center for Fluid Mechanics Seminar Series

Ryan Poling-Skutvik, Assistant Professor at the University of Rhode Island, will present a talk: “The Yield Transition in Gels: Accounting for Structural Breakdown.”

Abstract: Gels are materials comprised of a majority fluid phase, but which exhibit solid-like mechanical properties arising from a percolating network structure. When subjected to external stresses or strains, these materials undergo a yield transition in which the material no longer elastically deforms but viscously flows. This transition is often accompanied by a commensurate breakdown in the material structure to a state that depends on the shear history. For many classes of gels, the structural recovery from this yielded state back into a percolating network occurs over long time scales, resulting in time-dependent mechanical properties that obscure the yield transition and deleteriously affects the performance of gels in applications ranging from additive manufacturing to tissue engineering and drug delivery. Here, we develop a novel rheological protocol to account for structural breakdown and to precisely quantify the yield transition in a variety of materials, including colloidal gels, physical gels, and a physicochemical gel comprised of polymer-linked emulsion droplets. The gelation of these materials is characterized through standard linear oscillatory rheology, but the yield transition is measured through a series of stress-controlled creep measurements in which the time it takes to yield the sample depends on the quiescent recovery time and applied stress. From these measurements, we quantify the structural evolution of the gels through a bifurcation in the creep response and unambiguously define the yield transition according to the divergence of yield times. Our findings elucidate the unique mechanisms of structural recovery that depend on gel physicochemistry, provide insight into the origins of the yield transition, and quantifies the thixotropic recovery of mechanical properties.

Center for Fluid Mechanics Seminar Series

Albane Théry, Simons Postdoctoral Fellow in Mathematical Biology, University of Pennsylvania, will present a talk, “Swimming into the wall: patterns and self-organization of microswimmers in confinement.” 

Abstract: Biological and artificial microswimmers often swim in confined environments, and their interaction with boundaries can play an important role in their ability to swim and form collective structures. We will discuss our recent results on boundary-mediated propulsion and patterns of microswimmers, highlighting how different theoretical and numerical approaches can be used depending on the complexity and geometry of the environment. We will relate our results with some recent experimental findings, demonstrating how hydrodynamic and contact interaction shape single swimmer propulsion and collective dynamics.

Center for Fluid Mechanics Seminar Series

Mauro Rodriguez, Assistant Professor of Engineering, will host a panel discussion: “On building academic competitiveness when building when building the American professoriate.”

Abstract: The Center of Fluid Mechanics seminar this week shall be on workforce development towards the professoriate. Entering the professoriate is a competitive process. It is of key importance that applicants be knowledgeable of the mechanics and insights of the process to be competitive. A comprehensive overview of the faculty search process and application will be discussed in this workshop. In-depth details and guidance regarding preparing the research, teaching and commitments to diversity statements for the faculty application and steps and processes after application submission, such as faculty interview, will be discussed. The session is aimed to be a highly interactive discussion. A facilitated discussion with a panel of recently hired Brown University faculty members to the School of Engineering will be provided to provide different perspectives and insights. Graduate students and postdocs exploring careers in the professoriate are strongly encouraged to attend.

You are encouraged to send questions in advance to the facilitator at mauro_rodriguez@brown.edu .

Center for Fluid Mechanics Seminar Series

Ching-Yao Lai, Professor at Princeton University, will present a talk, “Physics-informed neural networks for the Euler equations and ice dynamics.”

Abstract: Physics-informed neural networks (PINNs) have recently emerged as a new class of numerical solver for partial differential equations which leverage deep neural networks constrained by equations. I’ll discuss two applications of PINNs in fluid dynamics developed in my group. The first concerns the search for self-similar blow-up solutions of the Euler equations. The second application uses PINNs as an inverse method in geophysics. Whether an inviscid incompressible fluid, described by the 3-dimensional Euler equations, can develop singularities in finite time is an open question in mathematical fluid dynamics. We employ PINNs to find a numerical self-similar blow-up solution for the incompressible 3-dimensional Euler equations with a cylindrical boundary. In the second part of the talk, I will discuss how PINNs trained with real world data from Antarctica can help discover flow laws that govern ice-shelf dynamics. These ice shelves play a role in slowing the flow of glaciers into the ocean, which impacts global sea level rise. However, the effective viscosity of the ice, a crucial material property, cannot be directly measured. By using PINNs to solve the governing equations for the ice shelves and invert for their effective viscosity, we were able to calculate flow laws that differ from those commonly assumed in climate simulations. This suggests the need to reassess the impact of these flow laws on sea level rise projections.

Center for Fluid Mechanics Seminar Series

Bianca Viggiano, Postdoctoral Researcher, Johns Hopkins, will present a talk, “Lagrangian intermittency in a non-homogeneous turbulent shear flow built from homogeneous isotropic turbulenceLagrangian intermittency in a non-homogeneous turbulent shear flow built from homogeneous isotropic turbulence.”

Abstract: Limitations of the applicability of universality in turbulence, as it was introduced by Kolmogorov in 1941, are often observed, especially in non homogeneous flows deviating from idealized conditions, e.g. jets, wakes, canopies, etc. In 1957, Batchelor proposed a method to “stationarize” Lagragnian trajectories of flow fields which are spatially inhomogeneous. An extension of such a transformation is herein applied to experimental data from a turbulent jet. Analysis of the stationarized jet statistics is performed and results are compared to validate the methods presented.
We then invert this method to build an inhomogeneous turbulent jet based on velocity signals from homogeneous, isotropic and stationary turbulence. The transformation of stationary turbulent velocity signals into a jet is performed on Lagrangian velocity signals obtained from a stochastic process and from direct numerical simulation data. The modeled jets are compared to Lagrangian experimental data at four downstream locations along the centerline of the jet. Both modeled jets show good agreement for Eulerian and Lagrangian statistics, including higher-order moments, and thus demonstrating the ability of the model to capture intermittent behavior.

Center for Fluid Mechanics Seminar Series

Xiaoyu Tang, Assistant Professor at Northeastern, will present a talk, “Dynamics and manipulation of multiphase systems involving droplets and particles.”

Abstract: Multiphase flows, involving droplets and/or particles, is ubiquitous in many applications, ranging from oil recovery, additive manufacturing, to drug delivery. My research efforts have been devoted to understanding the fundamental physics of multiphase flow and investigating new ideas for applications in energy, environment, to healthcare. In this talk, I will first discuss the dynamics of drop impact on liquid films and demonstrate various phenomena orchestrated by the complex interplay among impact inertia, surface tension, and viscosity. Special attention will be paid to non-Newtonian effect caused by corn starch suspension. Our experiments and scaling analyses have led to new insights into optimizing operating conditions in various applications. In the second part, I will focus on the migration of colloidal particles driven by solute concentration gradient, known as diffusiophoresis. Utilizing the interaction between solute-emitting particles and surface charge heterogeneity, I will demonstrate a strategy and its theoretical foundation to pattern particle distribution on surfaces.

Center for Fluid Mechanics Seminar Series

Jeff Oishi, Associate Professor of Physics at Bates College, will present a talk: “Computing Fluid Flows.” 

Abstract: “Computing Fluid Flows” is a series of lectures aimed at treating fluid mechanics from a physicist’s perspective, with the goal of learning how to compute solutions to complex problems ranging from climate to clean energy to biological fluids. Central to our understanding of fluids are numerical solutions to linear and non-linear model equations. Starting from a solid foundation in the numerical analysis of partial differential equations (PDEs), I will discuss how to build models in order to increase understanding of complicated fluid systems using the Dedalus project, an open-source framework for PDEs.

In this first of a series of lectures, I will introduce fluid mechanics with an emphasis on the mathematical and physical details that make it such a challenging field in which to compute solutions. Starting from the Navier-Stokes equations, I will discuss aspects of PDEs and give a brief overview of some of the major classes of numerical techniques to solve them. From there, I will focus on one particular family of numerical methods, the spectral methods. I will discuss why they are an enduring choice, particularly among physicists interested in theoretical studies, and illustrate the directions in which spectral techniques continue to be developed. Finally, I will introduce the Dedalus framework, a tool for solving PDEs using spectral methods which I will use throughout the lecture series.

This is the first in a series of about 5 lectures on computational fluid dynamics that Prof. Oishi will offer this spring (the others are tentatively scheduled for Fridays at 2pm in the 2nd floor seminar room of Barus building at 340 Brook Street — note that it is a different building than Barus & Holley). The lectures are supported by the Tony and Pat Houghton Conference Fund.

If you would like to install Dedalus before the lectures, please do so by following the instructions here: https://dedalus-project.readthedocs.io/en/latest/pages/installation.html#full-stack-conda-installation-recommended

A set of example scripts will be posted at https://github.com/jsoishi/Brown2023_dedalus_examples
in advance of the first lecture.

Igor Schevchenko, Research Associate at the Imperial College of London, will present a virtual talk, “An Alternative Approach to the Ocean Eddy Parameterisation Problem.”

Abstract: It is typical for low-resolution ocean simulations to miss not only small- but also large-scale patterns of the flow dynamics compared with their high resolution analogues. It is usually attributed to the inability of coarse-grid models to properly reproduce the effects of the unresolved small-scale dynamics on the resolved large scales. In part, the reason for that is that coarse-grid models fail to at least keep the coarse-grid solution within the region of phase space occupied by the reference solution (the high-resolution solution projected onto the coarse grid).

In this talk we discuss three methods to solve this problem: (1) computation of the image point in the phase space restricted to the region of the reference flow dynamics, (2) reconstruction of a dynamical system from the available reference data, and (3) constrained dynamics. The proposed methods show encouraging results for both low- and high- dimensional phase spaces. One of the important and general conclusions that can be drawn from our results is that not only mesoscale eddy parameterisation is possible in principle but also it can be highly accurate (up to reproducing individual vortices). This conclusion provides great optimism for the ongoing parameterisation studies.

Center for Fluid Mechanics Seminar Series

Sam Pellone, Postdoctoral Researcher, Los Alamos National Laboratory, will present a talk, “Shock-induced variable-density turbulence at disparate scales.” 

Abstract: Shock-driven turbulence may arise when a shock wave interacts multiple times with a perturbed interface separating two fluids of different densities. Such shock-interface interaction is referred to as the Richtmyer-Meshkov (RM) instability and is important in many applications such as supersonic combustion engines, supernova formation, and inertial confinement fusion. In this work, we report results of recent experiments and simulations of the RM instability encountered in two disparate regimes. The first operates under High-Energy-Density (HED) conditions characterized by extreme pressures (∼ Mbar) and temperatures (∼ eV), while the second operates under standard atmospheric conditions (∼1 bar and ∼ 273 K). The time-, length-, and velocity-scales are drastically different in both regimes: nanoseconds, millimeters, and kilometers-per-second for the first regime; seconds, meters, and meters-per-second for the second. The objective is to investigate turbulence and mixing ensuing from the RM instability at such disparate scales. Because of the transient nature of the flow, the presence of discontinuities, and the wide range of temporal/spatial scales, performing direct numerical simulations of such flows is impractical. Instead, the experiments are simulated in the context of a Reynolds-Averaged, variable-density turbulence mix model known as the Besnard-HarlowRauenzahn (BHR) model.

