Bulletin of the American Physical Society
APS March Meeting 2023
Volume 68, Number 3
Las Vegas, Nevada (March 5-10)
Virtual (March 20-22); Time Zone: Pacific Time
Session B01: Active MatterFocus
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Sponsoring Units: GSNP Chair: Daphne Klotsa, UNC Chapel Hill Room: Room 124 |
Monday, March 6, 2023 11:30AM - 12:06PM |
B01.00001: Active fluids under deformable confinement: lessons from particle-based models Invited Speaker: Aparna Baskaran We will review the current state of our understanding of active fluids confined by deformable boundaries. The two principle effects, curvature induced accumulation of active particles and the rectification and enhancement of boundary fluctuations will be laid out. Then, we will discuss recent work on understanding the dynamics of a GUV enclosing a dilute microtubule active nematic. |
Monday, March 6, 2023 12:06PM - 12:18PM |
B01.00002: Shortcuts to adiabaticity for Active Brownian Particles Marco Baldovin Among the many physical systems exhibiting active behaviour, some have the remarkable property that the degree of activity can be externally controlled. Systems of Janus particles have been realized, for instance, such that the chemical reactions providing self-propulsion are activated by the external light: by tuning the light intensity, it is thus possible to switch the system from a passive-like state to an active phase. The transition is not istantaneous: the thermalization to the final state requires a time that depends on the chosen protocol for the light intensity and the other external parameters, i.e. on the way they are modified in time. It is thus natural to search for "shortcuts to adiabaticity", i.e. protocols bringing the system to the desired final state in a given (short) time interval. |
Monday, March 6, 2023 12:18PM - 12:30PM |
B01.00003: Long-range velocity correlations in a passive system with active dopants Leila Abbaspour, Rituparno Mandal, Peter Sollich, Stefan Klumpp Active matter systems exhibit fascinating collective behaviour across a wide range of lengths and time scales. Active Brownian particles are a canonical example of such active matter systems. Such systems show surprisingly long-ranged correlations in the velocity field in the dense phase during motility-induced phase separation. |
Monday, March 6, 2023 12:30PM - 12:42PM |
B01.00004: Activity-Enhanced Colloidal Materials Assembly Jin Lee, Daphne Klotsa The emerging field of active matter has given us the ability to intentionally drive and dynamically manipulate individual particles in synthetic systems. Chemically-coated colloids and nanoparticles can self-propel or 'swim' in a directed way via a chemical fuel source and/or light activation. Doping of material with active particles can speed up the assembly process by overcoming kinetic barriers, can increase the "effective local temperature" causing a rearrangement and annealing of defects, and can drive the system to completely new structures and pathways inaccessible through equilibrium processes. Our computational studies of these active colloids will map out a wide parameter space to understand the dynamics of heterogeneous, nonequilibrium colloidal systems. As a result, together with our experimental collaborators, our goal is to provide unprecedented particle-level control over the assembly process and thus materials properties through controlled and targeted dynamic manipulation of individual particles. |
Monday, March 6, 2023 12:42PM - 12:54PM |
B01.00005: Phase behavior and surface tension of soft active Brownian particles Nicholas J Lauersdorf, Thomas M Kolb, Moslem Moradi, Ehssan Nazockdast, Daphne Klotsa Active-matter systems consist of components that locally consume energy to move, exert forces or perform chemical reactions, thus being inherently out of equilibrium. Simple models have been developed to capture their emergent behavior, including the active Brownian particle (ABP) where each colloid is self-propelled by an active force. One question that we aimed to resolve was how should we define stress in active systems and its balance at steady state? Unique to active systems, active forces align at the interface. Extending equilibrium statistical mechanics to our non-equilibrium systems by using a volume-averaged swim pressure results in unrealistic surface tensions. We derived a continuum theory to investigate the relationship between the interparticle pressure, swim pressure, and macroscopic pressure in the momentum equation. We found that formulating the point-wise macroscopic pressure as the interparticle pressure and modeling the particle activity through a spatially variant body force-as opposed to a volume-averaged swim pressure-results in a surface tension that is negligible and intrinsic to all ABP steady states. Furthermore, we discuss the extension of our monodisperse theory to active mixtures. |
Monday, March 6, 2023 12:54PM - 1:06PM |
B01.00006: On the Einstein relation between mobility and diffusion in an active bath Jordan M Horowitz, Alexandre Solon The Einstein relation relates the diffusivity and the mobility of a tracer particle immersed in an equilibrium fluid to the bath’s temperature. It is one the major predictions of near-equilibrium linear response theory. Active baths, made of self-propelling units, are driven far from equilibrium and as such we would not expect a similar relation. We show that for large, heavy tracers the diffusivity and mobility can be related to correlation functions, just like for an equilibrium fluid. Moreover, to good approximation, an Einstein relation does hold in an active bath upon using a different temperature defined mechanically through the pressure exerted on the tracer. |
Monday, March 6, 2023 1:06PM - 1:18PM |
