Bulletin of the American Physical Society
APS March Meeting 2021
Volume 66, Number 1
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session Y17: Interacting Active ParticlesLive
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Sponsoring Units: GSNP Chair: Bryan VanSaders, University of Chicago |
Friday, March 19, 2021 11:30AM - 11:42AM Live |
Y17.00001: Substrate-mediated interaction of active agents on an elastic membrane Shengkai Li, Hussain Gynai, Yasemin Ozkan-Aydin, Steven Tarr, Pablo Laguna, Daniel I Goldman Substrate-mediated interaction is observed in many biological and artificial systems. To discover principles by which collections of active agents can influence each other solely via environmental interaction, we study the dynamics of two robotic vehicles (10 cm in diameter, 150 gram in mass) on a deformable spandex membrane with a radius of 1.2 m; each vehicle’s motion is affected by both the global curvature of the membrane and local deformations due to the other vehicle. The vehicles are driven forward by a motor/differential system which enforces constant (but variable) speed movement; the vehicles turn according to the local slope of membrane. We initiated two vehicles at different locations and orientations on the membrane, monitored the vehicle trajectories, and recorded the time for them to collide (and remain in contact) due to the membrane-mediated coupling. While dynamics and trajectories could be complex, in general, increasing vehicle masses and/or decreasing speeds decreased the merger time. A mathematical model which incorporates the reciprocal interactions captures these dynamics and facilitates design of a controller that avoids collisions by changing a vehicle’s speed as a function of its locally sensed slope. |
Friday, March 19, 2021 11:42AM - 11:54AM Live |
Y17.00002: Collective motion in large deviations of active particles Yann-Edwin Keta, Etienne Fodor, Frederic van Wijland, Michael Cates, Robert L. Jack We analyse collective motion that occurs during rare (large deviation) events in systems of active particles. We discuss the associated dynamical phase transition to collective motion, which occurs when the work of non-conservative active forces is biased towards larger values. We show numerically for 2D active Brownian particles that a finite biasing field is needed to induce spontaneous symmetry breaking, even in large systems, and analyse this symmetry breaking by an optimal-control representation of the biased dynamics. |
Friday, March 19, 2021 11:54AM - 12:06PM Live |
Y17.00003: Dry Active Matter Exhibits a Self-Organized Cross Sea Phase Rüdiger Kürsten, Thomas Ihle The Vicsek modell is known in three different phases: a disordered phase for high noise (or small density), a phase of regularly arranged high density bands with strong polar order that are sourrounded by low density regions with almost no polar order for intermediate noise (or densities), and a (on large scales) spatially homogeneous phase with global polar order for small noise (or high densities). We show that for large systems, there is a fourth phase. It looks like a superposition of bands, tilted in two directions similar to the 'cross sea' phenomenon, sometimes observed in the sea. Here however, the pattern is self-organized and not only a superposition of tilted bands. The density at the crossing points of the pattern is much larger than twice the band density. We show that cross sea phase is separated from the band phase and from the homogeneous polar ordered phase by two discontinuous transitions. We use a local integral of the pair correlation function as an order parameter. Due to the differnt spatial arrangement It can distinguish all four phases. Alternatively, a lattice order parameters leads to similar results. |
Friday, March 19, 2021 12:06PM - 12:18PM Live |
Y17.00004: Magnetic microswimmers exhibit Bose-Einstein-like condensation Fanlong Meng, Daiki Matsunaga, Benoit Mahault, Ramin Golestanian We study an active matter system comprised of magnetic microswimmers confined in a microfluidic channel and show that it exhibits a new type of self-organized behavior. Combining analytical techniques and Brownian dynamics simulations, we demonstrate how the interplay of non-equilibrium activity, external driving, and magnetic interactions leads to the condensation of swimmers at the center of the channel via a non-equilibrium phase transition that is formally akin to Bose-Einstein condensation. We find that the effective dynamics of the microswimmers can be mapped onto a diffusivity-edge problem, and use the mapping to build a generalized thermodynamic framework, which is verified by a parameter-free comparison with our simulations. Our work reveals how driven active matter has the potential to generate exotic classical non-equilibrium phases of matter with traits that are analogous to those observed in quantum systems. |
Friday, March 19, 2021 12:18PM - 12:30PM Live |
Y17.00005: Analyzing Collective Motion Using Graph Fourier Analysis Kevin Schultz, Marisel Villafane-Delgado, Elizabeth P Reilly, Grace M Hwang, Anshu Saksena Collective motion in animal groups, such as swarms of insects, flocks of birds, and schools of fish, are some of the most visually striking examples of emergent behavior. Empirical analysis of these behaviors in experiment or computational simulation primarily involves the use of "swarm-averaged" metrics or order parameters such as velocity alignment and angular momentum. Recently, tools from computational topology have been applied to the analysis of swarms to further understand and automate the detection of fundamentally different swarm structures evolving in space and time. Here, we show how the field of graph signal processing can be used to fuse these two approaches by collectively analyzing swarm properties using graph Fourier harmonics that respect the topological structure of the swarm. This graph Fourier analysis reveals hidden structure in a number of common swarming states and forms the basis of a flexible analysis framework for collective motion. |