Bio: Sam Pellone received his Ph.D. in Mechanical Engineering in 2020 from the University of Michigan. Under the supervision of Eric Johnsen, he focused and specialized in the computational modeling of hydrodynamic instabilities occurring at material interfaces such as the Richtmyer-Meshkov (RM), Rayleigh-Taylor, and Kelvin Helmholtz instabilities. He is now a postdoctoral researcher at Los Alamos National Laboratory performing experiments and simulations of the RM instability in HighEnergy-Density (HED) as well as classical fluids conditions. Using state-of-the-art fluids diagnostics (planar laser induced fluorescence and particle image velocimetry) and turbulence modeling, he focuses on investigating turbulence and mixing in highly compressible environments. 

Center for Fluid Mechanics Seminar Series

Baylor Fox-Kemper, Professor, Dept. of Earth, Environmental and Planetary Sciences at Brown University, will present a talk: “Connecting atmospheric and oceanic boundary layer turbulence to global warming: regional mixed layer depth as an emergent constraint.”

Abstract: The global ocean modulates the climate’s temperature response to forcing. Ocean turbulent mixing processes mediate this relationship, and global climate models (GCMs) often struggle to simulate this mixing, but most research has ignored ocean processes as an avenue for constraining predicted warming. Here we show that regional mixed layer depth (MLD) strongly constrains climate sensitivity because of its relationship to oceanic heat capacity, heat uptake, and the depth of the Atlantic Meridional Overturning Circulation. We fit a two-layer energy balance model (EBM) to each GCM in a 25-member CMIP6 ensemble, and correlate the parameters of the EBMs with average pre-forcing mixed layer depths in the northern (55N–75N), tropical (26S–26N), and southern oceans (65S–45S). Using these correlations and observations from the Argo float network, we revise the ensemble mean and narrow the 66% range of equilibrium climate sensitivity (ECS) for the particular CMIP6 model collection from 4.51 (3.13–5.71)C, to 4.66 (3.88–5.43)C, amounting to a 40% reduction in the span of the uncertainty range. Such a reduction in uncertainty rivals the impact of other critical processes, e.g., clouds, in their effect on overall surface warming projections. 

Center for Fluid Mechanics Seminar Series

Michelle DiBenedetto, Assistant Professor, University of Washington, will present a talk: “Microplastics at the ocean surface: waves, turbulence, and particles.”

Abstract: Plastic pollution poses a critical threat to the world’s oceans, but critical gaps in knowledge surrounding plastic fate and transport impede remediation and prevention efforts. Much of the plastic in the ocean exists as microplastics. Predicting the behavior of microplastics is non-trivial for two primary reasons: their physical properties (size and density) are fundamentally different from traditionally studied environmental particles like sediment and bubbles, and complex interactions among waves, turbulence, and particle inertia in the ocean surface boundary layer (where most microplastics reside) are not well-understood, especially for buoyant particles such as microplastics. In this talk, I will discuss the importance of surface waves in predicting the transport and distribution of microplastics. I will present results from both an analytical study and laboratory experiments of particles in wavy flows, and discuss these in the context of microplastic transport in the ocean.

Bio: Michelle DiBenedetto is an Assistant Professor at University of Washington Seattle in the Mechanical Engineering department. She received her B.S. from Cornell University in 2014 in Environmental Engineering, and her Ph.D. from Stanford University in 2019 in Civil & Environmental Engineering where she studied the behavior and transport of non-spherical particles in surface gravity waves. For her dissertation work, she was awarded the Andreas Acrivos Dissertation Award in Fluid Dynamics from the American Physical Society. Prior to her current appointment, she was a postdoctoral scholar at Woods Hole Oceanographic Institution in the biology and physical oceanography departments. Her lab at University of Washington uses laboratory experiments, observations, and mathematical modelling to study problems at the intersection of environmental fluid mechanics and particle-laden flows.

Center for Fluid Mechanics Seminar Series

Thomas Sykes, Oxford, will present a talk, “Droplet Splashing on Solid and Liquid Surfaces.”

Abstract: The dynamics of droplets impacting surfaces at high speed are not only fascinating from a fundamental perspective, but they are also important for a variety of applications including spray processes, forensics, and inkjet printing. We are especially interested in cases where droplets break up, with a view to reducing the formation of small fast secondary droplets with indiscriminate trajectories (splashing). In this talk, we will consider two such surfaces from our recent and current projects: dry curved substrates (spheres and concave surfaces) and shallow pools.
For the former, using high-speed imaging experiments we have shown that it is harder for droplets to splash on small spheres during axisymmetric impact. We propose a physical mechanism to explain this behaviour and incorporate it into state-of-the-art splashing theory to attain a consistent parameterisation of the splashing threshold across dry concave, convex, and flat surfaces. We will briefly touch-on some effects of asymmetry using unpublished color high-speed imaging results.
On shallow pools, we have uncovered a well-defined depth transition that strongly affects both the propensity for, and dynamics of, splashing at high Reynolds number. This transition will be delineated throughout a wide Weber number/pool depth parameter space, using numerical simulations to reveal the underlying physics. We will examine the incredibly diverse range of dynamics that can occur and compare the results to those seen on the dry (curved) surfaces discussed prior.

Center for Fluid Mechanics Seminar Series

Michael Howland, MIT, will present a talk, “Modeling and Optimization for Collective Wind Farm Flow Control at Utility-scale.”

Abstract: Wind turbines located in wind farms are operated to maximize only their own power production. Individual operation results in wake losses that reduce typical farm energy 10-20%. Wake steering, the intentional yaw misalignment of turbines to deflect wakes, has demonstrated potential as a wind farm flow control approach to increase collective power production. To achieve the maximum farm power production, the models used for wind farm decision-making must be both accurate and computationally efficient. The potential for wake steering depends, in part, on the power reduction of yaw misaligned turbines. In the atmospheric boundary layer (ABL), the sheared wind speed and direction may change significantly over the rotor area, resulting in a relative inflow wind to the blade airfoil which depends on the radial and azimuthal positions. In order to predict the power production for an arbitrary yaw misaligned turbine based on the incident ABL velocity profiles, we develop a blade element model which accounts for wind speed and direction changes over the rotor area, and the model is validated using experimental data from a utility-scale wind farm. The new blade element model is coupled with an aerodynamic wake model to establish a collective flow control model. Leveraging the flow control model, we designed a physics-based, data assisted wake steering control method to increase the power production of wind farms, which utilizes data assimilation and gradient-based optimization. The method was first validated, demonstrating that it predicts the true power maximizing operation, and then tested in a multi-turbine array at a utilityscale wind farm, where it statistically significantly increased the energy production over standard, individual operation. Collective control can increase the generation potential of wind farms through software modifications, without additional turbines or hardware. The developed and validated predictive model can enable a wider adoption of collective wind farm optimization.

Center for Fluid Mechanics Seminar Series

Kausik Sarkar, Professor, George Washington University, will present a talk: “Bubbles for ultrasound imaging, therapeutics and tissue engineering: Interfacial rheology, jet and microstreaming flows.”

Abstract: Intravenously injected microbubbles are used as contrast-enhancing agents in diagnostic ultrasound imaging. They are coated by a nanometer-thick shell of lipids, proteins, or polymers which stabilizes them against premature dissolution. In 2003, we proposed that the shell be modeled as an interface with intrinsic interfacial rheology, characterized by properties such as interfacial viscosities and elasticities. This was a sharp contrast to the prevalent practice of modeling the microbubble coating using ad hoc parameters or as a finite-thickness layer with bulk rheological properties. We applied interfacial rheological models to commercial contrast agents, determined the values of their characteristic interfacial properties, and validated the model using in vitro acoustic experiments. Over the years, we have developed a hierarchical approach to contrast agent modeling where models were progressively refined as warranted by validating experiments and the underlying physics. We have built an in-house facility for synthesizing lipid-coated microbubbles and micro- and nanodroplets of volatile perfluorocarbon liquid, and have been investigating fundamental phenomena such as acoustic droplet vaporization (ADV) and bioeffects of ultrasound and microbubbles in cancer therapy and stem cell tissue engineering.

In this talk, I will present an overview of our research emphasizing recent efforts on microbubbles and ultrasound-assisted bone and cartilage tissue engineering in 3D printed scaffolds. Low-intensity pulsed ultrasound (LIPUS) in conjunction with microbubbles has been shown in our lab to facilitate bone and cartilage formation from mesenchymal stem cells. We will discuss nonlinear shape oscillations of microbubbles and acoustic microstreaming that are responsible for such bioeffects. We studied them using boundary element (BEM) simulation and perturbative analysis of an encapsulated microbubble near a vessel wall. I will discuss the resulting stresses and geometry of streaming vortices.

If time permits, I will briefly present our research in CFD simulations of viscous and viscoelastic emulsions, their rheology, and shear-induced diffusion.

Center for Fluid Mechanics Seminar Series

Robert Hunt, Brown University, will present a talk: “Diffusion-induced flow, aggregation, and settling of porous particles in density-stratified fluid.”

Abstract: Particles in density-stratified fluids are ubiquitous and relevant to many important processes, such as the carbon cycle. First, this talk will present  mechanism for particle aggregation in stratified fluids, in the absence of adhesion, which is due to the interplay of solute diffusion, boundary conditions, and gravity. A neutrally-buoyant particle immersed in a quiescent stratified fluid self-induces a flow which mediates an effective force between other immersed particles. Direct numerical simulations characterize the influence of particle size, shape, and material parameters on this induced flow and compare favorably with experiments. Further, a numerically derived pairwise force law is implemented in modified Stokesian dynamics simulations which quantitatively agree with experiments for two-particle interactions and reproduce emergent features of the many-body system.

Second, I will discuss the settling of porous particles which carry excess solid density. In a particular regime, these particles reach a terminal settling velocity which is dominated by mass exchange. A model for this settling velocity as a function of the fluid and solid parameters will be introduced which quantitatively agrees with experiments.

Center for Fluid Mechanics Seminar Series

Linda Abriola, Joan Wernig and E. Paul Sorensen Professor of Engineering at Brown University, will present a talk: “Modeling the Transport and Retention of Surface-Active Contaminants in Multiphase Subsurface Systems.”

Abstract: Aqueous film forming foams (AFFFs), employed for firefighting, have been recognized as a major source of groundwater contamination by per- and polyfluoroalkyl substances (PFAS) throughout the US. In addition to the application of aqueous PFAS solutions, firefighting training and emergency response activities have typically involved the intentional or accidental co-release of hydrocarbon fuels and chlorinated solvents (nonaqueous phase liquids (NAPLs)), creating complex contaminant mixtures. Due to their amphiphilic properties, PFAS accumulate at interfaces, and thus, the presence of air-water and NAPL-water interfaces within AFFF release areas can influence both the transport and distribution of PFAS in soils and groundwater. This presentation provides an overview of recent collaborative research related to the development and application of mathematical models for the description of PFAS transport and retention beneath complex AFFF source areas. Model simulations are compared to packed column experimental observations to assess model performance. Model applications to field-scale AFFF release scenarios illustrate the potential influence of release history, soil texture, and PFAS characteristics on the spatial distribution of contaminants within the unsaturated soil zone and on contaminant migration to the water table.