B01.00007: Detecting active clustering on quenched disorder with unsupervised machine learning Danielle M McDermott, Cynthia Reichhardt, Charles M Reichhardt Phase transitions can be difficult to characterize in active matter due to the soft interparticle interactions and the inherent disorder. Using large-scale numerical simulations of active disks driven far from equilibrium, we demonstrate that principal component analysis, a dimensionality reduction technique popular in machine learning, can detect dynamical phase transitions in systems driven far from equilibrium. We model active agents as monodisperse disks executing run-and-tumble motion in two regimes - motility induced phase transitions in a clean environment, and subject to an external driving force across an environment of quenched disorder. The machine learning order parameter is derived from a system wide measure of interparticle distance, that distinguishes homogeneous versus inhomogeneous arrangements of particles. Using this tool, we identify a variety of order-disorder transitions such as clustering and depinning and disorder-disorder transitions including clogging and laning which are not readily distinguished with traditional measurements. We highlight particularly the detection of incipient clustering, where the machine learning order parameter performs better than measurements based on interparticle contact. |
Monday, March 6, 2023 1:18PM - 1:30PM |
B01.00008: Spinners on an air table as an inertial chiral fluid Shengkai Li, Trung V Phan, Gao Wang, Liyu Liu, Ramzi R Khuri, Robert H Austin Conventionally, studies of active matter are concerned with overdamped objects. Recently, researchers have gradually picked up interest in the less studied underdamped active matter and have found interesting effects caused by the delay of speed gain, such as the mitigated phase separation. Previous experimental studies on inertial active matter have tried to increase the mass of the driven objects to increase the inertia. On the contrary, in our study, we increase the inertia by significantly decreasing the damping. To do so, we float rotationally-driven disks (spinners) on an air table. Each spinner is an acrylic disk with 6 cm in diameter and is powered by two blowers blowing in opposite directions at the ends of a diameter. The damping is so small that a spinner takes a few seconds to reach the terminal angular speed from zero. Gears are added to the rims of the spinners to promote the conversion between translational and rotational energy upon collisions. The difference between collisions of the same-handedness and opposite-handedness spinner pairs creates different emergent spin rate distributions and spatial currents depending on the ratio between the left-handed and right-handed spinners. |
Monday, March 6, 2023 1:30PM - 1:42PM |
B01.00009: Interfacial dynamics in chiral active matter. SJ Kole, Ananyo Maitra, Cesare Nardini, Gareth Alexander, Sriram R Ramaswamy We construct an active field theory for a non-conserved pseudoscalar field in a uniaxial medium. Then we use this to obtain the stochastic dynamics of a domain wall separating regions of opposite chirality. This dynamics turns out to be equivalent to that of a steadily forced polymer. The steady-state probability distribution of the one-dimensional shape of the domain wall is the same as the passive Edwards-Wilkinsons model. However, surprisingly, the dynamical behaviour of the domain wall shape reveals its activity. A nonlinearity -- which by scaling arguments is marginal in one dimension -- turns out to lead to anomalous growth. We examine this numerically and using a two-loop RG calculation. |
Monday, March 6, 2023 1:42PM - 1:54PM |
B01.00010: Natural swarms in 3.99 dimensions Mattia Scandolo, Andrea Cavagna, Luca Di Carlo, Irene Giardina, Tomas S. Grigera, Stefania Melillo, Leonardo Parisi, Giulia Pisegna The dynamical critical exponent z of natural swarms of insects is calculated using the Renormalization Group (RG) to order ε=4-d. The hydrodynamic theory of swarms combines the effects of activity with the presence of inertial behavior, which has been found to be a key ingredient in the description of the dynamics of natural swarms in the field. A novel RG fixed point emerges, where both off-equilibrium activity and inertial mode-coupling inertial interactions are relevant. In three dimensions we predict a critical exponent z = 1.35, in excellent agreement with the experimental value, zexp = 1.37 ± 0.11. |
Monday, March 6, 2023 1:54PM - 2:06PM |
B01.00011: Evidence of fluctuation-induced first-order phase transition in active matter Luca Di Carlo In Vicsek-like active matter systems the interplay between density fluctuations and ferromagnetic/imitation interactions gives rise to a rich phenomenology. In particular, the ordering transition to the flocking state seems to be similar to a first-order phase transition. Intuitively this happens because of the positive feedback between density fluctuations and ferromagnetic interaction, however, a theoretical explanation is still missing. We address this problem by studying the Malthusian Toner-Tu theory (MTT) in its near-ordering phase. Because of the birth/death process, characteristic of this Malthusian model, density fluctuations are partially suppressed but still strong enough to give rise to a non-trivial phenomenology. We study the MTT model with perturbative renormalization group techniques. A one-loop calculation shows that the renormalization group flow drives the system in an unstable region, suggesting a fluctuation-induced first-order phase transition. This calculation could provide a rigorous explanation of why the disorder/order transition is first order in active matter systems. |
Monday, March 6, 2023 2:06PM - 2:18PM |
B01.00012: Active Brownian dynamics in a Random Lorentz gas Mingyuan Zheng, Patrick Charbonneau Active Brownian particles are a minimal model of both synthetic and biological active matter. Despite the simplicity of the model, its theoretical description remains incomplete in many regimes, especially at high densities. Although a dynamical mean-field theory (DMFT) of the system is then available, its analysis is hindered by the difficulty of solving the resulting equations. Using high-dimensional simulations of an active Brownian random Lorentz gas model, we extract physical insights expected to emerge from the DMFT. Our results broadly enrich the understanding of active matter. |
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