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 J05: Active Matter in Complex Environments IILive
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Sponsoring Units: DSOFT DBIO GSNP DFD Chair: Tapomoy Bhattacharjee, Princeton University; Sujit Datta, Princeton University Room: 05 |
Tuesday, March 16, 2021 3:00PM - 3:12PM Live |
J05.00001: Nonequillibrium shape fluctuations and motility of a droplet enclosing active particles Gasper Kokot, Hammad Ali Faizi, Gerardo Pradillo, Alexey Snezhko, Petia Vlahovska Ensembles of motile colloids, driven by Quincke electrorotation to roll on a solid surface, have become a popular model of active matter since the direction of the individual colloid motion is not predetermined. We study the collective dynamics of the Quincke rollers in soft confinement by enclosing the rollers inside a liquid droplet sandwiched between two surfaces. We find that the rollers self-organize into a single vortex that uniformly fills the drop. The droplet interface exhibits strong fluctuations with power spectrum consistent with active fluctuation driven by particle-interface collisions. Broken detailed balance confirms the nonequilibrium nature of the shape dynamics. The rollers activity also gives rise to droplet net motion, which is superdiffusive. |
Tuesday, March 16, 2021 3:12PM - 3:24PM Live |
J05.00002: States of active nematics confined to annuli Zahra Zarei, Chaitanya Joshi, Michael M. Norton, Michael F Hagan, Seth Fraden Defects play an important role in active-matter systems. They are nucleated because of the bend instability and in steady-state are continuously created and annihilated. In this study, we investigate a 2D active nematic confined in an annulus in which the plus and minus half defects exhibit rich dynamical behavior. Confinement into an annulus effectively transforms the turbulent dynamics of the active nematic into a coherent flow. We measure the positional-orientational distribution of defects in different degrees of confinement, as well as the circulation order parameter, and finally, we classify the behavior into three distinct regimes. |
Tuesday, March 16, 2021 3:24PM - 3:36PM Live |
J05.00003: Optimal navigation strategies for microswimmers on curved manifolds Lorenzo Piro, Evelyn Tang, Ramin Golestanian Finding the fastest path to a desired destination is a vitally important task for microorganisms moving in a fluid flow. We study this problem by building an analytical formalism for overdamped microswimmers on curved manifolds and arbitrary flows. We show that the solution corresponds to the geodesics of a Randers metric, which is an asymmetric Finsler metric that reflects the irreversible character of the problem. Using the example of a spherical surface, we demonstrate that the swimmer performance that follows this "Randers policy" always beats a more direct policy. A study of the shape of isochrones reveals features such as self-intersections, cusps, and abrupt nonlinear effects. Our work provides a link between microswimmer physics and geodesics in generalizations of general relativity. |
Tuesday, March 16, 2021 3:36PM - 3:48PM Live |
J05.00004: Slip and ypsotaxis of catalytically self-propelled particles near surfaces Stefania Ketzetzi, Joost de Graaf, Daniela Kraft Catalytically propelled microswimmers have a strong affinity for surfaces, but little is known about how surfaces affect their behavior. In this talk, I will demonstrate that the choice of the substrate material has a strong influence on the microswimmer speed through slippage [1]. Then, using a new height analysis approach, I will show that the microswimmer-wall separation is surprisingly robust for a range of salt concentrations, swimmer surface charges, and swimmer sizes [2]. These striking, activity-induced findings have furthermore important implications for the still-debated propulsion mechanism. |
Tuesday, March 16, 2021 3:48PM - 4:00PM Live |
J05.00005: Spontaneous propulsion of an isotropic colloid in a phase-separating medium Jeanne Decayeux, Vincent Dahirel, Pierre Illien, Marie Jardat We consider an isotropic colloid in a bath of small solute particles. Far away from the colloid, we assume that the solute particles interact via purely repulsive interactions. However, within a given distance from the colloid, their interaction potential becomes attractive, yielding local phase separation and significant density fluctuations in the vicinity of the colloid. We perform Brownian dynamics simulation of this system, which account explicitly for the interactions between all the particles. We show that the fluctuations of the position of the colloid are transiently superdiffusive, and that its long-time diffusion coefficient is significantly enhanced by the attraction between solute particles. We determine the range of parameters where this spontaneous propulsion occurs, and we relate analytically and numerically the dynamics of the position of the colloid to the density fluctuations in its environment. Our study reveals that the interactions between solute particles, which are often neglected in the context of active colloids, can play a crucial role in their propulsion mechanism. |
Tuesday, March 16, 2021 4:00PM - 4:12PM Live |
