Bulletin of the American Physical Society
75th Annual Meeting of the Division of Fluid Dynamics
Volume 67, Number 19
Sunday–Tuesday, November 20–22, 2022; Indiana Convention Center, Indianapolis, Indiana.
Session Z04: Active Matter IV: Active Gels, Drops, and Particles |
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Chair: Heeseok Koo Room: 131 |
Tuesday, November 22, 2022 12:50PM - 1:03PM |
Z04.00001: Self-propulsion of a rigid shape in active gels and active nematics Thomas R Powers, Pranay Sampat, Wan Luo, Robert A Pelcovits Since active liquids flow spontaneously, it is natural to ask if a passive rigid object can propel itself using the forces of a surrounding active liquid. We present simple theoretical arguments showing that a Janus particle in an active nematic or an active gel moves spontaneously. For example, imposing boundary conditions that favor parallel alignment of the active nematic rods on one side of the particle and that favor perpendicular (homeotropic) alignment on the other side leads to a net active force on the particle, and propulsion. Likewise, imposing a weak homeotropic boundary condition on only one side of a particle in active gel also leads to net active force on the particle. In both cases, the propulsion mechanism is analogous to the way that Marangoni forces propel a camphor boat on a water surface. |
Tuesday, November 22, 2022 1:03PM - 1:16PM |
Z04.00002: Activity driven droplet shape fluctuations and motility Sho Kawakami, Petia M Vlahovska The presence of active particles, such as bacteria or motile colloids, within a soft confinement changes both the collective behaviors of the particles and the shape dynamics of the confinement. We model and simulate active particles encapsulated in 2D droplet, whose boundary is modeled as a ring of spring connected beads. We explore the behavior of both the active particles and the confinement to identify their effects on each other. The collision of the active particles with the droplet boundary and their hydrodynamic interactions lead to the formation of a collective vortex motion inside the droplet as observed in experiments [1]. The power spectrum of the amplitude of the shape fluctuations, induced by the collision of the active particles, show a power spectrum decay of q^{-4}, which differs from the q^{-2} decay expected from purely thermal surface tension controlled fluctuations. The addition of random reorientation to mimic run and tumble bacteria, shows the active particles no longer form a vortex inside the confinement, although the power spectrum remains unchanged. The particle activity also gives rise to super-diffusive net motion of the droplet. |
Tuesday, November 22, 2022 1:16PM - 1:29PM |
Z04.00003: Active control of active fluids S.Arman Ghaffarizadeh, Gerald J Wang Active matter systems feature discrete particles that can self-propel by converting stored or ambient free energy into motion. This mechanism results in an array of novel structural and transport phenomena that are not observed in their equilibrium counterparts. Informed by our recent work on excess entropy scaling relationships for a simple model of active fluids, in this work, we investigate strategies for driving an active fluid to a particular non-equilibrium state in which the fluid has a specified value for its self-diffusion coefficient. These strategies leverage the degree of activity of active particles as the key control parameter. We calibrate and refine our strategies within a testbed of automated molecular-dynamics simulations, with variations in model for and extent of activity, volume fraction of active particles, and interaction between inactive and active particles. We also briefly discuss strategies for accelerated sampling of transport coefficients, which aid in our efforts to achieve on-the-fly control of fluid transport properties. |
Tuesday, November 22, 2022 1:29PM - 1:42PM |
Z04.00004: To turn or not to turn: Slender body analysis for a self-propelling axially asymmetric bent rod Arkava Ganguly, Ankur Gupta Synthetic microswimmers have gained attention due to their ability to mimic biological motion. However, the interplay between phoretic and geometric effects isn’t fully understood for axially asymmetric particles. To this end, we investigate the self-diffusiophoretic motion of a two-dimensional bent rod by employing slender body theory. |
Tuesday, November 22, 2022 1:42PM - 1:55PM |