Bio: Linda M. Abriola is the Joan Wernig and E. Paul Sorensen Professor of Engineering at Brown University. Prior to joining Brown in 2021, she held the positions of University Professor, director of Tufts Institute of the Environment and inaugural dean of the Tufts School of Engineering. Professor Abriola is an international expert on the multiphase transport, fate, and recovery/destruction of contaminants in the subsurface and the author of more than 170 refereed publications. She is a member of the American Academy of Arts and Sciences and the National Academy of Engineering and a Fellow of the American Geophysical Union. She has been the recipient of numerous awards, including the National Ground Water Association’s Distinguished Darcy Lectureship, the Strategic Environmental Research and Development Program Project of the Year Award in Remediation, and a Presidential appointment as a U.S. Science Envoy. Prior to joining Tufts, Dr. Abriola was the Horace Williams King Collegiate Professor of Environmental Engineering at the University of Michigan. She received her Ph.D. and M.S degrees from Princeton University and a B.S. degree from Drexel University, all in Civil Engineering.

Center for Fluid Mechanics Seminar Series

Aswin Gnanaskandan, Assistant Professor, Worcester Polytechnic Institute, will present a talk: “Modeling Microbubbles in Ultrasound for Therapeutic Applications.”

Abstract: Microscopic gas bubbles in the form of Ultrasound Contrast Agents have been recently shown to have therapeutic applications under the action of ultrasound. High Intensity Focused Ultrasound (HIFU) has been approved by FDA in 2016 for treatment of sub-surface prostate cancer using the ablative effect created by focusing the ultrasound on the affected area. However, this technique is not suitable for deep-seated cancer where very high intensity ultrasound is needed to achieve acoustic penetration which has deleterious side effects due to pre-focal heating. This talk explores the idea, primarily through modeling combined with experiments, of injecting microbubbles in the vicinity of the tumor to enhance the heat deposition due to ultrasound while maintaining moderate ultrasound intensity levels. The interaction between the injected bubbles and the HIFU field is investigated using a recently developed 3-D numerical model. The propagation of non-linear ultrasonic waves in the tissue or in a phantom medium is modeled using the compressible Navier-Stokes equations on a fixed Eulerian grid, while the microbubbles dynamics and motion are modeled as discrete singularities, which are tracked in a Lagrangian framework. These two models are coupled to each other such that both the acoustic field and the bubbles influence each other. The resulting temperature rise in the field is calculated by solving a heat transfer equation applied over a much longer time scale. The presence of microbubbles modifies the ultrasound field in the focal region and significantly enhances heat deposition. The various mechanisms through which heat deposition is increased are then examined. The effects of the microbubble cloud size and its location in the focal region are studied, and the effects of these parameters in altering the temperature rise and the location of the temperature elevation are discussed.

Center for Fluid Mechanics Seminar Series

Lorène Champougny, Postdoctoral Research Associate, Universidad Carlos III de Madrid, Spain, will present a talk: “Characterization and Handling of Multiphase Fluid Dispersions in Microfluidics.”

Abstract: In this seminar, I will present some experimental developments on the handling and characterization of multiphase dispersions in the context of microfluidics, that is to say when at least one of the dimensions confining the dispersion is sub-millimetric.

In the first part, I will describe an original route to achieve spatial control of wettability in microchannels. Such wettability patterning is key to many applications of microfluidic devices, ranging from double emulsion generation to localized cell adhesion. Our approach consists in harvesting the natural hydrophobic recovery of poly (dimethylsiloxane) (PDMS) elastomers: hydrophilized PDMS surfaces tend to return spontaneously to hydrophobicity with time, mainly because of diffusion of low molecular weight silicon species to the surface. I will show how space-resolved hydrophobic patterns can be produced by thermally enhancing hydrophobic recovery of PDMS at local scale. Importantly, I will also discuss how to locally quantify wettability in microchannels using a fluorescent probe. 

In the second part, I will show how ultrasound transmission may be used to characterize the structure of a liquid-gas dispersion in microfluidics. Previous studies (Pierre et al., 2013, 2014, 2017) qualitatively revealed that ultrasound propagation is sensitive to the structure of three-dimensional polydisperse liquid foams. Here, I will introduce an original setup to study experimentally the transmission of ultrasound through a single layer of monodisperse bubbles generated by microfluidics techniques. Careful and independent characterization of this 2D microfoam will allow us to quantify how the ultrasonic transmission depends on the gas contents and structure of the bubble monolayer. 

Center for Fluid Mechanics Seminar Series

Rui Ni, Assistant Professor, Johns Hopkins, will present a talk: “Bubble breakup in turbulence driven by small eddies”.

Abstract: From air-sea gas exchange to flotation bioreactors, fragmentation of bubbles in turbulence constitutes one of the most basic and practically important processes in turbulent multiphase flow. Most phenomenological models and simulations for this problem have been developed based on the classical
Kolmogorov-Hinze framework, even though some of the key assumptions have never been tested and challenged. In this talk, I will first introduce a new experimental framework that measures the geometry of breaking bubbles and their surrounding turbulence simultaneously in 3D. From this new result, I will discuss two issues that we found in the classical framework: (i) the Kolmogorov’s classical theory of turbulence is not sufficient for quantifying the turbulent stresses on the bubble interface, and (ii) the assumption that the most relevant and energetic scale of the flow is at the bubble diameter is incorrect. Our work underlines the importance of two missing mechanisms and paves the foundation for future studies on the fragmentation of bubbles, droplets, and particles in turbulence.

Bio: Dr. Ni joined the Department of Mechanical Engineering at Johns Hopkins University as an Assistant Professor in 2018 and was appointed as the DOE ORISE professor since then. Prior to joining JHU, he was the endowed Kenneth K. Kuo Early Career Professor at Penn State University. He received his Ph.D. in the Department of Physics from the Chinese University of Hong Kong in 2011, and worked as a postdoctoral scholar at Yale and Wesleyan University. He has received the NSF CAREER award in fluid dynamics, ACS-PRF New Investigator Award, and NASA Early Stage Investigation award. His primary research focus is the development of advanced experimental methods for multiphase flows.

Center for Fluid Mechanics Seminar Series

Sangwoo Shin, Professor, University of Buffalo, will present a talk: “Delivering and removing colloidal particles from porous media with a pinch of salt.”

Abstract: The transport of colloidal suspensions containing various solutes in porous media is involved in many important applications such as drug delivery, oil extraction, CO2 sequestration, bioseparation, soil remediation, and water purification. Such systems often exhibit spatiotemporal inhomogeneities in the solute and colloid distributions, which may lead to unique colloidal dynamics via non-equilibrium processes. One of which is diffusiophoresis, which refers to the spontaneous migration of colloidal particles induced by solute gradients. In this talk, I will discuss three examples in which diffusiophoresis can be helpful for systems that require accelerated colloid transport in confined porous media: 1) delivering therapeutic nanoparticles into a compressed extracellular matrix, 2) removing oil emulsion from deep rock pores, and 3) getting cleaner laundry.

Bio: Sangwoo Shin received his BS and PhD in Mechanical Engineering from Yonsei University in 2005 and 2012. He is currently an Assistant Professor in the Department of Mechanical Engineering at the University at Buffalo. Prior to joining UB, he was a Postdoctoral Research Associate at Princeton University from 2013 to 2016 and an Assistant Professor at the University of Hawaii from 2017 to 2021. His research involves diverse problems in the areas of complex fluids, interfacial processes, and transport phenomena in small-scale systems.

Center for Fluid Mechanics Seminar Series

Dominique Legendre, Professor, Toulouse Fluid Mechanics Institute, France, will present a talk: “Two Bubble Tales: Skirts and Chains.”

Abstract: The seminar addresses two topics related to bubble dynamics. The first one is about skirt bubbles. For viscous liquids and large bubbles, a thin layer of fluid, commonly referred to as a “skirt”, can be observed issuing from the rim of bubbles. First reported for drops by Thomson and Newall (1885), the appearance of skirts requires a sufficiently viscous continuous phase. Different theories have been proposed for the skirt description. The significant differences between experimental observations and proposed theories have motivated this work. Direct numerical simulations of skirt bubbles are presented and investigated to clarify skirt bubble dynamics. In the second part of the seminar, the stability of bubble chain is discussed. For example, bubbles appear when a carbonated drink is poured in a glass. Very stable bubble chains are clearly observed in champagne, showing an almost straight line from microscopic nucleation sites on the glass surface from which they are continuously formed. In some other drinks such as soda, such chains are not straight (not stable). Considering pair interactions for spherical clean bubbles, bubble chains should not be stable which contradicts some observations. The aim of this work is to explain the condition of stability of bubble chains. For this purpose, experiments and direct numerical simulation are presented.

Bio: Prof. Dominique Legendre graduated with a PhD in Fluid Mechanics in 1996 in IMFT (Institut de Mécanique des Fluides de Toulouse), France. He is currently Professor in Fluid Mechanics Professor since 2007 and is a senior researcher at IMFT one leading Fluid Mechanics Institute in Europe. His main line of research is bubble and drop dynamics including, heat and mass transfert, icing, wetting phenomena. His investigations are mainly based on the development of numerical methods for direct numerical simulations and collaborations with experimentalist. He is co-chairman of the International Congress of Multiphase Flows (ICMF).

Center for Fluid Mechanics Seminar Series

Jake Socha, Samuel Herrick Professor, Virginia Tech, will present a talk: “How flying snakes glide: turning the body into a wiggling wing.”

Abstract: Flying snakes are perhaps the world’s most unconventional gliders, turning their body into a wing by changing shape and undulating in the air. In this talk, I’ll discuss our experimental and theoretical efforts to understand the biomechanical features that underly this unique form of flight. In particular, I’ll highlight recent work that aims to understand the relative contributions of the snake’s inertial mechanics and aerodynamics. Our lab uses engineering tools to study fundamental questions about how animals work, but we also aim to transfer that knowledge to bio-inspired design. I’ll briefly discuss such efforts, in which we’ve mostly been forced to extract lessons from things that don’t work. I’ll also briefly mention a few other ongoing projects related to how animals move.

Bio: Dr. Jake Socha is the Samuel Herrick Professor in the Department of Biomedical Engineering and Mechanics at Virginia Tech. He earned B.S. degrees in physics and biology from Duke University in 1994 and a Ph.D. in biology (with a focus on biomechanics) from the University of Chicago in 2002. After graduate school, he was the Ugo Fano Postdoctoral Fellow at Argonne National Laboratory, studying internal flow systems in insects using synchrotron x-ray imaging at the Advanced Photon Source. His research program at Virginia Tech combines both interests, investigating the biomechanics and functional morphology of flows in and around organisms. Prior to entering science, he was a member of the Teach for America national teacher corps, serving as the sole high school science teacher at Centerville High School in southern Louisiana.

Center for Fluid Mechanics Seminar Series

ABSTRACT: Multiphase flows require special attention to prevent run-away
computational costs. These costs stem from the broad range of dynamically relevant spatial and temporal scales that the flow entails. For example, small bubbles rapidly oscillate compared to the acoustic waves that set them in motion. Sub-grid scale models can average over these effects, allowing for coarser grids and larger time steps. Analytic closure models are only available in special cases, and computing them can be costly. This talk presents the state of these models from both perspectives. I address how we close analytic models numerically without bottle-necking simulations and hope to create valid models without strict assumptions.

BIO: Spencer Bryngelson is an assistant professor in the School of Computational Science and Engineering at Georgia Tech. He has been a senior postdoc at Caltech, a visiting researcher at MIT, and a postdoc at the Center for Exascale Simulation of Plasma‐Coupled Combustion. He received his Ph.D. in 2017 from UIUC and his B.S. in 2013 from the University of Michigan. Spencer’s research group, Computational Physics @ GT, develops models, fast numerics, and scalable software for problems in health and defense.

