Bulletin of the American Physical Society
77th Annual Meeting of the Division of Fluid Dynamics
Sunday–Tuesday, November 24–26, 2024; Salt Lake City, Utah
Session C10: Interact: Active Matter |
Hide Abstracts |
Chair: David Saintillan, University of California, San Diego Room: Ballroom J |
Sunday, November 24, 2024 10:50AM - 11:20AM |
C10.00001: INTERACT FLASH TALKS: Active Matter Each Interact Flash Talk will last around 1 minute, followed by around 30 seconds of transition time. |
|
C10.00002: Bacterial Barriers: How bacterial activity enhances transport barriers in chaotic flows Paulo E. Arratia, Ranjiangshang Ran In thisstalk, we will discuss the effects of bacterial activity on the mixing and transport properties of a passive scalar in time-periodic flows in experiments and in a simple model. We focus on the interactions between swimming Escherichia coli and the Lagrangian coherent structures (LCSs) of the flow, which are computed from experimentally measured velocity fields. Experiments show that such interactions are non-trivial and can lead to transport barriers through which the scalar flux is significantly reduced. Using the Poincaré map, we show that these transport barriers coincide with the outermost members of elliptic LCSs known as Lagrangian vortex boundaries. Numerical simulations further show that elliptic LCSs can repel elongated swimmers and lead to swimmer depletion within Lagrangian coherent vortices. A simple mechanism shows that such depletion is due to the preferential alignment of elongated swimmers with the tangents of elliptic LCSs. Our results provide insights into understanding the transport of micro-organisms in complex flows with dynamical topological features from a Lagrangian viewpoint. |
|
C10.00003: Instability of an active droplet in a Hele-Shaw geometry Federico Cao, Bennett C Sessa, Guillaume Duclos, Robert Alan Pelcovits, Thomas R Powers Our experiments have uncovered a fingering-like instability for active droplets surrounded by a viscous, passive fluid in a Hele-Shaw geometry. In these experiments, a microtubule-driven active liquid is embedded in a binary, polymeric mixture that phase separates. Light-activated ATP is used to control the onset of the instability. The interface becomes unstable once a critical activity and droplet size are reached. |
|
C10.00004: Deformable bodies in active nematics Thomas Graham John Chandler, Saverio Eric Spagnolie Fluid anisotropy can be observed in biofluids like mucus or, at a larger scale, self-aligning swarms of bacteria. A model fluid used to investigate such environments is a nematic liquid crystal. Large deformable bodies tend to be stretched when immersed in these complex environments, offering a passive means of measuring cell material properties. Active stresses in these systems can also generate flows, either due to molecular activity or active bodies suspended within them. We will discuss a complex variables approach to analytically solve for the activity-induced flow of a liquid crystal with immersed bodies. We will demonstrate that the activity-induced stresses on the surface of the bodies compete with the elastic stresses from the passive liquid crystal, putting the shape of deformable bodies in the balance. Anchoring conditions, topological defects, and the geometry and composition of the bodies will all considered. |
|
C10.00005: Manifolds, swimming microbes and propagating fronts in a vortex array flow Thomas H Solomon, Matthew C D'Andrea, Gwynne K Aull, Kevin A Mitchell We present experiments that measure the speeds of moving fronts and swimming organisms in a two-dimensional, vortex array flow. The fronts in these experiments are produced by the excitable Belousov-Zhabotinsky (BZ) reaction, and the swimming organisms are either tumbling brine shrimp nauplii or an active dye composed of swimming Tetraselmis microbes. The motion of the reaction fronts is determined by burning invariant manifolds (BIMs) that have been previously shown to act as one-way barriers. These BIMs serve to roughen the front, with rougher, more folded fronts (with larger perimeter Lf) for smaller non-dimensional propagation speed V0/U, where V0 is the speed at which fronts move in the absence of a flow and U is the maximum flow speed. We test predictions that the coarsened front speed scales linearly with the roughness Lf/La where La is the circumference of a circle with the same overall area as the roughened front. Swimming organisms in the same flow are also bound by the same BIMs, even if the organism is tumbling. |
|
C10.00006: Deformation and migration of a two-dimensional droplet enclosing active particles Sho Kawakami, Petia M. Vlahovska Active particles enclosed inside of a droplet or a vesicle induce shape fluctuations [1,2] and in some cases cause spontaneous motion of the “soft container” [2]. In the experiment [2], the droplet is quasi two-dimensional (2D). To understand the propulsion mechanism, we derive and analyze the dynamics of a 2D droplet enclosing active particles modeled as Stokes-flow singularities. We derive an analytical solution for the fluid flow and drop deformation driven by one singularity in the asymptotic limit of small shape distortions. Boundary integral method is implemented to explore the scenario of large drop shape deformations and many enclosed particles. We compare the motion of the enclosed active particle and the drop dynamics in 2D with the previous analysis in 3D [3]. |