Friday, March 19, 2021 12:30PM - 12:42PM Live |
Y17.00006: Systematically elucidating the non-equilibrium steady states of active brownian particles Samuel Cameron, Majid Mosayebi, Tanniemola Liverpool In equilibrium systems, the steady-state probability distribution (e.g., the Boltzmann distribution in the canonical ensemble) is known a priori. This is generally not true for non-equilibrium systems which can have non-vanishing currents. In this talk we demonstrate that one can perturbatively compute a steady-state probability distribution and an associated probability current for certain non-equilibrium systems, which then allows one to compute macroscopic averages in analogy to the equilibrium case. We apply this framework to a paradigmatic non-equilibrium system (active brownian particles in 2D) and compare our results with simulations. We demonstrate a quantitative agreement between our calculations and simulations for low enough Peclet number and average density. |
Friday, March 19, 2021 12:42PM - 12:54PM Live |
Y17.00007: Odd viscosity in Stokes flows Tali Khain, Colin Scheibner, Vincenzo Vitelli In standard fluids, the viscosity converts the mechanical energy provided by external forces into heat. If, however, a fluid breaks microscopic time-reversal symmetry, for example by being composed of active spinning particles, its viscosity tensor may acquire an additional “odd” contribution that does not dissipate energy. In this work, we elucidate the effect of odd viscosity on Stokes flows in three dimensions. Strikingly, we discover that the Stokeslet solution for fluids with cylindrical symmetry contains an azimuthal component of velocity that originates from singularities in the limit of vanishing dissipative viscosity. In the small odd viscosity limit, we directly solve for the viscous flow past a sphere and compare with the Stokeslet solutions. Our work reveals the significant effect of odd viscosity on flow at low Reynolds number and suggests sedimentation-based probes of odd viscosity in three dimensions. |
Friday, March 19, 2021 12:54PM - 1:06PM Live |
Y17.00008: Quantifying Dissipation from Structure in Active Matter Laura Tociu, Gregory Rassolov, Etienne Fodor, Suriyanarayanan Vaikuntanathan Active matter systems, driven by non-conservative forces acting on individual particles, show a variety of behaviors and structures not seen in equilibrium systems. However, precisely connecting the structure of active matter to the dissipation of energy is a significant challenge, particularly for systems driven far out of equilibrium. We tackle this problem by developing a perturbative mean field theory that works surprisingly well in predicting structural information even for strongly interacting systems, unlike existing approaches. Significantly, this theory requires no more than the direct correlation function at equilibrium. We show that our approach works well even in moderately driven systems with hard interaction potentials. Then, we extend this theory to develop an expression for the rate of dissipative work and show that a robust relationship with the activity-induced deviation in the correlation function exists. This relationship holds even as the system approaches an activity-induced phase transition very far from equilibrium. Finally, we construct a neural network that maps snapshots of active matter to the energy dissipation encoded in them, consolidating our findings on the connection between static structural information and dissipative work. |
Friday, March 19, 2021 1:06PM - 1:18PM Live |
Y17.00009: Predictive modeling of interacting active Brownian particles Jens Bickmann, Julian Jeggle, Stephan Bröker, Joakim Stenhammar, Raphael Wittkowski A predictive field-theoretical modeling of the dynamics of interacting active Brownian particles (ABPs) is challenging, but highly desirable since it allows for quantitative results. In this talk, we present an analytic representation for the full pair-distribution function of a homogeneous suspension of ABPs [1] and show how it can be used to derive predictive field theories for the collective dynamics of ABPs. We present predictive local models for both ordinary ABPs [2,3] and active Brownian circle swimmers [4]. The models yield analytic expressions for the density-dependent mean swimming speed, the spinodal corresponding to the onset of motility-induced phase separation, and the associated critical point as well as a mapping between circle swimmers and ordinary ABPs. All analytic results are shown to be in very good agreement with results of corresponding Brownian dynamics simulations and findings from the literature. |
Friday, March 19, 2021 1:18PM - 1:30PM Live |
Y17.00010: Significance of the effective temperature obtained from the Einstein relation in a system of interacting active Ornstein-Uhlenbeck particles Alireza Shakerpoor, Elijah Flenner, Grzegorz Szamel Systems consisting of athermal active Ornstein-Uhlenbeck particles (AOUPs) are inherently non-equilibrium and generally cannot be characterized by a unique temperature-like variable. Different formulas that are equal to the temperature for equilibrium systems may give different values for the temperature for systems of AOUPs. Here we show that the effective temperature obtained from the Einstein relation between the self-diffusion and mobility coefficients determines the spatial distribution of a tagged particle under the influence of a slowly varying in space external potential. We analytically show that the tagged particle distribution has the standard Boltzmann form with the temperature equal to the effective temperature obtained from the Einstein relation. We verify this prediction using computer simulations. Thus, we show that this effective temperature, defined through a fluctuation-dissipation relation, is valid outside the linear response regime for a slowly varying external potential. |