Center for Fluid Mechanics Seminar Series

Colin R. Meyer, Professor, Dartmouth, will present a talk: “A thermomechanical model for frozen sediments.”

Abstract: Ice-infiltrated sediment, known as a frozen fringe, leads to phenomena such as frost heave, ice lenses, and meters of debris-rich ice under glaciers. Understanding the dynamics of frozen fringe development is important as frost heave is responsible for damaging infrastructure at high latitudes; frozen sediments at the base of glaciers can modulate glacier flow, influencing the rate of global sea level rise; and frozen water ice exists within the sediments of the top several meters on Mars and in places on the Moon. Here we study the fluid physics of interstitial freezing water in sediments and focus on the conditions relevant for subglacial and planetary environments. We describe the thermomechanics of liquid water flow through and freezing in ice-saturated frozen sediments. The force balance that governs the frozen fringe thickness depends on the weight of the overlying material, the thermomolecular force between ice and sediments across premelted films of liquid, and the water pressure within liquid films that is required by flow according to Darcy’s law. Our model accounts for premelting at ice-sediment contacts, partial ice saturation of the pore space, water flow through the fringe, the thermodynamics of the ice-water-sediment interface, and vertical force balance. We explicitly account for the formation of ice lenses, regions of pure ice that cleave the fringe at the depth where the interparticle force vanishes.

Center for Fluid Mechanics Seminar Series

Zehao Pan, Complex Fluids Group, Princeton University, will present a talk: “From focused electric field to hydrodynamic triple point: applications in biosensing and biomaterial.”

Abstract: Digital PCR allows detecting a single target DNA molecule from biological samples, a technology essential for sensitive disease detection ranging from cancer to COVID-19. One of the hurdles of wider adoption of digital PCR hinges upon the cost of generating large numbers of uniform emulsion using complex microfluidic chips. Harnessing the power of electric stress, I will report the first instance of generating uniform water-in-oil emulsion using electrospray where the chip is made of a single micropipette. Along the same vein of generating microemulsion, I will next focus on a parallelization strategy based on selective withdrawal, where a suction flow is introduced near a gravitationally separated liquid-liquid interface. The lowest flow rate for selective withdrawal is at a hydrodynamic triple-point that reveals the interplay between the viscosity, density and capillary force.

Center for Fluid Mechanics Seminar Series

Ousmane Kodio, Applied Mathematics, MIT, will present a talk: “Emergence and Evolution of Dynamic Buckling Patterns in Soft Matter.”

Abstract: In this seminar, we will discuss the evolution of patterns that emerge from elastic and fluids instabilities. Indeed, many phenomena encountered in nature may be understood through the paradigm of buckling instabilities, for example: the supercoiling in DNA, the wrinkles on skin, the waves of leaves and the petals of flowers, the folding of geological formations, the design of columns in structural engineering. In engineering, buckling and wrinkling are traditionally considered nuisance, but more recently they have proven useful as a potential tool for controlling pattern formation, particularly at small scales. In general, buckling and wrinkling are dynamical processes. While there is a long history of studies devoted to the theory of static elastic structures, the dynamical theory of how these structures evolve in time presents many open challenges at the theoretical and computational level. Here, we will discuss two such problems: First, how wrinkle patterns evolve in time under confinement. Specifically, we will illustrate how integral constraints can slow down the evolution of patterns and break down the self-similarity in contrast to what is frequently observed in fluid mechanics. Second, we will demonstrate how non-trivial buckling patterns may emerge when motion happens quickly. We will show how quenching, namely the rapid change of the control parameter can be tuned for the spontaneous selection of buckling modes. This is reminiscent of the Kibble-Zurek mechanism for the size of defects observed during a continuous non-equilibrium phase transition which are thought to describe the structure formation of the early universe.

Center for Fluid Mechanics Seminar Series

Davide Masiello, University of  Edinburgh, will present a talk: “Bubbles, sound and frequency: An investigation of acoustic cavitation drive by audible sound.”

Abstract: The dynamics of bubble collapse are of great interest in a variety of fields across science and engineering. Amongst these, sonochemistry has shown to be a particularly promising technology in several applications, ranging from the degradation of pollutants to the synthesis of valuable chemicals.

The ensemble of studies concerned with sonochemistry carried out up to the present day suffer from a lack of exploration of the lower end of the frequency spectrum. In fact, almost no study at all is available at frequencies lower than 20 kHz. This has led many researchers to believe the existence of a set of reasons why sonochemistry is practically feasible only in the ultrasonic range of frequency. However, such reasons do not seem to be backed up by solid evidence. The scarce interest in the lower frequency range has had an impact on the number of fundamental studies on the single bubble dynamics, which remains little. This seminar entails the dynamics of a single spherical bubble driven by audible sound. The aim of this investigation is to evaluate the possibility to carry out sonochemistry using low frequency sound. First, a numerical analysis of the mechanisms of mass and heat transfer is carried out to elucidate significant differences between the low and high ends of the frequency spectrum. Lowering the sound frequency has the dramatic effect of causing larger bubble expansions. Consequently, greater amounts of the surrounding liquid’s vapour intrude in the gas phase with significant impact on the intensity of the bubble collapse. It is argued that the predictions offered by existing studies are incorrect because of flaws in the basic assumptions of the used theories. A novel model is developed to provide more accurate predictions maintaining a reasonable level of computational efficiency. The validity of any discussed reduced-order model is inferred by comparison with the results of the accurate equations of the bubble dynamics that, although being the most accurate tool available for simulations, are very time-expensive.

The new proposed model is extended to account for chemical reactions of the gas phase and a large parametric study is carried out across many acoustic amplitudes, frequencies, and bubble sizes. The production of ammonia is chosen as a case study, and the effect of the driving frequency is explored in relation to the number of molecules produced during one acoustic cycle. The studies carried out show that for acoustic amplitudes lower than 1.2 atm, production of ammonia is observed only in the lower end of the frequency spectrum. Some of the theoretical predictions are backed up by an experimental validation that has been carried out on a custom-made apparatus designed to take high-speed pictures of inertially collapsing bubbles.

Center for Fluid Mechanics Seminar Series

Dr. Nathan B. Speirs, Naval Undersea Warfare Center, Newport, will present a talk: “Smashed into Vapor, Cavitation from Water Slamming.”

Abstract: Cliff diving can really hurt. The pain stems from the high pressure generated at impact, which von Karman showed approaches infinity when striking flat (von Karman, 1929). Yet in contrast to the high pressures found from hard experience we show that a cylinder impacting on a water surface can decrease the local pressure enough to cavitate the liquid in the very early moments (~100 μs). The liquid cavitates because its slight compressibility allows large pressure waves to form that reflect and create negative pressure regions. Impact velocities as low as ~3 m/s suffice to cavitate the liquid. We formulate a new cavitation number to predict the onset of cavitation in these low-speed water slamming scenarios. These findings imply that cavitation is possible in a variety of free-surface impacts such as boats slamming, cliff diving, and ocean landing of spacecraft. 

Center for Fluid Mechanics Seminar Series

Zoe Zhu, Harvard University, will present a talk, “Topological origin of stratified fluids and plasma waves.”

Abstract: Topology and symmetries have been applied to explain phenomena at the quantum mechanical scale such as the quantum Hall states and the robustness of surface states in topological insulators against disorder. Remarkably, topology also plays an important role in the Earth’s climate system and other classical fluids. Poincaré gravity modes described by the shallow water equations in a rotating frame have nontrivial topology, providing a new perspective on the origin of equatorially trapped Kelvin and Yanai waves that are an important component of the El Niño Southern Oscillation. In this talk, I will show that non-trivial topology in rotating shallow water equations and continuously stratified primitive equations persist in the presence of a background sinusoidal shear flow, which not only breaks the Hermiticity, isotropy, and homogeneity of the system but also leads to instabilities. We also predict that topologically nontrivial waves can arise in magnetized plasmas, and I will describe a planned experiment at the Large Plasma Device at the UCLA Basic Plasma Science Facility to look for the topological waves.

Center for Fluid Mechanics Seminar Series

Jie Feng, Assistant Professor, Mechanical Science and Engineering, University of Illinois, will present a talk “Bubble Bursting with a Compound Interface: from Submicron Dispersal to Multi-Phase Jetting.”

Abstract: Bursting of bubbles at a liquid surface is ubiquitous in a wide range of physical, biological, and geological phenomena, as a key source of aerosol droplets for mass transport across the interface. However, how a structurally complex interface, widely present in nature, mediates the bursting process remains largely unknown. In this talk, we will describe our studies on bubble bursting dynamics with an oil-covered aqueous surface, which typifies the sea surface microlayer as well as an oil spill on the ocean. First, we will show that bubbles bursting at an air/oil/water-with-surfactant interface can disperse submicron oil droplets in water. Dispersal results from the detachment of an oil spray from the bottom of the bubble towards water during bubble collapse. Surprisingly, the droplet size is selected by physicochemical interactions between oil molecules and the surfactants rather than by hydrodynamics. The implications of the dispersal mechanism for oil-spill remediation will also be demonstrated. Our system may provide an energy-efficient route, with potential upscalability, for applications in drug delivery, food production and materials science. Secondly, we will focus on the bubble-bursting jet dynamics at such a compound interface. The jet tip radius and velocity are altered with even a thin oil layer, and oily aerosol droplets are produced. We show that the coupling of oil spreading and cavity collapse dynamics results in a multi-phase jet and the follow-up droplet size change. The oil spreading influences the effective viscous damping of the capillary waves, and scaling laws are proposed to quantify the jetting dynamics. Our study not only advances the fundamental understanding of bubble bursting dynamics, but also may shed light on the airborne transmission of organic matters in nature related to aerosol production.

Center for Fluid Mechanics Seminar Series

Meghana Ranganathan, Ph.D. candidate, Massachusetts Institute of Technology, will present a talk: “A microstructural view of glacier dynamics.”

Abstract: Ice shelves (floating regions of an ice sheet) play a key role in stabilizing the Antarctic Ice Sheet (AIS) due to their ability to act as a “cork”, preventing rapid flow of ice towards the ocean. Observations have shown significant ice deformation and ice fracture in the margins of ice shelves, where there the ice is shearing significantly, and current studies suggest that this deformation and fracture may destabilize the ice shelf. Here, we seek to characterize the physical processes leading to rapid ice flow and fracture in the margins of ice shelves. We derive a model to show that processes that occur on the microphysical scale (at the scale of individual ice crystals within the glacier) may be responsible. These results propose a mechanism that may allow for instabilities in the AIS and demonstrate the need for ice-flow models to incorporate micro-scale processes into projections of ice sheet behavior.

Center for Fluid Mechanics Seminar Series

Benjamin McDonald, Assistant Professor of Chemistry, Brown University, will present a talk: “Designer Polymers as Building Blocks for Hierarchically Structured Soft Materials and Enzyme Mimetic Systems.”

Abstract: The function of nature’s structural material systems, such as wood, silk, and bone, is intrinsically tied to the spatio-temporal organization of chemical functionality within these materials. This functionality is embedded in nanoscale building blocks with uniform structure and chemical decoration that are further organized into repetitive hierarchical assemblies spanning nano- to macroscopic length scales. Thus, from relative chemical simplicity, arise materials with remarkable mechanical and optical properties. Inspired by this bottom-up paradigm, the McDonald lab seeks to develop polymeric materials as modular and scalable synthetic building blocks with programable nanostructure, assembly, and reactivity. The development of these chemical tools will be driven by organic and polymer chemistries, but the realization of their rich potential will require multidisciplinary engagement across the biomedical, fluid, and material sciences. This lecture will discuss fundamental studies towards such polymeric systems and their applications, as well as opportunities to collaboratively engage critical challenges in areas such as tissue engineering, energy storage, and active matter.