J05.00006: Interplay of flow and swimmer’s polar orientation in confined active suspensions Paarth Gulati, Suraj Shankar, M Cristina Marchetti We examine the dynamics of a continuum model of confined active bacterial suspensions where flows driven by active stresses are coupled to the tendency of microorganism to propel themselves along their orientation axis. Fluid flow is dissipated by both friction with the substrate and internal viscous forces. The relative importance of these two dissipation mechanisms, as quantified by the hydrodynamic screening length relative to the system size, modifies the nature of the spontaneous flow transition in the channel geometry. It also affects the nature of the crossover between laminar flows at low activity to periodic or oscillatory flow structures at high activity. In both disk and channel geometries, we also examine the role of the anchoring strength of the polar liquid crystals at the boundary. |
Tuesday, March 16, 2021 4:12PM - 4:24PM Live |
J05.00007: Unravel the transport and rotation properties of a squirmer in viscoelastic fluids Kai Qi, Marco de Corato, Ignacio Pagonabarraga Microorganisms such as bacteria and algae naturally inhabit complex environments. Understanding their ubiquitous behavior in, e.g., biofilms, is fundamental for medical and industrial applications. We study the rotational motion and transport peculiarities of a single swimmer in a viscoelastic fluid via Lattice Boltzmann (LB) simulations. Here, the generic squirmer model is employed and fluid viscoelasticity is achieved by added flexible polymer chains. For systems with low viscosity, due to no-slip boundary condition, heterogenous collisions between squirmer and polymers yield additional torques, which enhance squirmer’s rotational motion by more than an order of magnitude. However, asymmetric collisions barely have evident contributions to the rotational enhancement in the high viscosity regime, where the viscous force damps the transport of polymers so that the momentum and angular momentum exchanges between squirmer and polymers are suppressed. Our results can facilitate the understanding of the behavior of the microorganisms in the complex systems and guide the design of microfluidic devices by harnessing these properties. |
Tuesday, March 16, 2021 4:24PM - 4:36PM Live |
J05.00008: Viscoelasticity enables self-organization of bacterial active matter Song Liu, Suraj Shankar, M Cristina Marchetti, Yilin Wu Simultaneous control of spatial and temporal organization of active matter is challenging and generally requires complex interactions, such as reaction-diffusion hierarchies or genetically engineered cellular circuits. Here we found that tuning the rheological properties of bacterial active fluids enables large-scale spatial and temporal self-organization. As the viscoelasticity of the suspending fluid is varied, a confined bacterial active fluid first self-organizes in space into a millimeter-scale rotating vortex; then displays temporal organization as the giant vortex switches its global chirality periodically with tunable frequency, reminiscent of a self-driven torsional pendulum. The phenomenon can be explained in terms of the interplay between active forcing and viscoelastic stress relaxation. Our findings advance the understanding of bacterial behavior in complex fluids and demonstrate experimentally that rheological properties can be harnessed to control active matter flows. |
Tuesday, March 16, 2021 4:36PM - 4:48PM Live |
J05.00009: Effective thermodynamic properties of inertial active microswimmers with alignment interaction Soumen Karmakar, Rajaraman Ganesh Self propelling feature, resulting from the associated internal or external energy sources, distinguishes microswimmers or micromotors from equilibrium passive systems of similar scales. Properties of background medium and the associated scales, often, make the microswimmers a low Reynolds number system (Re ≈ 10−4 − 10−5). However, there are many instances where Re is orders of magnitude higher, where inertial effects play major role. Recently, we have demonstrated[1] that active Langevin system without explicit alignment exhibits thermodynamic equilibrium-like properties despite inherent out-of-equilibrium nature. |
Tuesday, March 16, 2021 4:48PM - 5:00PM Live |
J05.00010: Dynamic structures in concentrated suspensions of spherical squirmers Douglas Brumley, Takuji Ishikawa, Timothy J Pedley Concentrated suspensions of micro-organisms exhibit striking collective motions, but characterisation of the bulk rheological properties have been limited by a lack of suitably efficient numerical methods. A vertical monolayer of identical spherical squirmers, which may be bottom-heavy, is modelled computationally by two different methods: Stokesian dynamics, and a lubrication-theory-based method. The effective suspension viscosity is calculated for monolayers subjected to either a linear shear flow or pressure-driven flow, as functions of the areal fraction of spheres, the squirmers’ swimming properties, and external torque conditions. We characterise ephemeral structures, force chains and jamming events within the suspension, and reconcile these with the bulk rheological properties. This work suggests that lubrication theory, based on near-field interactions alone, contains most of the relevant physics, and successfully connects microscale structural features in the suspension with bulk rheological attributes. |
Tuesday, March 16, 2021 5:00PM - 5:12PM Live |
