Bulletin of the American Physical Society
APS March Meeting 2024
Monday–Friday, March 4–8, 2024; Minneapolis & Virtual
Session T31: Phase Separation in Active MatterFocus
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Sponsoring Units: GSNP DSOFT Chair: Daphne Klotsa, UNC Chapel Hill; Corey O'Hern, Yale University Room: 102C |
Thursday, March 7, 2024 11:30AM - 12:06PM |
T31.00001: Biofilm formation in confined environments Invited Speaker: Amir Pahlavan Biofilms are microbial communities that develop when free-swimming (planktonic) bacteria transition to a non-motile, surface-attached state, sharing similarities with motility-induced phase separation. This transition is modulated by environmental factors such as nutrient availability, chemical and mechanical stresses, as well as collective response of bacteria through quorom sensing. Despite the inherent heterogeneity in their natural environments, biofilm formation has often been studied in unconfined settings. Here, using microfluidic experiments, we study the influence of confinement on the spatiotemporal evolution of biofilm formation and growth. Our observations and theoretical modeling demonstrate that the interplay between bacterial growth and self-generated nutrient gradients can modulate the spatial patterning of biofilms in confinement, pointing to the importance of geometric effects on biofilm formation and growth in heterogeneous environments. |
Thursday, March 7, 2024 12:06PM - 12:18PM |
T31.00002: Softening and Enhanced Transport of Colloidal Chains in a Bacterial Bath Devadyouti Das, Bipul Biswas, Manasa Kandula, Shuang Zhou Semi-flexible filaments immersed in an active environment are ubiquitous in biological systems. Therefore, understanding their dynamics is of fundamental importance to both biology and physics. Recent theoretical and simulation studies of semi-flexible filaments in an active bath predict a rich diversity of phenomena such as activity-induced collapse and enhanced diffusion. However, there is a notable lack of experimental investigations on this topic. To address this issue, we construct a model system consisting of colloidal chains and the motile bacteria Bacillus subtilis. We experimentally study the non-equilibrium dynamics of the chains by controlling the concentration and the activity of the bacteria and the rigidity of the chains. We observe 'softening' of the colloidal chains: the effective persistence length of the chains is reduced by over an order of magnitude. Consequently, we see a variety of conformations adopted by the colloidal chains which we quantitatively describe using shape parameters including radius of gyration and acylindricity. We also observe enhanced diffusion of the centers of mass of the colloidal chains. We attribute both effects to the large-scale, structured flow in the active bath. |
Thursday, March 7, 2024 12:18PM - 12:30PM |
T31.00003: Effects of hydrodynamic interactions in Motility-Induced Phase Separations. (Edmond) Tingtao Zhou, John F Brady Hydrodynamic interactions (HI) strongly influence the collective behavior of microswimmers, such as motility-induced phase separation (MIPS). We systematically study the collective dynamics of so-called squirmers using Active Fast Stokesian Dynamics simulations. We focus on ``stealth'' swimmers with 2D orientations but with fully 3D flow fields. We discuss different swimming modes, or ``hydrodynamic ensembles'', such as fixed swim force or fixed swim velocity, and show that the collective behaviors depend not only on the squirming modes but also strongly on both the hydrodynamic ensemble and the boundary conditions for the flow in the system. Specifically, we show that ``stealth'' swimmers at fixed swim velocity only exhibit crystalline clusters when the flow is confined by either no-slip walls or stress-free liquid-liquid interfaces. These results help to reconcile previous theoretical and numerical investigations. From the simulations, we assemble a comprehensive phase diagram of active squirmers. |
Thursday, March 7, 2024 12:30PM - 12:42PM |
T31.00004: Motility Induced Phase Separation and Frustration in Active Matter Swarmalators Charles M Reichhardt, Cynthia Reichhardt, Andras Libal, Biborka Adorjani We introduce a system of active matter swarmalators composed of elastically interacting run-and-tumble active disks with an internal phase ϕi. The disks experience an additional attractive or repulsive force with neighboring disks depending upon their relative difference in ϕi. In the absence of the internal phase, the system forms a Motility-Induced Phase Separated (MIPS) state, but when the swarmalator interactions are present, a wide variety of other active phases appear depending upon whether the interaction is attractive or repulsive and whether the particles act to synchronize or anti-synchronize their internal phase values. These include a gas-free gel regime, arrested clusters, a labyrinthine state, a regular MIPS state, a frustrated MIPS state for attractive anti-synchronization, and a superlattice MIPS state for attractive synchronization. |
Thursday, March 7, 2024 12:42PM - 12:54PM |
T31.00005: Collective behavior of active cell-like "flexicle" microrobots Philipp Schönhöfer, Sharon C Glotzer In recent years the distinction between the functionality of synthetic active microparticles and of their biological counterparts has become increasingly blurred. However, we still lack the fundamental understanding needed to recreate the key facets of autonomous behavior exhibited by microorganisms or macroscopic robots comprised of colloidal particles. In this study, we propose a model for a three-dimensional self-driven, deformable cellular robot composed of self-propelled particles confined to a flexible membrane - a superstructure we call a “flexicle”. Using molecular dynamics simulations, we investigate the collective behavior of dense systems comprised of many flexicles, as well as the behavior of individual flexicles within the collective. We show that individual flexicles exhibit intricate shape changes depending on the membrane’s bending modulus. These differences in shape deformability give rise to a diverse set of motility-induced phase separation phenomena and the spontaneous flow of flexicles, akin to the migration of cells. Our findings establish a foundation for controlling the migration of cell-like active particles and for developing strategies for achieving other autonomous robotic swarm behaviors. |
Thursday, March 7, 2024 12:54PM - 1:06PM |