Bio: Ben was born in Bangor, Maine, but grew up in rural central Ohio. Eager to study chemistry at a small liberal arts college, he received his undergraduate degree in chemistry from University of North Carolina Asheville in 2012, conducting research with Prof. Herman Holt. He then pursued his PhD at Northwestern University under the guidance of Prof. Karl Scheidt. As a NIH NRSA predoctoral fellow, he developed new catalytic methods for the construction of small molecules. Subsequently, he was a Martin Luther King postdoctoral scholar in the lab of Prof. Tim Swager at MIT. Here he developed a passion for organic macromolecules and their use as dynamic and reactive materials. Ben began his appointment in the Department of Chemistry at Brown as an assistant professor in 2021. Ben enjoys climbing plastic simulations of rocks and lavishing love upon his cats.

Center for Fluid Mechanics Seminar Series

Shengze Cai, Postdoctoral Research Associate, Applied Math, Brown University, will present a talk: “Data-driven and physics-informed deep learning for flow visualization technologies.”

Abstract: Flow visualization, the process of making the fluid flows visible, plays a significant role in modern experimental fluid mechanics. Despite the advances in experimental methods, the use of visualization data to quantify the fluid dynamic is still challenging. In this talk, I will mainly introduce two examples on how deep learning techniques can be involved in flow visualization. First, we develop a data-driven learning strategy for particle image velocimetry (PIV). A convolutional neural network, trained by using an artificial PIV dataset, is employed to extract 2D velocity fields from particle images with high accuracy and efficiency. Second, we propose a method based on physics-informed neural networks (PINNs), which can integrate the underlying physics of the observed fluid flow and the experimental data (e.g., sparse measurements, temperature or images), to infer the continuous velocity and pressure fields. The method has been applied in particle tracking velocimetry, background oriented schlieren and in-vitro imaging, showing the flexibility to deal with various experimental data.

Center for Fluid Mechanics Seminar Series

Margaret Byron, Assistant Professor of Mechanical Engineering, Penn State University, will present a talk: “Life in transition: the fluid dynamics of locomotion at physical and figurative interfaces.”

Abstract: Animals use a wide variety of strategies to move through their environment. Flying and swimming animals, who move by manipulating the surrounding fluid, experience a unique transition: at very small lengthscales and slow speeds, viscosity plays a primary role, but at larger scales and speeds, inertial forces dominate. Studies of animal locomotion in air and water have historically focused on organisms that move at the limits of viscous- or inertia-dominated flow, but many animals move in such a way that both inertia and viscosity play an important role (the intermediate Reynolds number regime). We will focus on two different groups of animals who swim at intermediate Reynolds numbers, from both marine and freshwater environments. We will first focus on comb jellies (ctenophores), a group of marine zooplankton who are among the oldest animals on the planet. They are also the largest animals on Earth which use cilia to swim. These flexible hair-like structures commonly occur at the scale of microns, but in ctenophores they appear as millimeter-scale structures that are bundled together into paddle-like structures called ctenes. We will explore the scaling of these structures from the viscous- to inertia-dominated flow regime, and discuss implications for bioinspired devices, sensors, and vehicles. Second, we will examine two aquatic insects in the infraorder Nepomorpha: water boatmen and backswimmers. These insects are unique not only because they swim in the intermediate Reynolds number regime, but because they are trimodal: they can swim, walk, and fly, and transition quickly and easily between locomotor modes. We will look at their transition across the figurative interface (from low to high Re) as they swim, as well as their transition across the literal air-water interface as they move between swimming and flight.

Bio: Margaret L. Byron is an Assistant Professor of Mechanical Engineering at Penn State University, where she directs the Environmental and Biological Fluid Mechanics (EBFM) Laboratory. She received her B.S.E. from Princeton University in Mechanical and Aerospace Engineering (2010), and her M.S. and Ph.D. from the University of California Berkeley (2012/2015). From 2015 – 2017 she was an NSF Postdoctoral Fellow in Biology at the University of California Irvine. She is a recipient of the National Science Foundation Graduate Research Fellowship, the American Chemical Society Doctoral New Investigator Award, and the Arnold and Mabel Beckman Foundation Young Investigator Award. Dr. Byron’s group studies the interactions between organisms and particles in environmental flows, with a particular focus on intermediate scales where inertial and viscous fluid forces are both important. She is interested in how animals control their position and orientation in turbulence, how swimming strategies scale with size and speed, and what this implies for their overall behavior and distribution in aquatic environments. She is also exploring the effects of particles’ size, shape, and mass properties on their kinematics in environmental flows; these problems have implications for sediment and pollutant transport. Specific projects investigate the behavior of microplastics, marine snow, and oil-mineral aggregates in turbulence.

Center for Fluid Mechanics Seminar Series

Isabel Scherl ’17, Ph.D. student, Research Assistant
University of Washington

Title: Experimental Fluid Mechanics with Machine Learning

Abstract: The ability to understand unsteady fluid flows is foundational to advancing technologies in energy, health, transportation, and defense. We use cutting edge data-driven methods (i.e. machine learning) to interpret and control unsteady fluid flows through experiments in the following three cases: 1. We use robust principal component analysis (RPCA) to improve flow-field data by leveraging global coherent structures to identify and replace spurious data points. In all cases, both simulated and experimental, we find that RPCA filtering extracts dominant coherent structures and identifies and fills in incorrect or missing measurements. 2. We optimize a two cross-flow (i.e. vertical-axis) turbine array using a hardware-in-the-loop approach and find that arrays with well-considered geometries and control strategies can outperform isolated turbines by up to 30%. 3. Using similar turbines, we create an experimental framework to more efficiently explore arrays’ high-dimensional parameter space. Our data-driven approach allows us to model parameter spaces using sparse data. As a result, we are able to map turbine system dynamics with orders of magnitude fewer data points.

Center for Fluid Mechanics Seminar Series

Title: Capillary Flows of Suspensions

Abstract: Interfacial flows of multiphase systems containing a dispersed solid or liquid phase occur in a broad range of manufacturing, environmental, and bioengineering processes. However, the classical capillary dynamics is strongly modified when the length scale of the liquid becomes comparable to the particle size. This configuration may lead to a failure of classical models based on a rheological approach. For instance, particles can destabilize thin-films, reduce transport efficiency, and result in the contamination of substrates.
In this talk, I will  present some of our recent studies that characterize the role of interfaces in suspension dynamics. I will first characterize the formation of a thin-film of suspension on a substrate to illustrate how the particles are entrained and deposited depending on the flow configuration and suspension properties. I will discuss how these results can be used to develop passive capillary filtering and sorting mechanisms. The second part of the talk will characterize how particles can modify the atomization of suspension sheets and ligaments.
Our approach, bridging different length and time scales, describes how the bulk behavior and local heterogeneities contribute to the dynamics of multiphase capillary objects.

Bio: Alban Sauret is an Assistant Professor in the Department of Mechanical Engineering at UC Santa Barbara. He graduated with a BS and an MS in Physics from ENS Lyon (France) and earned a Ph.D. in Mechanical Engineering from the University of Aix-Marseille (France) in 2013. During his graduate studies, he was awarded a Geophysical Fluid Dynamics Fellowship from the Woods Hole Oceanographic Institution. He then worked as a Postdoctoral Fellow at Princeton University from 2013 to 2014 and then spent four years as a tenured CNRS Research Scientist in a joint academic and industrial laboratory, while also being a visiting research scholar at NYU Tandon School of Engineering. He joined UC Santa Barbara in 2018. His research aims at understanding the dynamics of multiphase systems. He is particularly interested in the couplings between the fluid dynamics, interfacial effects, and particle transport mechanisms involved in environmental and industrial processes. Alban Sauret was named a Soft Matter Emerging Investigators in 2017, was elected a UC Regents Junior Faculty Fellow in 2019, and received the NSF CAREER Award in 2020. His past results were highlighted in various media, including the Los Angeles Times, the Wall Street Journal, and Science Friday.

Center for Fluid Mechanics Seminar Series

Title: Vortex tubes and vortex rings: reconnections and the turbulent cascade

Abstract: We numerically simulate the vortex ring collision experiment of Lim and Nickels (Nature, 357:225- 227, 1992), in an attempt to understand the rapid formation of very fine scale turbulence (or ‘smoke’) from relatively smooth initial conditions. Reynolds numbers of up to Re = |Gamma/|nu = 4000 are reached, where |Gamma is the vortex ring circulation, and |nu the kinematic viscosity of the fluid. These coincide with the highest-Reynolds number case of the experiments. Different perturbations to the ring vortex are added, and their effect on the generation and amplification of turbulence is quantified. The underlying dynamics of the vortex core is analyzed and the presence of Crow and elliptic instabilities is used to explain the different dynamics: either turbulent reconnection or cloud formation. We show how we can control these instabilities by using different initial conditions. The asymptotic behaviour of the collision is analyzed, and a link to the turbulent energy cascade is revealed through which a cascade of iterative instabilities is shown to transfer energy through spatial scales.

Bio: Rodolfo Ostilla Mónico is an assistant professor at the University of Houston in the department of Mechanical Engineering since Fall 2017. He obtained his bachelor degree in Aerospace Engineering from the University of Sevilla (Spain) and an MSc in Aerospace Dynamics from Cranfield University (UK). His PhD thesis was obtained at the University of Twente in the Netherlands, under the supervision of Roberto Verzicco and Detlef Lohse. From there, he moved to Harvard University for a postdoc under Michael Brenner. His research focuses on computational fluid mechanics at high Reynolds numbers from the fundamental to the applied.

Center for Fluid Mechanics Seminar Series

Title: Locomotion of flagellated bacteria: From the swimming of single bacteria to the collective motion of bacterial swarm

Abstract: I will discuss two recent experimental works in my group on the locomotion of flagellated bacteria. First, we study the motility of flagellated bacteria in colloidal suspensions of varying sizes and volume fractions. We find that bacteria in dilute colloidal suspensions display the quantitatively same motile behaviors as those in dilute polymer solutions, where a size-dependent motility enhancement up to 80% is observed accompanied by a strong suppression of bacterial wobbling. By virtue of the well-controlled size and the hard-sphere nature of colloids, this striking similarity not only resolves the long-standing controversy over bacterial motility enhancement in complex fluids, but also challenges all the existing theories using polymer dynamics in addressing the swimming of flagellated bacteria in dilute polymer solutions. We further develop a simple hydrodynamic model incorporating the colloidal nature of complex fluids, which quantitatively explains bacterial wobbling dynamics and mobility enhancement in both colloidal and polymeric fluids. Second, we study the collective motion of dense bacterial suspensions as a model of active fluids. Using a light-powered bacterial strain, we map the detailed phase diagram of bacterial flows and image the transition kinetics of bacterial suspensions towards collective motions. The effect of collective motions on the density fluctuation and rheological response of bacterial flows will also be discussed. Together, our study sheds light onto the puzzling motile behaviors of bacteria in complex fluids relevant to a wide range of microbiological processes, and provides a basis for engineering collective bacterial swimming in various physical and biomedical applications.