|
C10.00007: Active flows in dense suspensions of motile colloids Rui Luo, Alexey Snezhko, Petia M. Vlahovska Dense bacterial suspensions generate a distinct turbulent-like motion at low Reynolds numbers, energized by the microswimmers’ activity. The energy spectrum of bacterial turbulence is predicted to scale with the wavenumber k as E ∼ k^(−8/3), which contrasts the E ∼ k^(−5/3) observed in classical inertial turbulence, and reflects an energy flow from micro to macro scales. |
|
C10.00008: Microtubule-based active nematics and the necessity of chaotic flows Kevin A Mitchell, Mainul Sabbir, Brandon Klein, Daniel A Beller Active nematics are fluids with local orientational order and an internal energy source driving fluid motion. Examples include dense swarms of motile bacteria, generating coherent flows, and dense 2D layers of rod-like microtubules cross-linked by molecular motors powered by ATP. We focus on the latter, microtubule system, for which experimental flows always exhibit chaotic advection. This is not the case for general active nematics. Theoretical models of the microtubule system can generate both chaotic flows and static flows that are not chaotic. We seek an explanation for why experiments always exhibit chaos. To this end, we propose a theoretical principle that guarantees chaotic advection. In particular, under reasonable physical assumptions we find that the Lyapunov exponent of the flow must be greater than the rate at which kinematic energy is dissipated by the system. This implies a strictly positive Lyapunov exponent and hence chaotic advection. The same argument does not hold for bacterial systems, which may explain why they can have non chaotic experimental states. |
|
C10.00009: Topology controls flow patterns in active double emulsions Giuseppe Negro, Davide Marenduzzo, Tyler N Shendruk, Livio N Carenza, Louise C Head, Adriano Tiribocchi, Giuseppe Gonnella Active emulsions and liquid crystalline shells are intriguing and experimentally realisable types of topological matter. Here we numerically study the morphology and spatiotemporal dynamics of a double emulsion, where one or two passive small droplets are embedded in a larger active droplet. We find activity introduces a variety of rich and nontrivial nonequilibrium states in the system. First, a double emulsion with a single active droplet becomes self-motile, and there is a transition between translational and rotational motion: both of these regimes remain defect-free, hence topologically trivial. |
|
C10.00010: Bacterial chemotaxis to a finite localized source Meera Ramaswamy, Ipsita Tingi, Jenna Anne Ott, Yaxin Duan, Sujit S Datta Chemotaxis, the ability to sense and respond to chemical gradients, enables microorganisms to forage for food, colonize new environments, and locate hosts and symbionts. This phenomenon occurs across various length scales, environments, and cell concentrations, from oceans to the human body. Previous research has examined factors like environmental porosity, fluid flow, and cell concentration, often focusing on dense bacterial populations responding to abundant, global attractant sources. However, the response to patchy attractant sources remains less understood. In this study, we conduct experiments and simulations to investigate bacterial chemotaxis in a porous environment with a local, ephemeral attractant source. We find that cell density and attractant size influence whether bacteria accumulate or form chemotactic bands. These findings enhance our understanding of chemotaxis in real-world scenarios and offer valuable tools for engineering bacterial suspensions in bioremediation applications. |
|
C10.00011: Worm drop experiment : Fluid like behavior of tangled worm assemblages Paulami Sarkar, Ishant Tiwari, Prathyusha K. R. PhD, Saad Bhamla
|
|
C10.00012: Bioconvection in the wild Oscar Sepulveda Steiner, Damien Bouffard, Tobias Sommer, Alfred Wüest, George Constantinescu, Nicola Storelli, Hugo N Ulloa Bioconvection is an intriguing example of collective biological activity in which microorganisms drive hydrodynamic processes that modify their host fluid environment. This phenomenon has been thoroughly documented in the laboratory. Here, we provide an example of its occurrence in a natural system. A community of motile and heavy, purple sulfur bacteria –Chromatium okenii– drive bioconvection in the permanently stratified Lake Cadagno (Switzerland). Measurements of the lake's physical, chemical and biological properties and a mathematical model of the bacteria's vertical migration provide compelling proof of concept. The lake exhibits a chemical gradient zone separating an oxygen-rich upper layer (top ~10 m) from anoxic and sulfide-rich deep waters. This environment offers an optimal niche for bacteria to accumulate at the uppermost anoxic zone, where also light and sulfide are available to perform anoxygenic photosynthesis – their primary metabolic process. The collective behavior driving bioconvection consists of bacteria vertically migrating upward towards light and stopping at the oxic-anoxic transition. This leads to a locally unstable density excess that fuels convection and mixing, demonstrating the ability of bacteria-inducing bioconvection to modify their surroundings. |