Friday, March 19, 2021 1:30PM - 1:42PM Live |
Y17.00011: Stochastic replicator equation for understanding traffic congestion Leonardo Apaza, Mario Sandoval We study traffic congestion in multilines at moderate densities. Here, we consider that the congestion arises when drivers wait for passengers no matter the traffic condition. This situation is modeled by using a stochastic replicator equation. Its numerical solution shows that in the transient state, and although the traffic lines are empty, the variance of the system increases until it reaches a maximum point, which results in the maximum traffic congestion. After this critical point, the variance decreases until it recovers the Nash equilibrium and the traffic lines become free again. In addition, the variance is calculated analytically by solving the corresponding Fokker-Planck equation using the homotopy-Padé approximation. |
Friday, March 19, 2021 1:42PM - 1:54PM Live |
Y17.00012: Clumping in a model of self-propelled particles in one dimension Jacob McConley, Narayanan Menon We introduce a model for a one-dimensional system of self-propelled particles with periodic boundary conditions. The self-propulsion is represented by endowing each particle with a natural velocity drawn from a distribution. Following elastic collisions between particles, each collision partner’s velocity decays to its natural velocity over a specified time scale. Numerical simulations of this model generically show that in the long-term, the particles tend to clump spatially, with a rapidly increasing collision rate. This behavior is reminiscent of inelastic collapse but has a completely different origin. The two-particle system was analyzed for the asymptotic form of particle separation and velocity difference in the limit of large collision number. For the multi-particle system, we study analogous measures of the emergent clumping behavior, and we will discuss the scaling of the time to clump as a function of the only two time-scales available in the model. |
Friday, March 19, 2021 1:54PM - 2:06PM Not Participating |
Y17.00013: Active Hard Spheres in Infinitely Many Dimensions Thibaut Arnoulx de Pirey, Gustavo Lozano, Frederic van Wijland Few equilibrium—even less so nonequilibrium—statistical-mechanical models with continuous degrees of freedom can be solved exactly. Classical equilibrium hard spheres in infinitely many space dimensions are a notable exception. We show here that dimensionality is a powerful organizing device for exploring collective properties of active hard spheres evolving far from equilibrium. In infinite dimensions, we exactly compute the stationary state properties that govern and characterize the collective behavior of active hard spheres: the structure factor, the equation of state for the pressure and the self-propulsion velocity. We show that this allows to account for motility-induced phase separation. |
Friday, March 19, 2021 2:06PM - 2:18PM Live |
Y17.00014: Towards a statistical mechanics of chiral active gases Ming Han, Michel Fruchart, Colin Scheibner, Suriyanarayanan Vaikuntanathan, william Thomas Mark irvine, Juan De Pablo, Vincenzo Vitelli Statistical mechanics allows to describe materials near equilibrium using just a few thermodynamic variables. Extending this approach far-from-equilibrium is tempting but often unfeasible. In this talk, we present the footprints of a statistical mechanical treatment of chiral active fluids composed of self-spinning particles. The nature of self-spinning breaks time-reversal symmetry and detailed balance. Nevertheless, such active fluids converge to a non-equilibrium steady state exhibiting Boltzmann statistics with a universal effective temperature determined by the active torques. Beyond exhibiting analogues of common thermodynamic properties, the chiral active gas also displays a dissipation-less odd viscosity in addition to the shear viscosity. Both transport coefficients satisfy a Kubo relation in terms of our effective temperature. We show that the stochastic dynamics of this many body system can be represented as a chiral Brownian motion in shear-stress space. Using this assumption, we derive analytically the full frequency dependence of the viscosities in agreement with simulations. |
Friday, March 19, 2021 2:18PM - 2:30PM Live |
Y17.00015: Jannsen’s effect in 3D ant columns Alberto Fernandez-Nieves, Caleb Anderson When a liquid fills a cylindrical container, the pressure at the bottom increases linearly with height; we all experience this when we dive in the sea. In contrast, when a granular material fills a cylindrical container, this linearity is lost at a certain height. In this case, the pressure eventually saturates to a constant value due to the presence of frictional forces and the formation of force chains. Interestingly, for sufficiently narrow containers, these forces can be compressive rather than supportive for a range of heights, resulting in a pressure at the bottom that is larger than the fluid pressure .. We find that if fire ants, Solenopsis Invictae, are used instead of grains, this overshoot in pressure is washed away. Remarkably, we still observe the saturation typical of granular matter, indicating that ant collectives still form force chains to support their weight for sufficiently tall columns. These chains, however, form and break, reflecting the active character of the system. |
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