Center for Fluid Mechanics Seminar Series

Title: Heterogeneous soft materials: effects of local dynamics on transport and mechanics

Abstract: Many soft materials – such as polymer solutions and melts, colloidal suspensions, and biological fluids – contain nanometer or micron-sized structures. On these length scales, thermal energy is comparable to the strength of enthalpic and entropic interactions, causing the underlying structures to undergo Brownian motion. Whereas this motion leads to well-understood diffusive behavior in simple fluids, the structures present in soft materials make it difficult to predict how particles and other objects will move and transport. In this talk, I will discuss the interplay between structure and dynamics and the controlling physics across a variety of soft matter systems. We explain how hard nanoparticles transport orders of magnitude faster than expected by coupling to the dynamics of the surrounding environment. By grafting polymers onto the nanoparticle surface, we tune the softness of interactions to modify this coupling behavior and change the nanoparticle dynamics. Additionally, we will discuss how the fracture and self-healing of biomacromolecular gels depends on segmental dynamics to rebuild a stress-supporting network. These findings help to elucidate the enhanced mechanical properties of polymer nanocomposites, to improve the efficacy of targeted drug delivery and environmental remediation techniques, and to better predict the kinetics of biochemical reactions.

Bio: Ryan Poling-Skutvik is an Assistant Professor in the Department of Chemical Engineering at the University of Rhode Island. He received his B.E. in Chemical Engineering from the Cooper Union for the Advancement of Science and Art in 2013 and his Ph.D. in Chemical Engineering from the University of Houston in 2018. His research expertise combines rheology, microscopy, and scattering to identify hierarchical behavior in complex soft materials. 

Center for Fluid Mechanics Seminar Series

Title: Insights into tiny flow systems: how insects produce internal flows

Abstract: Insects can be viewed as exquisite microfluidic systems: they pump air, blood, and food through their bodies, all within one small package. Compared to engineered systems, they are far smaller, controllable, and efficient than anything that humans have designed. How do insects produce these flows? In this talk, I’ll describe some of our work on respiratory and circulatory flows in insects, driven primarily by imaging using synchrotron x-rays. Our efforts focus primarily on answering fundamental scientific questions about the animal, but I’ll also discuss some of our preliminary attempts at bio-inspired devices based on these systems.

Bio: Dr. Jake Socha is a professor in the Department of Biomedical Engineering and Mechanics at Virginia Tech. He earned a Bachelor of Science degree in physics and biology from Duke University in 1994 and a Ph.D. in biology (with a focus on biomechanics) from the University of Chicago in 2002. After graduate school, he was the Ugo Fano Postdoctoral Fellow at Argonne National Laboratory, studying insect flow systems using synchrotron X-ray imaging at the Advanced Photon Source. His research program at Virginia Tech combines both interests, investigating the biomechanics and functional morphology of flows in and around organisms, specifically flying snakes, frogs, and insects. Prior to entering science, he was a member of the Teach for America national teacher corps working in southern Louisiana.

Center for Fluid Mechanics Seminar Series

Title: Colloidal robotics: Engineering the autonomous behavior of self-propelled particles

Abstract: Living cells navigate complex environments to perform diverse functions by integrating the capabilities of sensing, computation, and actuation. In pursuit of similar capabilities in synthetic “microrobots”, we seek to understand the many mechanisms underlying the self-propulsion of colloidal particles through viscous fluids. Building on this understanding, we seek to design active particles capable of autonomous behaviors such as navigation of structured environments. In this talk, I discuss two recent projects – on Quincke oscillators and magnetic topotaxis, respectively – that highlight these complementary aims to understand and design active colloids. In part one, I explain how static electric fields drive the oscillatory motion of micron-scale particles commensurate with the thickness of a field-induced boundary layer in nonpolar electrolytes. In part two, I describe how spatially uniform, time-periodic magnetic fields can be designed to power and direct the migration of ferromagnetic spheres up local gradients in surface topography.

Center for Fluid Mechanics Seminar Series

Title: Computational microscopy for next-generation bio-imaging

Abstract: My research goal is to develop computational microscopy technologies for bio-imaging applications to drive scientific discovery. My research can be broadly applied across several basic-science, biomedical, and clinical fields, such as pathology, developmental biology, cancer-biology, immunology, physiology, neuroscience, etc.

Past decades have seen dramatic developments in optical imaging technologies, computing power, and data accessibility. These developments have recently combined to spawn the field of computational imaging, where optical systems and computational algorithms are jointly designed within data-driven imaging pipelines. This new imaging paradigm has found great success in consumer photography, where mobile phones routinely offer low-cost and high-resolution imaging with noise reduction, 3D imaging, digital refocus, etc. My research brings a similar paradigm shift into optical microscopy, and specifically explores big-data computational microscopy techniques for the biomedical and clinical sciences. The unique challenges associated with wavelength-scale scientific imaging distinguish microscopy from photography and make it an exciting application for cutting-edge computational research. In this talk, I will talk about some recent developments in computational microscopy that enable multi-dimensional super-resolution as well as 3D imaging into optically scattering biological specimens.

Center for Fluid Mechanics Seminar Series

Title: A Eulerian multi-component framework for fluid-structure interaction problems including cavitation

Abstract: The manipulation of cavitating, bubbly flows is a critical need across a wide range of applications including naval hydraulic, energy science, and biomedicine. Cavitation is a pressure-driven vaporization process that can take place in highly transient liquid flows. In naval hydraulic applications, cavitation erosion of propeller blades is undesired. The erosion inhibits performance and is the primary source of frequent, costly maintenance. To address this adverse effect and biofouling, viscoelastic coatings are applied to the blades. Nevertheless, cavitation erodes these coatings, which then also needs costly reapplication. Similarly, cavitation erosion limits the operation life cycle and performance of the Department of Energy’s Spallation Neutron Source experiments used to develop a wide range of technologies. On the other hand, cavitation is desired in biomedicine. An example is non-invasive, focused ultrasound therapy tools. These tools generate cavitation bubbles to erode kidney and gallbladder stones (lithotripsy) or ablate soft, pathogenic tissues (histotripsy). Understanding the bubble dynamics and material response interactions is needed to increase the efficacy of these tools.

To study these fluid-structure interaction problems, the challenge is to numerically simulate small bubbles in a bubble cloud with wave dynamics in the fluid(s) and nearby solid. Unlike the gas bubbles and surrounding liquid that lend themselves to a Eulerian framework, the nearby solid undergoes infinitely small to finite deformations that are well-suited to be captured in a Lagrangian framework. However, algorithmic complexity increases significantly with two separate solvers and coupling between them. Of the two current monolithic Eulerian approaches (i.e., hyperelastic and hypoelastic (conventional)) used to study wave dynamics and material deformations, the hypoelastic approach’s algorithm is well-suited to incorporate elasticity to existing multiphase/multi-component numerical solvers. During the seminar, a Eulerian numerical framework with a hypoelastic model to solve multi-component fluid-structure interaction problems including multi-scale cavitation will be presented. Physical insights involving two canonical flows, i.e., shock-induced collapse of a bubble near an elastic object and a confined bubble in a channel, will be detailed. Future directions and applications for numerically simulating multicomponent flows involving extreme deformations in and near viscoelastic materials will also be presented.

Center for Fluid Mechanics Seminar Series

Title: Tracing the New Arctic

Abstract: Sea ice is an important component of the Earth’s climate system. Following the observed long-term global warming trend, sea ice extent in the Arctic has continued to recede at unprecedented rates over the last decade. Understanding the dynamics of the Arctic sea ice field has, therefore, never been more pressing. However, progress on the development of next-generation climate models has been limited by the sparsity of in-situ field measurements and the limitations of remote sensing products to resolve small-scale features (<20 km). In this talk, I will present a new method for the automatic identification and tracking of ice plates in optical satellite imagery that provides a unique record of sea ice shape and size measurements (from which local concentration metrics can be derived) alongside unprecedented sea ice dynamic observations (drift, rotation rates, and dispersion characteristics). I will introduce recent advances by my group to leverage these observations to examine the dynamical structure of the sea ice field and describe how free-drifting ice plates can be used as a proxy to infer ocean eddy dynamics within the meso/sub-mesoscale range. Our ability to successfully retrieve daily observations at a 250-m resolution from a long-term satellite record (dating back to 2003) provides a road map to understand the dynamical structure of critical momentum and heat transfer regions in our polar oceans.

Center for Fluid Mechanics Seminar Series

Title: Bio-inspired Locomotion Strategies: From feather-inspired flow control to beetle-inspired power amplification

Abstract: Organisms have evolved various locomotion (self-propulsion) and shape adaptation (morphing) strategies to survive and thrive in diverse and uncertain environments. Unlike engineered systems, which rely heavily on active control, natural structures also rely on reflexive and passive control. Nature often exploits distributed flexibility to simplify global actuation requirements. These approaches to locomotion and morphing rely on multifunctional and passively adaptive structures. Two examples of multifunctional structures will be presented in this talk, namely avian-inspired deployable flow control structures and click beetle hinges, which enable legless jumping. Several flow control devices found on birds’ wings will be introduced as a pathway towards revolutionizing the current design of small-unmanned air vehicles. Wind tunnel results will be presented to show the aerodynamic benefits of these devices in delaying stall and improving flight performance. The discussion of avian-inspired flow control devices will be followed by discussing how click beetles can circumvent muscle limitation to achieve an impressive legless jumping maneuver. In this talk, I will present the jump’s kinematics and the non-linear dynamics governing the clicking maneuver that leads to the jump. These research topics do not only represent examples of how nature can inform engineering design, but they also highlight that engineering analysis can provide insights into the locomotion and adaptation strategies employed by nature.

Center for Fluid Mechanics Seminar Series

Title: Fingers, fractals, and flow in liquid metals

Abstract: A droplet of pure water placed on a clean glass surface will spread axisymmetrically, and a droplet of mercury will bead up into a spherical droplet. In both cases, the droplet is minimizing its surface energy – creating an object with a minimized surface area – and there is nothing to break the symmetry. Remarkably, droplets of the room-temperature liquid gallium-indium (EGaIn), which like all metals have an enormous surface tension, can nonetheless undergo fingering instabilities in the presence of an oxidizing voltage. I will describe how this oxide acts like a reversible surfactant, generating fingering instabilities, tip-splitting, and even fractals, through Marangoni instabilities. Remarkably, we find that EGaIn droplets placed in an electrolyte under an applied voltage can achieve near-zero surface tension. This effect can in turn be used to suppress the Rayleigh-Plateau instability in falling streams. Quantitative control of these effects provides a new route for the development of reconfigurable electronic, electromagnetic, and optical devices that take advantage of the metallic properties of liquid metals.

Center for Fluid Mechanics Seminar Series

Title: Phase transitions in geophysical turbulence

Abstract: Work in the last few decades has shown that the theory of turbulence due to Kolmogorov cannot describe turbulent behavior in the context of geophysical and astrophysical flows, which include the effects of rotation, stratification, ionization and large aspect ratios. In particular, recent work shows that turbulent behavior in geophysical contexts undergoes an unexpected phase transition as parameters such as rotation are varied, going from a state that forms large-scale coherent structures to one that only forms small-scale incoherent turbulent fluctuations. In this talk, I will show some of my work studying these transitions using direct numerical simulations and discuss the many questions that arise from these observations.

The University of Birmingham, as part of the UK Fluids Network, will host its final talk of 2020, with Brown University’s Tom Powers, Professor of Engineering at Brown University. Professor Powers will present the talk: “Imposed flow in active liquid crystals.”