Z04.00005: Collective behavior of Janus Particles Suspended in a Viscous Fluid Szu-Pei Fu, Rolf J Ryham, Bryan Quaife, Yuan-Nan Young Active colloidal systems with non-equilibrium self-organization is a long-standing challenge in biology. To understand how hydrodynamic flow may be used to actively control self-assembly of Janus particles (JPs), we use a model recently developed for the many-body hydrodynamics of amphiphilic JPs under a viscous background flow (JFM, 941, 2022) to investigate how various bilayer structures arise from tuning the hydrophobicity/hydrophilicity of the JPs. Focusing on three distributions of hydrophobicity we found JPs may assemble into uni-lamella, multi-lamella, and striated structures in a viscous fluid. Under a linear flow and a Taylor-Green mixing flow, we use three measures to quantify the collective dynamics of JP particles under a background flow: (a) Free energy from the hydrophobic interactions between the JPs, (b) an order parameter for the ordering of JPs in terms of alignment of their directors, and (c) a strain parameter that captures the deformation in the assembly. We found the dynamics of these three measures correlate well with the hydrodynamics of JPs. These numericals provide insights into dynamic control of non-equilibrium active biological systems with similar self-organization. |
Tuesday, November 22, 2022 1:55PM - 2:08PM |
Z04.00006: Data-driven continuum modeling of active nematics via sparse identification of nonlinear dynamics Connor Robertson, Anand U Oza, Travis Askham Data-driven modeling methods have recently shown great potential in determining accurate continuum models for complex systems directly from experimental measurements. One such complex system is the active nematic liquid crystal system consisting of microtubule-motor protein assemblies immersed in a fluid. This system exhibits rich non-equilibrium behavior, including spontaneous creation and annihilation of topological defects. Although several models have been proposed for the system, the governing equations remain under debate. |
Tuesday, November 22, 2022 2:08PM - 2:21PM |
Z04.00007: The role of the Marangoni effect in the formation of rotational equilibria in the system of hydrodynamically coupled micro-swimmers. Prajitha Mottammal, Sumesh P Thampi, Andrey Pototsky We study the motion of hydrodynamically coupled identical and non-identical micro-swimmers, in the bulk of a viscous fluid and at a stress-free liquid-air interface. Each swimmer is modeled in the far-field as a pusher or a puller with an intrinsic self-propulsion. Surfactant released by living bacteria such as Bacillus subtilis, Flavobacteria, Pseudomonas Aeruginosa, etc., alters the surface tension of the liquid-air interface thus generating the Marangoni flow in the surrounding fluid. Two identical pushers moving along a liquid-air interface may form an unstable rotational equilibrium orbit, whereby each pusher follows a circular path. The presence of the Marangoni flow is shown to stabilize the orbit, leading to the formation of three different types of rotational equilibria. For a pair of non-identical pushers or pullers in the absence of the Marangoni flow, we found circular and quasi-periodic localized states associated with the motion on a 2D torus. All planar equilibria are shown to be unstable with respect to three-dimensional perturbations. Finally, in the case of four identical pushers in the absence of the Marangoni flow, we observe a rich dynamical behavior, including the formation of two circular orbits with a slow drift and subsequent destruction. |
Tuesday, November 22, 2022 2:21PM - 2:34PM |
Z04.00008: Collective dynamics of squirmers with hydrophobicity Yuan-Nan Young, Bryan Quaife, Szu-Pei Fu, Rolf J Ryham Using a model recently developed for the many-body hydrodynamics of amphiphilic JPs suspended in a viscous background flow (JFM, 941, 2022), we investigate how various swimming dynamics of squirmers that interact with the solvent through a hydrophobic potential (HP) may vary from tuning the hydrophobicity/hydrophilicity of the squirmers. In the absence of such HP, several configurations of squirmers are known to be stable for squirmers to swim together. We numerically investigate how HP may help stabilize/destabilize these configurations. These results are further compared with the Vicek model for schooling and flocking of swimmers. We further investigate how active control of HP may be used for a cluster of swimmers to swim in specific fashions, indicating that the squirmers may actively change their surface properties so the collective of squirmers may swim in certain ways. |
Tuesday, November 22, 2022 2:34PM - 2:47PM |
Z04.00009: Transient behavior of a Marangoni swimmer Abhradeep Maitra, Jenna Rotheram, Anupam Pandey, Sunghwan Jung
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Tuesday, November 22, 2022 2:47PM - 3:00PM Author not Attending |
Z04.00010: Stiffening of an active elastic solid Mario Sandoval This work deals with the mechanical properties of an active elastic solid defined as a two-dimensional network of active stochastic particles interacting by nonlinear hard springs. It is numerically found that when activity in the system is turned on, the active solid stiffens. Interestingly, the active forces individually acting along the solid are stochastic; thus no preferred direction is imposed. This effect could be potentially used to construct novel active materials whose mechanical properties could be tuned according to their needs. |
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