|
C10.00013: Periodic Defect Trajectories in Confined Active Nematic Flows Spencer A Smith, Kevin A Mitchell In active matter systems, energy consumed at the small scale by individual agents gives rise to emergent flows at large scales. For 2D active nematic microtubule systems these flows are largely characterized by the dynamics of mobile defects in the nematic director field. As these defects wind about each other, their trajectories trace out braids, and the topological properties of these braids encode the most important global features of the flow. In bulk flow, defect motion is chaotic, however recent work has shown that confining the active nematic systems - via boundary geometry or the surface topology - leads to periodic defect braiding. Furthermore, the braids that result from confinement are special and appear to maximize a measure of topological entropy. Using recent advances in braid theory, we compare newly discovered maximal mixing braids to the emergent trajectories of active nematic defects found in experiments and simulations. |
|
C10.00014: Mechanisms of self-organization and aggregation of active particles in viscous membranes Fizza Usmani, Harishankar Manikantan Eukaryotic cell membranes are a crowded assembly of various biomolecular machines embedded in a viscous bilayer matrix. Often, such motors are active — they convert chemical energy into mechanical work and generate hydrodynamic disturbances in the surrounding medium. We investigate the collective behavior of such inclusions in viscous membranes surrounded by a shallow subphase ('confined' membranes). Using our computational platform based on a point-particle approach, we conduct large scale simulations to study the organization of the inclusions into either clusters and/or nematic strings. We quantify pair distribution functions and tie them to disturbance fields around a particle in the plane of the membrane to demonstrate the amplifying role of the surrounding 3D fluid in promoting strings or clusters. We then examine the stability of the string-like structures to demonstrate the role of membrane versus subphase hydrodynamics. We also illustrate the stability of triangular locked clusters that emerge upon increasing confinement or – equivalently – upon increasing subphase viscous stresses, which it emerges is the local arrangement that promotes larger system-spanning clusters. Put together, these results provide valuable insight into how geometric or fluidic parameters can be tuned to achieve desirable collective behavior within biological and biomimetic membranes. |
|
C10.00015: Active Healing of Microtubule-Motor Networks Fan Yang, Shichen Liu, Heun Jin Lee, Rob Phillips, Matt Thomson Cytoskeletal networks have a self-healing property where networks can repair defects to maintain structural integrity. However, both the mechanisms and dynamics of healing remain largely unknown. In this talk we report an unexplored healing mechanism in microtubule-motor networks by active crosslinking. We directly generate network cracks using a light-controlled microtubule-motor system, and observe that the cracks can self-heal. Combining theory and experiment, we find that the networks must overcome internal elastic resistance in order to heal cracks, giving rise to a bifurcation of dynamics dependent on the initial opening angle of the crack: the crack heals below a critical angle and opens up at larger angles. Simulation of a continuum model reproduces the bifurcation dynamics, revealing the importance of a boundary layer where free motors and microtubules can actively crosslink and thereby heal the crack. We also formulate a simple elastic-rod model that can qualitatively predict the critical angle, which is found to be tunable by two dimensionless geometric parameters, the ratio of the boundary layer and network width, and the aspect ratio of the network. Our results provide a new framework for understanding healing in cytoskeletal networks and designing self-healable biomaterials. |
|