Wednesday 16th December 2020
16.00 GMT / 17.00 Central European Time / 8.00 US West Coast / 11.00 US East Coast

https://bham-ac-uk.zoom.us/j/82240044618

Please contact Carolyn_Sherman@brown.edu for the password.

Abstract: Inspired by ongoing experiments on three dimensional active gels composed of sliding microtubule bundles, we study a few idealized problems in a minimal hydrodynamic model for active liquid crystals. Our aim is to use flow to determine the value of the coefficient of activity in a continuum theory. We consider the case of apolar active particles that form a disordered phase in the absence of flow, and study how activity affects the swimming speed of a prescribed swimmer, as well as the stability of a fluid interface. We also consider flows of active matter in channels or past immersed objects.

Center for Fluid Mechanics Seminar Series

Title: Active or electrophoretic particles in shear flow

Abstract: I shall discuss two pieces of recent work from my group. First, I present a model for the linear viscoelasticity of a dilute suspension of active (self-propelled) particles. Notably, the model predicts the particles to cause a negative increment to the suspension viscosity. Through a comparison with experiments on suspensions of Escherichia coli, I will demonstrate that biophysical parameters of these microorganisms can be inferred from our model. Second, I will discuss the dynamics of an electrophoretic particle in an ambient shear flow. This work is motivated by several experiments reporting cross-streamline migration of charged particles undergoing electrophoresis in Poiseuille microchannel flow. Specifically, I will demonstrate that the observed migration arises from the interaction of the electrophoretic particle with the weak inertia of the shear.

Center for Fluid Mechanics Seminar Series

Title: “Life in a tight spot: How bacteria move in porous media”

Abstract: Diverse processes in healthcare, agriculture, and the environment rely on bacterial motility in heterogeneous porous media; indeed, most bacterial habitats—e.g., biological gels, tissues, soils, and sediments—are porous media. However, while bacterial motility is well-studied in homogeneous environments, how confinement in a porous environment impacts bacterial transport remains poorly understood. To address this gap in knowledge, we combine microscopy, materials fabrication, and microbiology to investigate how E. coli moves in 3D porous media. By probing single cells, we demonstrate that the paradigm of run-and-tumble motility is dramatically altered by pore-scale confinement. Instead, we find a new mode of motility in which cells are intermittently and transiently trapped as they navigate the pore space; analysis of these dynamics enables prediction of bacterial transport over large length and time scales. Further, by developing a new 3D printing approach, we design multi-cellular communities with precise control over the spatial distribution of bacteria. Using this approach, we show that concentrated populations can collectively migrate through a porous medium—despite being strongly confined—and develop principles to predict and direct this behavior. 

The Center for Biomedical Engineering and the Center for Fluid Mechanics at Brown present a seminar:

Title: Integrating Machine Learning and Multiscale Modeling in Biomedicine and Engineering

Abstract: Machine learning has emerged as a powerful approach for integrating multimodality/multifidelity data, and for revealing correlations between intertwined phenomena and cascades of scales. However, machine learning alone does not explicitly take into account the fundamental laws of physics and thermodynamics and can result in ill-posed problems or non-physical solutions. Many human diseases are multiscale in nature, e.g., the sickle cell anemia, first characterized as molecular disease by Linus Pauling in 1949. Multiscale modeling is an effective strategy to integrate multiscale/multiphysics data and uncover mechanisms that explain the emergence of function, from the protein level to the organ level. However, multiscale modeling alone may fail to efficiently combine multimodality and multifidelity datasets. We believe that machine learning and multiscale modeling can naturally complement each other to create robust predictive models that integrate the underlying biophysics to manage ill-posed problems and explore massive design spaces. To this end, we will present a new approach to develop a data-driven, learning-based framework for predicting outcomes of biological systems and for discovering hidden biophysics from noisy data. We will introduce a deep learning approach based on neural networks (NNs) and generative adversarial networks (GANs). We will also introduce the DeepOnet that learns functionals and nonlinear operators from functions and corresponding responses for system identification. Unlike other approaches that rely on big data, here we “learn” from small data by exploiting the information provided by the physical conservation laws, reactive transport and thermodynamics, which are used to obtain informative priors or regularize the neural networks. Our multidisciplinary perspective suggests that integrating machine learning and multiscale modeling can lead to creation of medical digital twins, hence, providing new insights into disease mechanisms, help discover new treatments, and inform decision making for the benefit of human health.

Bio: George Karniadakis is from Crete. He received his S.M. and Ph.D. from Massachusetts Institute of Technology (1984/87). He was appointed Lecturer in the Department of Mechanical Engineering at MIT and subsequently he joined the Center for Turbulence Research at Stanford / Nasa Ames. He joined Princeton University as Assistant Professor in the Department of Mechanical and Aerospace Engineering and as Associate Faculty in the Program of Applied and Computational Mathematics. He was a Visiting Professor at Caltech in 1993 in the Aeronautics Department and joined Brown University as Associate Professor of Applied Mathematics in the Center for Fluid Mechanics in 1994. After becoming a full professor in 1996, he continues to be a Visiting Professor and Senior Lecturer of Ocean/Mechanical Engineering at MIT. He is an AAAS Fellow (2018-), Fellow of the Society for Industrial and Applied Mathematics (SIAM, 2010-), Fellow of the American Physical Society (APS, 2004-), Fellow of the American Society of Mechanical Engineers (ASME, 2003-) and Associate Fellow of the American Institute of Aeronautics and Astronautics (AIAA, 2006-). He received the Alexander von Humboldt award in 2017, the Ralf E Kleinman award from SIAM (2015), the inaugural J. Tinsley Oden Medal (2013), and the CFD award (2007) by the US Association in Computational Mechanics. His h-index is 103 and he has been cited over 52,000 times.

Center for Fluid Mechanics Seminar Series

Title: Individual and collective dynamics of biomimetic active colloids

Abstract: Suspensions of active living and artificial micro-particles exhibit diverse collective and large-scale dynamic structures. The emergence of these collective states from the motility pattern of the individual particles, typically a random walk, is yet to be probed in a well-defined synthetic system. In my talk, I present the experimental realization of tunable colloidal motion that reproduces a general run-and-turn motility pattern such as run-and-tumble of E-coli and Lévy trajectories. I utilize the Quincke electro-rotation instability effect and harness the relaxation dynamics of charge polarization to achieve controlled degrees of correlated random-walks. I show that the time scales during the run and tumbling phases play a major role in establishing different stable collective states reminiscent of bacterial suspensions such as dynamic clusters and vortex arrays. Our findings show the potential for dynamic transitioning between states at constant concentration and activity (speed) of active particles by solely tuning the kinematic time and length scales of individual random walkers.

Center for Fluid Mechanics Seminar Series

Title: Vibrations defy gravity making boats float under a levitating liquid

Abstract: A pendulum can be maintained in an upside-down position when shaken vertically. A similar phenomenon occurs with fluids. When placed over a less dense medium, a liquid layer will typically collapse downwards if it exceeds a certain size, as gravity acting on the lower liquid interface triggers a destabilizing effect called a Rayleigh–Taylor instability. Vertical shaking can stabilize the interface and prevent falling. In liquids, vibrations also induce other paradoxical phenomena on immersed or floating objects. Air bubbles can for instance sink in vibrated fluids and an air layer can be created to support a large liquid layer. Vertical shaking also creates stable buoyancy positions on the lower interface of the liquid, which behave as though the gravitational force were inverted. Bodies can thus float upside down on the lower interface of levitating liquid layers. In my talk, I will discuss these phenomena and show how stabilization is the result of the dynamical averaging effect of the oscillating effective gravity. We will also present experimental observations and a simple model to explain this inverted buoyancy and its characteristics.

Center for Fluid Mechanics Seminar Series

Title: Downshifting of Freely-Propagating, Surface-Gravity Waves
(collaborators: Isabelle Butterfield, John Carter, Harvey Segur)

Abstract: We consider the observed phenomenon of downshifting of freely-propagating, narrow-banded, surface-gravity waves. Downshifting is defined as either a decrease in the spectral peak or as a decrease in the average wavenumber (frequency) of the spectrum during evolution, and our experiments show that both dissipation and nonlinearity are ingredients. We summarize the available models: (i) nonlinear Schrödinger equation (NLS), (ii) dissipative NLS equation (dNLS), (iii) Dysthe equation, (iv) viscous Dysthe equation (vDysthe), (v) Gordon equation (Gordon), which has a free parameter, (vi) Islas-Schober equation (IS), which has a free parameter, and (vii) a new model, the dissipative Gramstad-Trulsen (dGT) equation. Comparisons with experiments show that the vDysthe, dGT, and IS models most accurately predict the observed evolution of the spectral peak and the spectral mean, and the IS and vDysthe models have the smallest overall errors.

Center for Fluid Mechanics Seminar Series

Title: Dynamic buckling in elastic bands and viscous bubble films

Abstract: The shooting of rubber bands and the popping of bubbles may seem like child’s play; however, the underlying dynamics can be subtle, if not counter-intuitive. This talk highlights two research topics within our group: the shape of a rubber band fired from a thumb and the wrinkling pattern that develops when a viscous bubble ruptures. Although the potential applications for these systems are quite distinct, we show experimentally and theoretically that both share an underlying dynamic dictated by inertia, compression, and bending. The competition of these effects leads to two distinct self-similar dynamics for the elastic retraction, as well as wrinkles on a collapsing viscous film that had previously been attributed to the film weight.

Center for Fluid Mechanics Seminar Series

Title: Sperm navigation in complex environments

Abstract: Microorganisms can swim in a variety of environments, interacting with chemicals and other proteins in the fluid. In this talk, we will highlight recent computational methods and results for swimming efficiency and hydrodynamic interactions of swimmers in different fluid environments. Sperm are modeled via a centerline representation where forces are solved for using elastic rod theory. The method of regularized Stokeslets is used to solve the fluid-structure interaction where emergent swimming speeds can be compared to asymptotic analysis. In the case of fluids with extra proteins or cells that may act as friction, swimming speeds may be enhanced and attraction may not occur. We will also highlight how parameter estimation techniques can be utilized to infer fluid and/or swimmer properties.

Center for Fluid Mechanics Seminar Series

Title: Inertial and viscous flywheel sensing of nanoparticles

Abstract: I will present two problems where hydrodynamic interactions between particles and microchannel walls reveal intriguing physics, and enable applications for sensing the physical properties of particles. First, I explore the interplay between inertial and viscous hydrodynamic effects on a particle suspended in fluid between rotating walls. Combining theory and experiments, I harness these effects to do triple measurement of single-particle mass, volume and density. I intuitively explain the fluid-particle interaction. Paradoxically, when viscous effects become dominant over inertia, particle inertia becomes relevant. A viscosity-driven hydrodynamic coupling turns the particle into a ‘viscous flywheel’. Second, I use a similar concept at the nanoscale to measure the DNA cargo of Adeno-Associated viruses (AAV), which are commonly engineered to deliver therapeutic DNA in gene therapy. Through experiments and advection-diffusion simulations of AAV particles, I demonstrate real-time mass measurement of their DNA cargo as a promising tool for quality control in AAV manufacturing.