J05.00011: Interaction of microswimmers with elastic interfaces Sankalp Nambiar, John Scott Wettlaufer The dynamics of systems constituting active microscopic agents that can propel themselves, such as swimming microorganisms or active colloids is often rich and non-trivial. For instance, dilute suspensions of these motile microswimmers have been known to exhibit viscosity reduction (when weakly sheared), enhanced fluid fluctuations, as well as collective and coherent motion, among others. Typically, however, the microswimmers are often constrained by the presence of an intervening boundary or an interface, that restricts their free-swimming. The detailed dynamics of such systems is then crucially dependent on the nature of the boundary, rigid versus deformable. Simulations and theories have shown that microswimmers subject to confinement between parallel rigid boundaries exhibit an excess accumulation near the boundary; in turn, there is an associated enhancement of the active pressure forces on the wall. Here we relax the rigidity constraint, and consider the motion of finite-sized microswimmers in the vicinity of an elastic interface. In particular, we analyze the character of the hydrodynamic stresses that develop due to the swimmer-interface interaction. |
Tuesday, March 16, 2021 5:12PM - 5:24PM Live |
J05.00012: Design principles for transport of vesicles by enclosed active particles Sarvesh Uplap, Michael F Hagan, Aparna Baskaran Active particles in the presence of boundaries exhibit non-trivial emergent states, including rectification, accumulation, and steady states with circulating currents. In this work, we seek to leverage these phenomena to identify design principles for optimal transport of flexible compartments that enclose active particles. By performing numerical simulations of simple models of self-propelled particles enclosed in a flexible elastic enclosure, we characterize the transport properties of the compartment as a function of the shape of the active agent and elastic properties of the enclosure. This work sheds light on organization of active stresses that aid transport, while potentially elucidating cellular transport processes and guiding the design of bio-derived and synthetic systems. |
Tuesday, March 16, 2021 5:24PM - 5:36PM Live |
J05.00013: The dynamic steady-states of polar self-propelled filaments confined in 2D flexible vesicles Yingyou Ma, Aparna Baskaran, Michael F Hagan Active materials exhibit spectacular spatiotemporal dynamics not possible in equilibrium systems. While it is well-known that these behaviors can be dramatically altered by boundaries, most such studies have focused on rigid boundaries or active agents that lack internal degrees of freedom. In this work we investigate the relationship between boundary and agent degrees of freedom, by simulating self-propelled semiflexible filaments confined within flexible vesicles in two dimensions. We observe a diverse array of emergent behaviors depending on filament properties (density, length, flexibility, and activity) as well as container size and flexibility. While some of these behaviors can be understood based on the intrinsic time and length scales associated with individual filaments or the boundary, others arise through a complex interplay between activity and deformations of the boundary or filamments, leading to new classes of dynamical self-organized states. |
Tuesday, March 16, 2021 5:36PM - 5:48PM Live |
J05.00014: Survival time of active Brownian particles on flat curves Pedro Herrera, Mario Sandoval We find the average time for an overdamped active Brownian particle (OABP) moving on any flat curve with metric, to first reach a given point on the curve (so called mean-first passage time, MFPT). This problem can also be interpreted as the survival time an OABP has, before it is absorbed at a give point on the curve. From this analysis, a general MFPT equation depending on the curve’s geometry is found and solved for three particular cases, namely, a circle, an ellipse, and a second order limacon. The results are also validated by using Brownian dynamics simulations. |
Tuesday, March 16, 2021 5:48PM - 6:00PM Live |
J05.00015: Spontaneous demixing of mixed active-passive suspensions Raphaël Jeanneret, Stephen Williams, Idan Tuval, Marco Polin Understanding the properties of active matter is driving a rapid growth in soft- and bio-physics. Some of the most important examples of active matter are at the microscale, and include active colloids and suspensions of microorganisms, both as a simple active fluid and as mixed suspensions of active and passive elements. In the latter class, recent work has started to reveal new phenomena including activity-induced depletion interactions, phase separation, and the extraction of work from active suspensions. Building on current research in our group on the physics of colloid-swimmer interactions [1,2] we are interested in how external control of the dynamics of the active component can be used to alter the transport of passive cargo. Here we report our new experiments on the behaviour of active-passive systems under spatial confinement. Confinement-induced inhomogeneities in swimmers' distributions influence dramatically the spatial distribution of passive particles, with the colloids accumulating either towards the boundaries or towards the bulk of the sample depending on the size of the container. We show that this can be used to induce the system to de-mix spontaneously. |
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