C10.00016: The motion of a circular object in a confined active suspension with wall-constrained nematic ordering Min Zhu, Jonathan Ben Freund The effect of wall-constrained nematic ordering on neutrally buoyant suspended object in an active nematic fluid within a closed circular container is studied with a simulation model. The active nematic fluid is described using the continnum model of Gao et al. (Phys. Rev. Fluids, 2017), which includes a slender-body strain response of rod-like agents, Maier-Saupe steric interaction, and a Bingham closure for fourth-moments of the orientation. Dirichlet boundary condition enforces orientation and alignment of the D field on both the object and container. This study considers apolar extensors. When both the object and the container have radial nematic ordering, above a critical activity, a stable limit-cycle solution is observed in which the object moves around the center in a circular trajectory. Below a critical activity level, the container center becomes a stable fixed point. When the nematic ordering of the container is changed to comet-type, which is similar to the director field of a +0.5 defect, the system shows two stable symmetric fixed points above a critical activity. For the activity just below the critical activity, two stable periodic trajectories appear and they are also symmetric to each other. As the activity decreases further, there exists only one symmetric periodic trajectory. In sum, these results show how wall-constrained nematic ordering causes the object into interesting behavior, potentially useful from the perspective of biological tasks. |
|
C10.00017: Squirmers with arbitrary shape and slip: modeling, simulation, and optimization Hai Zhu, Kausik Das, Marc Bonnet, Shravan Veerapaneni We consider arbitrary-shaped microswimmers of spherical topology and propose a framework for expressing their slip velocity in terms of tangential basis functions defined on the boundary of the swimmer. Given a microswimmer shape, we solve six Dirichlet boundary value problems and, exploiting the reciprocal theorem, show that their solution can be used to construct rigid body velocities for any prescribed time-independent slip profile. Moreover, we derive an analytical expression for the periodic motion of an isolated microswimmer suspended in free space. For a given swimmer shape, we then investigate which slip profile maximizes swimming efficiency. A six-dimensional eigenvalue problem is shown to encode the solution to this optimization problem, which can be solved easily. We showcase and analyze the slip profiles thus obtained for various shape families. |
|
C10.00018: Flow-mediated bacterial swimming near curved surfaces Akash Ganesh, Mobin Alipour, Amir A Pahlavan Swimming allows the bacteria to explore their environment in search for nutrients and new habitats. Near surfaces, hydrodynamic and steric interactions as well as flow shear modulate the bacterial swimming dynamics. These changes have important implications for their success in colonizing the surfaces. Here, we use microfluidic experiments to probe the influence of motility and surface curvature on the bacterial behavior. We discuss the implications of our findings for biological settings involving flows such as bacterial colonization of sedimenting marine snow in the oceans and nutrient hotspots in soils. |
|
C10.00019: Swimming near Deformable Membranes Adam Hitin Bialus, Bhargav Rallabandi, Naomi Oppenheimer Active particles often do not swim in a bare, infinite fluid but interact with the soft surfaces surrounding them, as seen in cases like active proteins or bacteria moving near a biomembrane. These particles generate flow fields that couple to the membrane, leading to non-trivial forces. In this work, we use the Lorentz reciprocal theorem and the method of images to derive an analytical expression for the hydroelastic force resulting from the coupling between an active particle, modeled as a symmetric force dipole, and a nearby elastic, bendable membrane. Compared to the repulsive force found near a rigid interface, we discover that in the limit of small deformations, the bending-induced force is opposite in sign and becomes attractive. Furthermore, the interaction strength increases quadratically with the dipole moment. |
|
C10.00020: Autophoretic skating along permeable surfaces Günther Turk, Rajesh Singh, Howard A Stone Janus particles are self-propelled colloidal locomotors that convert chemical energy into mechanical motion. Their asymmetric catalytic coating mediates chemical reactions in the surrounding fluid. The resulting solute concentration gradients drive slip flows along the particle surface, generating self-propulsion through diffusiophoresis. Via an interplay of chemical and hydrodynamic interactions, phoretic particles are known to swim along solid surfaces. In our analytical study we generalize this, investigating boundary guidance along a chemically permeable surface of two immiscible liquids, constituting a plausible model for autophoretic skating on biofilms or hydrogels. We also consider how particle-particle interactions affect the dynamics of multiple skaters. This opens exciting possibilities for applications in microfluidics and targeted drug delivery, where collective navigation through complex microscale environments is crucial. |
Sunday, November 24, 2024 11:20AM - 12:50PM |
C10.00021: INTERACT DISCUSSION SESSION WITH POSTERS: Active Matter After each Flash Talk has concluded, the Interact session will be followed by interactive poster or e-poster presentations, with plenty of time for one-on-one and small group discussions. |
|
C10.00022: Abstract Withdrawn |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2025 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700