Center for Fluid Mechanics Seminar Series

Title: Two problems in wall-bounded flow: fluid energy extraction in wind farms, and surfactant effects in superhydrophobic drag reduction

Abstract: In this talk, we consider two fluid problems directly linked to decarbonization efforts. In the first part, we investigate fundamental limits to the performance of large wind farms. Since wind turbines are often deployed in arrays of hundreds of units, wake interactions can lead to drastic losses in power output. Remarkably, while the theoretical “Betz” maximum has long been established for the output of a single turbine, no corresponding theory appears to exist for a generic, large-scale energy extraction system. We develop a model for an array of energy-extracting devices of arbitrary design and layout, first focusing on the fully-developed regime, which is relevant for large wind farms. We validate our model against data from field measurements, experiments and simulations. By defining a suitable ideal limit, we establish an upper bound on the performance of a large wind farm. This is an order of magnitude larger than the output of existing arrays, thus supporting the notion that large performance improvements may be possible.

Title: Synthetic swimmers: microorganism swimming without microorganisms

Abstract: The effect of non Newtonian liquid rheology on the swimming performance of microorganisms is still poorly understood, despite numerous recent studies. In our effort to clarify some aspects of this problem, we have developed a series of magnetic synthetic swimmers that self-propel immersed in a fluid by emulating the swimming strategy of flagellated microorganisms. With these devices, it is possible to control some aspects of the motion with the objective to isolate specific effects. In this talk, recent results on the effects of shear-thinning viscosity and viscoelasticity on the motion of helical swimmers will be presented and discussed. Also, a number of other new uses of the synthetic swimmers will be presented including swimming across gradients, swimming in sand, interactions and rheometry.

The Journal of Fluid Mechanics invites you to the Fluid Mechanics Webinar Series. Organizers: Leeds Institute for Fluid Dynamics, the Department of Applied Mathematics and Theoretical Physics at the University of Cambridge, the UK Fluids Network, and the Journal of Fluid Mechanics.

Brown University’s Baylor Fox-Kemper, Associate Professor of Earth, Environmental and Planetary Sciences, will present a talk: “Affronting Ocean Models: Submesoscale Interactions between Fronts, Instabilities, and Waves” 

New attendees can register for the fluid mechanics webinar series here. Please note that registration closes at 12:00 eastern time on the Thursday prior to the webinar. Zoom links and passwords will be sent prior to the event.

Abstract: Ocean fronts - sharp horizontal gradients in temperature, salinity, and density - are a key feature of the upper ocean that affect the transport of pollutants and the nature of near surface flows. I will highlight some of the recent modeling and theoretical work our group and collaborators have taken on to understand how fronts, frontal instabilities and turbulence, and surface waves interact. Traditional geophysical boundary layer theory neglects horizontal variations, and so is unable to capture frontal dynamics. Some consequences of these features found in large scale modeling and observations of oil, plastics and biological tracer dispersion; boundary layers; fluid energy cycling and dissipation statistics; and finally climate sensitivity will be elucidated.

Scalar excursions in large-eddy simulations
Georgios Matheou
Department of Mechanical Engineering, University of Connecticut
ABSTRACT:
The range of values of scalar fields in turbulent flows is bounded by their boundary values, for passive scalars, and by a combination of boundary values, reaction rates, phase changes, etc., for active scalars. In practice, as a result of numerical artifacts, this fundamental constraint is often violated with scalars exhibiting unphysical excursions. Passive-scalar excursions, a form of numerical model error, are studied in large-eddy simulations (LES) of a shear flow, and methods for diagnosis and assessment of the problem are examined. The analysis of scalar-excursion statistics provides support of the central hypothesis of the study that unphysical scalar excursions in LES result from dispersive errors of the convection-term discretization where the subgrid-scale model (SGS) provides insufficient dissipation to produce a sufficiently smooth scalar field. In the LES runs three parameters are varied: the discretization of the convection terms, the SGS model, and grid resolution. Two types of excursion diagnostics are studied: global excursions, which violate the boundary values of the scalar transport equation; and local excursions, which violate local scalar bounds. Global excursions are analyzed by considering the minimum and maximum in the entire computational domain and the volume of fluid with scalar values exceeding an excursion threshold. Local excursions are primarily used to obtain unphysical scalar-excursion information in the mixed fluid. As expected, unphysical scalar-excursion statistics strongly depend on the SGS model and model parameters. The excursions are significantly reduced when the characteristic SGS scale is set to double the grid spacing in runs with the stretched-vortex model, following similar trends to momentum-transport model error.

Experimental Reaseaches on Interfacial Flow : 1. Double emulsion droplet under high electric field; 2. Tree-inspired pump and actuator
Jinkee Lee, PhD
School of Mechanical Engineering, Sungkyunkwan University,
Suwon, Gyeonggi-do 16419, South Korea
ABSTRACT:
In this presentation, I want to show the research results focus on (1) emulsion under high electric field and (2) development of tree mimicking mechanical devices. (1) We investigate numerically, theoretically, and experimentally how EHD deformation and breakup of double emulsion droplet can occur under DC electric fields. Based on comprehensive experiments, we observe four different breakup modes for double emulsion droplet depending on various physical constraints, i.e. viscosity, conductivity, permittivity, and volume fraction between the core and shell fluid. The breakup modes are classified such as a unidirectional breakup mode, two different bidirectional breakup modes, and tip-streaming continuous breakup mode. We obtained phase diagram to depict the different breakup modes which can contribute to control the emulsion droplet shape. Furthermore, we employed a theoretical study to predict the core droplet migration inside the shell. (2) An artificial leaf mimicking structure using hydrogel, which has a nanoporous structure is fabricated. The cryogel method is used to develop a hierarchy structure on the nano- and microscale in the hydrogel media that is similar to the mesophyll cells and veins of a leaf, respectively. The suction pressure of the artificial leaf is affected by several variables (e.g., pore size, wettability of the structure, nano particle modification). Finally, by decreasing the pore size and increasing the wettability, the maximum negative pressure of the artificial leaf, 7.9 kPa is obtained. Also, We have developed a hygromorphic metallic oxide monolayer film capable of actuation by electrochemically producing superhydrophilic free-standing nano-capillary forest of titanium oxide with an anatase crystal structure of high aspect ratio (~80) nano capillaries. This hygromorphic metallic monolayer is activated by the generation of forces from the spreading and capillary-driven imbibing into nano gaps during hydration and evaporation. This system possesses a great stability and repeatability for long time usage and has a high bending energy density of ~1250 kJ/m3. The results suggest that these hygromorphic structures could possibly exhibit high energy densities and therefore potentially play important roles for external stimuli-responsive materials that are efficient energy converters and actuators.
Biography:
Prof. Jinkee Lee received B.S. and M.S. degrees in Mechanical Engineering from Korea Advanced Institute of Science and Technology (KAIST), Korea in 1997 and 1999, respectively, and Ph.D. degree from Brown University in 2008, where he held the prestigious Simon Ostrach Fellowship. Following his graduate studies, he was a Postdoctoral Research Fellow at jointly in School of Engineering and Applied Science and Department of Organismic and Evolutionary Biology in Harvard University from 2008 to 2009, then moved back to Brown University as an Assistant Professor (Research) in School of Engineering from 2009 to 2011. In 2012, he joined Sungkyunkwan University (SKKU), where he is currently Associate Professor and Director of Multiscale Fluid Mechanics Laboratory in School of Mechanical Engineering.
His research interests is the Interfacial Flow & Transport Phenomena and their Applications falls under the area of Mechanical Engineering, Chemical Engineering, Material Science, Physics and Micro-/Nano-Technologies. He has published 48 peer-reviewed journal articles. He was a recipient of the SKKU Teaching Award 2016 awarded by President of SKKU, which is chosen by evaluating level of contribution, innovation for education and passions for teaching.

New laser-imaging technology elucidates form, function, and ecological impact of deep sea, giant larvacean mucus houses
Dr. Kakani Katija
Research and Development, Monterey Bay Aquarium Research Institute,
Moss Landing, CA, USA
ABSTRACT:
The midwater region of the ocean (below the euphotic zone and above the benthos) is one of the largest ecosystems on our planet, yet remains one of the least explored. Little-known marine organisms that inhabit midwater have developed life strategies that contribute to their evolutionary success, and may inspire engineering solutions for societally relevant challenges. Although significant advances in underwater vehicle technologies have improved access to midwater, small-scale, in situ fluid mechanics measurement methods that seek to quantify the interactions that midwater organisms have with their physical environment are lacking. Here we present DeepPIV, an instrumentation package affixed to a remotely operated vehicle that quantifies fluid motions from the surface of the ocean down to 4000 m depths. Utilizing ambient suspended particulate, fluid-structure interactions are evaluated on a range of marine organisms in midwater (and the benthos). Initial science targets include larvaceans, biological equivalents of flapping flexible foils that create mucus houses to filter food. Little is known about the structure of these mucus houses and the function they play in selectively filtering particles, and these dynamics can serve as particle-mucus models for human health. Using DeepPIV, we reveal the complex structures and flows generated within larvacean mucus houses, and elucidate how these structures function.

A touch of non-linearity at intermediate Reynolds numbers: where spheres “think” collectively and swim together.
Daphne Klotsa
University of North Carolina at Chapel Hill
ABSTRACT:
From crawling cells to orca whales, swimming in nature occurs at different scales. The study of swimming across length scales can shed light onto the biological functions of natural swimmers or inspire the design of artificial swimmers with applications ranging from targeted drug delivery to deep-water explorations. In this talk, I will present experiments and simulations of how oscillating spheres, universally simple geometric objects, can utilize non-linearities to demonstrate complex pattern formation in a granular system, or different swimming behaviors in a spherobot (robot made out of spheres) when placed in a fluid at intermediate Reynolds numbers. I will talk about how a simple swimmer transitions from rest to motility and then switches direction as a function of the Reynolds number.

Locomotion in Generalized Newtonian Fluids: Living Organisms & Active Colloids
Dr. David Gagnon
Georgetown University
ABSTRACT:

Biofilm formation, mammalian reproduction, and infection typically occur in environments where surrounding fluids comprise suspensions of polymers. These polymeric suspensions possess non-Newtonian rheological properties, such as rate-dependent viscosity and viscoelasticity, and present numerous experimental and modeling challenges. Using well-studied polymeric fluids, we aim to systematically investigate the effects of generalized Newtonian (rate-dependent) fluids on locomotion.
In this talk, I will discuss the effect of rate-dependent viscosity on (i) the swimming behavior of the nematode Caenorhabditis elegans and (ii) the rheology of active kinesin-driven microtubule suspensions. First, we investigate the swimming behavior of the low Reynolds number swimmer C. elegans using tracking methods and flow velocity measurements. With knowledge of the local flow behavior, we then address the important question of whether rate-dependent viscosity modifies the nematode’s cost of swimming. We find the cost of swimming in shear-thinning fluids is less than or equal to the cost of swimming in Newtonian fluids of the same zero-shear viscosity; furthermore, the cost of swimming in shear-thinning fluids scales with a fluid’s effective or average viscosity and can be predicted using rheological properties and simple swimming kinematics.
Second, we explore the rheology and dynamics of an active suspension of microtubules and kinesin motors in a dilute polymeric suspension using a confocal rheometer, which provides both rheological measurements and fluorescent imaging of microscale dynamics. We find the activity of microtubules enhances both the zero-shear viscosity and the shear-thinning behavior of the suspension. Using velocimetry techniques, we examine local mesoscale flow dynamics for insight into the underlying mechanisms responsible for this macroscale rheological behavior.

David Lentink, Stanford
Fluids at Brown and Fluids and Thermal Sciences Joint Seminar Series
Tuesday, March 6, 2018
12:00pm
BioMed Center, Eddy Auditorium