Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session G08: Statistics of Active MatterRecordings Available
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Sponsoring Units: GSNP Chair: Dani Bassett, University of Pennsylvania Room: McCormick Place W-179B |
Tuesday, March 15, 2022 11:30AM - 11:42AM |
G08.00001: Backing Yourself into a Corner: Self-avoidant Memory Leads to Self-trapping of Swimming Droplets Katherine Daftari, Katherine A Newhall Microscale swimming droplets have been shown experimentally to respond to local, self-produced chemical gradients that mediate self-avoidance or self-attraction. Via this mechanism, we investigate a physically-inspired stochastic model constructed to encode this observed self-avoidant memory. Surprisingly, we find that the enhanced diffusion is substantially suppressed by the self-avoidant memory relative to that predicted by the commonly used active Brownian model with equivalent velocity and angular decorrelation timescale. We attribute this suppression to transient self-caging that we propose is novel for self-avoidant systems. |
Tuesday, March 15, 2022 11:42AM - 11:54AM |
G08.00002: Interfaces and coarsening in chiral active matter Sriram R Ramaswamy, S. J Kole, Ananyo Maitra, Gareth Alexander, Cesare Nardini Chirality is ubiquitous in the living world. We study ordering and phase separation in chiral active matter in the framework of pseudo-scalar active field theories. For the two-dimensional case we then extract the 1+1-dimensional dynamical equation for the height field of an interface. Our predictions include unidirectional interface waves -- a hallmark of parity breaking. We elucidate active chiral effects on coarsening and on instabilities of strips, derive the coarse-grained equation from a microscopic model, and investigate forces and torques between inclusions in a chiral active medium. |
Tuesday, March 15, 2022 11:54AM - 12:06PM |
G08.00003: Ringkinetic Theory of Vicsek-like Systems of Self-Propelled Particles Rüdiger Kürsten, Thomas Ihle We consider aligning self-propelled point particles in two dimensions. |
Tuesday, March 15, 2022 12:06PM - 12:18PM |
G08.00004: Persistent random walkers with shocks Matthew J Metson
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Tuesday, March 15, 2022 12:18PM - 12:30PM |
G08.00005: Negative drag route to anomalous diffusion in active fluids Yongjoo Baek, Ki-won Kim, Yunsik Choe Various experiments have reported that a tracer immersed in a baterial bath initially exhibits a non-ballistic superdiffusion. The origin of this behavior has been variously ascribed to hydrodynamic interactions among the bacteria or to the direct contact between each bacterium particle and the tracer. In this study, we present a simple model of the tracer dynamics in one dimension which shows that the non-ballistic superdiffusion can be induced entirely without the hydrodynamic effects. We find that the swimming bacteria can exert a "negative" drag force on the tracer in the sense that the drag force applies in the direction of the tracer's velocity. This implies that the bacteria can impart their motility to symmetric tracers through a spontaneous symmetry-breaking mechanism. At the onset of this symmetry-breaking tracer motility, the critical slowdown effects give rise to a non-ballistic superdiffusion with a power-law exponent close to those reported in experiments. While our arguments are based on a mean-field analysis of the one-dimensional model, we also numerically check their validity in more general situations, including the two-dimensional case typically encountered in experiments. |
Tuesday, March 15, 2022 12:30PM - 12:42PM |
G08.00006: Two-dimensional noninteractive active Fokker-Planck equation Pedro E Herrera Avila, Mario Sandoval We solve the noninteractive active Fokker-Planck equation (NAFP) in two dimensions by introducing a perturbation parameter containing the inertia of the system. From this NAFP and in velocity space, we obtain a 'Maxwell-Boltzmann' velocity distribution in the stationary state. The shape of this velocity distribution is the result of a bimodal distribution rotated about its symmetry axis. This distribution is used to calculate the system's mean-square speed and the results are validated by means of Langevin dynamics simulations. |
Tuesday, March 15, 2022 12:42PM - 12:54PM |
G08.00007: Three-dimensional motion of noninteractive active matter with inertia Omar V Espinosa, Dr. Mario Sandoval It is well known that inertia in active matter at macroscopic scales cannot be neglected. Up to now, theoretical work in this regard has been reported only in two dimensions. Here, we extend previous models and consider a three-dimensional ABPs system of noninteracting spherical active particles with both translational and rotational inertia. Several important physical quantities from the system (effective diffusion, swim and Reynolds pressure, mean-square speed) are theoretically obtained and numerically validated. |