T31.00006: Characterizing Phase Separation and Critical Behavior in Active Matter with Machine Learning and Noise Power Cynthia Reichhardt, Danielle M McDermott, Charles M Reichhardt We examine motility-induced phase separation (MIPS) in two-dimensional run and tumble disk systems using both machine learning and noise fluctuation analysis. Our measures suggest that within the MIPS state there are several distinct regimes as a function of density and run time, so that systems with MIPS transitions exhibit an active fluid, an active crystal, and a critical regime. The different regimes can be detected by combining an order parameter extracted from principal component analysis with a cluster stability measurement. The principal component-derived order parameter is maximized in the critical regime, remains low in the active fluid, and has an intermediate value in the active crystal regime. We demonstrate that machine learning can better capture dynamical properties of the MIPS regimes compared to more standard structural measures such as the maximum cluster size. The different regimes can also be characterized via changes in the noise power of the fluctuations in the average speed. In the critical regime, the noise power passes through a maximum and has a broad spectrum with a 1/f1.6 signature, similar to the noise observed near depinning transitions or for solids undergoing plastic deformation. |
Thursday, March 7, 2024 1:06PM - 1:18PM |
T31.00007: Thermodynamics of active phase separation and patterns Massimiliano Esposito, Timur Aslyamov, Franceco Avanzini, Étienne Fodor I will present a nonequilibrium thermodynamics for non-ideal mixtures undergoing reaction-diffusion dynamics. I will use it to study the energetic cost of sustaining spatial organization in the form of phase separations and patterns. The nature of the instabilities in these systems will also be discussed (PRL 131, 138301). |
Thursday, March 7, 2024 1:18PM - 1:30PM |
T31.00008: Emergence of collective oscillations in dense crowds Francois Gu, Benjamin Guiselin, Nicolas Bain, Iker Zuriguel, Denis Bartolo Building upon a combination of quantitative observations and theoretical insights, we unveil and elucidate the emergence of collective oscillations within densely packed pedestrian crowds. |
Thursday, March 7, 2024 1:30PM - 1:42PM |
T31.00009: Thermodynamically consistent lattice Monte Carlo simulation for active particle system Ki-Won Kim, Euijoon Kwon, Yongjoo Baek Active particles are known to exhibit various novel collective phenomena, such as motility-induced phase separation and current rectification. Recently, there has been a surge of interest in identifying the role of energy dissipation in maintaining such large-scale nonequilibrium structures. Since the models of active particles are typically very challenging to solve analytically, developing an efficient numerical method for investigating their properties is a task of great importance. |
Thursday, March 7, 2024 1:42PM - 1:54PM |
T31.00010: Collective flow and defect dynamics of active nematic liquid crystals under an electric field Yutaka Kinoshita, Nariya Uchida Active nematic liquid crystals have attracted much attention as a model of mesoscale active turbulence in microtubule-kinesin suspensions and other biological systems. The chaotic flow is controlled by confinement and friction as has been demonstrated both experimentally and theoretically. An external electric field can be also used to manipulate the collective dynamics by inducing reorientation of the active elements. Here we numerically demonstrate the relatively unexplored effects of an electric field on the dynamics of two-dimensional active nematics [1]. We found transitions among three states, which are characterized by different degrees of flow anisotropy: the active turbulence, laning state, and uniformly aligned state. The average flow speed and its anisotropy are maximized in the laning state. We also found the localization of topological defects associated with the simultaneous creation and annihilation of two pairs of defects. It results in periodic oscillations between the active turbulence and laning state, which partly resemble the ones experimentally observed in a friction-controlled system. Our results provide insights into the way of controlling the flow patterns of active nematics with external fields. |
Thursday, March 7, 2024 1:54PM - 2:06PM |
T31.00011: Defect self-propulsion in active nematic films with spatially-varying activity Jonas Rønning, M Cristina Marchetti, Mark J Bowick, Luiza Angheluta We study the active flow caused by an isolated topological defect of charge ± 1/2 in an active nematic film subject to both friction with a substrate (Γ) and viscus dissipation (η). The two dissipation mechanisms give rise to a length scale ld= (η/Γ)1/2, which sets the scale for the self-propulsion velocity of the +1/2 defect and the scale for the decay of the flow velocity. When confined to a disc, spanning vortices of alternating vorticity are formed around the defects, and the self-propulsion velocity becomes determined by the ratio between the disc's radius R and ld. For small discs the self-propulsion velocity is proportional to R, while it is set by the dissipation length for large discs. |
Thursday, March 7, 2024 2:06PM - 2:18PM |
T31.00012: Agent-based simulation study of confined active nematic filaments Matthew J Deutsch, Michael P Varga, Robin L Selinger To model self-organized dynamics in a confined active nematic, we perform agent-based simulations of flexible filaments driven by inter-filament shear propulsion. Each filament is represented as an elastic bead-spring chain. Inter-filament forces mimic the action produced by ATP-powered kinesin motor clusters and produce extensile stresses like those observed in experiment (see, e.g. Opathalge et al, PNAS 2018). Depletion forces between nearby filaments are introduced to drive bundle formation. As a novel way to introduce hydrodynamic forces, we model interaction with a background fluid represented as a coarse-grained liquid sublayer. Active filaments are thermostatted only via interaction with the fluid sublayer, which is subject to a momentum-conserving thermostat. Adjustable model parameters include: inter-filament active driving force, driving force against the confining wall, filament bead-spring stretch and bend elastic parameters, fluid interaction intensity, temperature and dissipation parameters of the thermostat, filament and fluid sublayer densities, and depletion forces. We consider confinement in both disk-shaped and more complex confinement geometries, and examine resulting topological defect trajectories and flow patterns. The confining boundary is represented by an assembly of immobile particles with short-range, repulsive interactions, allowing us to model confining geometries of arbitrary shape. We compare results to relevant experiments. |
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