Tuesday, March 15, 2022 12:54PM - 1:06PM |
G08.00008: Information transfer, flocking and defects in a non-reciprocal XY model Pankaj Popli, Sriram R Ramaswamy We study the classical dynamics of a lattice of XY spins, i.e., planar rotors with inertia and damping, characterized by a unit vector pi at site i, where the torque on the i th rotor due to the j th depends on whether site j is ahead or astern of i, as defined by the direction of pi. We study analytically and numerically the consequences [Dadhichi et al., Phys Rev E 101, 052601 (2020)] of this non-mutuality of interaction, including self-advection of the orientation and the resulting bend waves of flocking models, directed information transfer and, at large enough asymmetry, a bending instability and spatiotemporal chaos. |
Tuesday, March 15, 2022 1:06PM - 1:18PM |
G08.00009: Static structure of active fluids reflects energy dissipation Gregory V Rassolov Active matter systems are driven out of equilibrium by local non-conservative forces, giving rise to unique behaviors and providing access to structures with potentially useful material properties. However, a significant obstacle for controlling the structure of active systems is their incompletely understood relationship to the dissipation of energy induced by this local driving. In this talk, I will outline our attempts to overcome this by using tools from liquid-state theories and machine learning. Our main results are a non-equilibrium mean field framework which elucidates the connection between dissipation and structure. We show how our results may be applicable to a wide range of systems, from isotropic active Ornstein-Uhlenbeck particles to more complex anisotropic active rotors, and we demonstrate that a neural network can learn this connection even without access to information about the underlying dynamics. Our results outline a new perspective on the underlying relationship between system organization and dissipation in far-from-equilibrium systems and point towards the use of similar techniques for more complex and physically relevant systems such as active nematics. |
Tuesday, March 15, 2022 1:18PM - 1:30PM |
G08.00010: Fluctuation-Induced First-Order Transition to Collective Motion David Martin The transition to collective motion is of paradigmatic importance within active matter, with implications in fields as diverse as animal behavior or biology. However, it remains difficult to assess the nature of this transition as it seems to vary depending on the microscopic interactions at play. Starting from mean-field hydrodynamics predicting a continuous emergence of collective motion, we show that fluctuations generically induce a density-dependent shift of the onset of order, which, in turns, changes the nature of the transition into a phase-separation scenario. Our results apply to a range of systems, including 'metric' models where alignment occurs up to a finite distance. They also hold for 'metric-free', or 'topological', models in which particles interact with their k nearest neighbors and for which the onset of order was so far believed to be continuous. Our analytical predictions are confirmed by numerical simulations of fluctuating hydrodynamics and microscopic models. |
Tuesday, March 15, 2022 1:30PM - 1:42PM Withdrawn |
G08.00011: Universality class and effective field theory of the motility-induced critical point Claudio Maggi In this talk I will present the results of large-scale simulations of a two-dimensional model of active particles close to the motility-induced critical point. I will show how finite-size scaling analysis provides exhaustive evidence that the critical behaviour of this active system belongs to the Ising universality class. In addition to the scaling observables that are also typical of passive systems, in the active system we find a kinetic temperature difference emerging between the two cohexisting phases. This quantity, which is always zero in equilibrium, is instead singular in the active system and it is well described by the same exponent of the order parameter in agreement with mean-field theory. In the seond part of the talk I will focus on the critical non-equilibrium dynamics of this model, which is studied by analyzing the breakdown of the Fluctuation-Dissipation Theorem (FDT). The FDT-violation manifests in the short time and wavelength regime, where the response function has a much larger amplitude than the fluctuation spectrum. Conversely, at larger spatiotemporal scales, the FDT is restored and the critical slowing-down is compatible with the dynamic Ising universality class. Building on these results, we develop a novel field-theoretical description employing a space-time correlated noise which qualitatively captures the numerical results already at the Gaussian level. A one-loop renormalization group analysis of such a field theory shows that the correlated noise does not change the critical exponents with respect to the equilibrium. I will discuss how these results demonstrate that a correlated noise field is a fundamental ingredient to capture the features of critical active matter at the coarse-grained level. |
Tuesday, March 15, 2022 1:42PM - 1:54PM |
G08.00012: Novel critical phenomena in compressible polar active fluids: A functional renormalization group approach Patrick Jentsch, Chiu Fan Lee Renormalization group (RG) methods provide physicists a systematic way to categorize macroscopic behaviour of a dynamical system, be it equilibrium or nonequilibrium, into distinct universality classes (UCs). For instance, dynamic RG analyses together with the perturbative epsilon-expansion method have uncovered diverse novel nonequilibrium UCs in dry polar active fluids (PAF), albeit only in the incompressible and infinitely compressible (or Malthusian) limits. Indeed, the study of compressible PAF has been hampered by technical difficulties in the applications of perturbative RG methods. Here, we use functional RG methods to bypass these difficulties and unveil for the first time novel critical behavior in compressible PAF. Specifically, we focus on the multicritical point that emerges when the critical order-disorder transition coincides with critical motility-induced phase separation, and uncover three novel UCs and their associated scaling behavior near the upper critical dimension dc = 6. |
Tuesday, March 15, 2022 1:54PM - 2:06PM |
G08.00013: Interfacial Polar-Nematic Ordering in Motility-Induced Phase Separation Chiu Fan Lee Motility-induced phase separation (MIPS) is a purely non-equilibrium phenomenon in which self-propelled particles aggregate without any attractive interactions. One surprising feature of MIPS is the emergence of polar-nematic order at the interfacial region, whose underlying physics remains poorly understood. Here, I will use a density-dependence speed model to obtain the corresponding exact polar-nematic ordering structure at the MIPS interfacial region. |
Tuesday, March 15, 2022 2:06PM - 2:18PM |
G08.00014: MIPS Beyond Simple Repulsive Interactions: from Attractive Forces to Aligning Torques Gianmarco Spera, Julien Tailleur, François Graner, Marc Durand Among active matter phenomena, motility-induced phase separation (MIPS), which leads to condensed active matter in the absence of cohesive forces, is a paradigmatic example of how activity overcomes the constraints imposed in equilibrium [1]. MIPS starts to be well understood for self-propelled spherical particles interacting via purely repulsive forces [2,3,4]. Its interplay with other interactions, from attractive tails to aligning torques, leads to a rich physics that we illustrate with numerical simulations [5,6,7]. We then show how a generalized stress tensor formalism accounts for MIPS in the presence of these more general interactions, accounting both for re-entrance phenomena as well as alignment-induced MIPS. |
Tuesday, March 15, 2022 2:18PM - 2:30PM |
G08.00015: Equation of State and Reentrant Collision of Active Dumbbells under Stiff Wall Interactions Xin Yong, Emad Pirhadi, Xiang Cheng Active systems of energy-consuming components are persistently out of equilibrium. It is known that pressure is not always a state function for generic active matter. Torque interaction with confinement affects the distribution of active particles throughout the system. Thus, the mechanical pressure of anisotropic active particles depends on their microscopic interactions with a solid wall. We perform numerical simulations of self-propelled dumbbells to explore how variations in the wall stiffness influence the mechanical pressure of dry active matter. In contrast to previous findings, we find that mechanical pressure can be independent of the wall interaction, even in the presence of intrinsic torques for dumbbells. Particularly, the dependency of pressure on the wall stiffness vanishes when the stiffness is above a critical level. In such a limit, the dynamics of dumbbells near the walls are randomized due to the large torque experienced by the dumbbells, leading to the recovery of pressure as a state variable of density. We also observe anomalous collision dynamics at high wall stiffnesses induced by inertial effects, in which a dumbbell exhibits multiple collisions with the wall in a very short time before leaving the wall indefinitely. |
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