Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session F29: Active Matter in Complex Environments IV |
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Sponsoring Units: DSOFT DBIO DFD GSNP Chair: Tapomoy Bhattacharjee, Princeton University Room: 501 |
Tuesday, March 3, 2020 8:00AM - 8:12AM |
F29.00001: Nonequilibrium phases in mixtures of active and passive particles Junang Li, Shreyas Gokhale, Alexandre Solon, Jeffrey Gore, Nikta Fakhri Active systems composed of self-propelled particles are intrinsically out of equilibrium. Due to the breaking of detailed balance at the individual particle level, they exhibit complex dynamic phenomena not observed in their equilibrium counterparts. For instance, distinct nonequilibrium phases such as a demixed state and motility induced phase separated state (MIPS) can arise in a binary mixture of passive and active particles. Here, we map the computational phase diagram of a binary mixture of active and passive particles as a function of orientational interactions and Peclet number and observe new emergent behavior of dynamical clustering and a polar band of active particles. This computational study helps us to illuminate observations of experiments we developed consisting of passive colloid particles immersed in a bath of bacteria. By quantifying fluctuations and correlations between passive particles in both simulations and experiments, we quantify how far the system is from equilibrium. |
Tuesday, March 3, 2020 8:12AM - 8:24AM |
F29.00002: Tuning the self-organization of confined active particles by the steepness of the trap Shubhashis Rana, Md. Samsuzzaman, Arnab Saha We consider the collective dynamics of self-propelling particles in two dimensions. They can align themselves according to the direction of propulsion of their neighbours, together with small rotational fluctuations. They also interact with each other via soft, isotropic, repulsive potentials. The particles are confined in a circular trap. The steepness of the trap is tuneable. The average packing fraction of the particles is low. When the trap is steep, particles flock along its boundary. They form a polar cluster that spreads over the boundary. The cluster is not spatially ordered. We show that when the steepness is decreased beyond a threshold value, the cluster becomes round and compact and eventually spatial order (hexagonal) emerges in addition to the pre-established polar order. We investigate the kinetics of such ordering. We find that while rotating around the centre of the trap along its circular boundary, the cluster needs to roll around its centre of mass to be spatially ordered. We have studied the stability of the order when the trap is suddenly switched off. We find that for the particles with velocity alignment interaction, the decay of the spatial order is much slower than the particles without the alignment interaction. |
Tuesday, March 3, 2020 8:24AM - 8:36AM |
F29.00003: Active matters: broken symmetries and response Sara Dal Cengio, Demian Levis, Ignacio Pagonabarraga We address the question of how interacting active systems in a non-equilibrium steady-state respond to an external perturbation. We establish an extended fluctuation-dissipation theorem for Active Brownian Particles (ABP) which highlights the role played by the local violation of detailed balance due to activity. By making use of a Markovian approximation we derive closed Green-Kubo expressions for the diffusivity and mobility of ABP and quantify the deviations from the Stokes-Einstein relation. We compute the linear response function to an external force using unperturbed simulations of ABP and compare the results with the analytical predictions of the transport coefficients. Our results show the importance of the interplay between activity and interactions in the departure from equilibrium linear response. |
Tuesday, March 3, 2020 8:36AM - 8:48AM |
F29.00004: Spontaneous symmetry breaking induced unidirectional rotation of a chain-grafted colloidal particle in the active bath WENDE TIAN Exploiting the energy of randomly moving active agents such as bacteria is a fascinating way to power a microdevice. Here we show, by simulations, that a chain-grafted disk-like colloidal particle can rotate unidirectionally and hence output work when immersed in a thin film of active particle suspension. The collective spontaneous symmetry breaking of chain configurations is the origin of the unidirectional rotation. Long persistence time, large propelling force and/or small rotating friction are keys to sustaining the collective broken symmetry and realizing the rotation. In the rotating state, we find very simple linear relations, e.g. between the mean angular speed and the propelling force. The time-evolving asymmetry of chain configurations reveals that there are two types of non-rotating state. The basic phenomena are also observed in the macroscopic granular experiments, implying the generic nature of these phenomena. Our findings provide new insights into the collective spontaneous symmetry breaking in active systems with flexible objects and also open the way to conceive new soft/deformable microdevices. |
Tuesday, March 3, 2020 8:48AM - 9:00AM |
F29.00005: Temperature dependence of microtubule-based active fluid Teagan Bate, Edward J Jarvis, Megan Varney, Kun-Ta Wu Kinesin molecular motors step along microtubules in cells to transport cargos with a stepping speed that increases with temperature. This temperature-dependent speed was reported to follow the Arrhenius Law. Kinesin was also used to drive microtubule-based active fluid; however, whether the flow of the fluid follows the Arrhenius Law to accelerate with temperature remains unclear. Here, we systematically characterized the flow speed of kinesin-driven active fluid as a function of temperature, followed by fitting the data to the Arrhenius equation. We characterized the temperature dependence of the flow speed in three different motor systems: (1) processive motors, (2) non-processive motors, and (3) an even mixture of both, finding that these systems followed the Arrhenius Law and developed faster flows with increasing temperature. However, we found that the relation between the flow speed and temperature could be reversed by introducing a depleting agent that had a temperature-dependent micelle formation, causing the active fluid to flow more slowly when heated. Finally, we heated and cooled the fluid repeatedly to accelerate and decelerate the flow sequentially, demonstrating the ability to control the flow speed in real time. |
Tuesday, March 3, 2020 9:00AM - 9:12AM |
F29.00006: Elasticity mediated attraction between active polar particles Rahul Gupta, Raushan Kant, Harsh Soni, Ajay Sood, Sriram Ramaswamy We investigate in experiments and simulations the interaction between a pair of motile granular rods moving through a dense monolayer of beads. Vibration of the supporting surface provides the energy source for motility. Above a critical value of the bead area fraction we find that the rods reorient in such as way as to be drawn towards each other. We present a theory for this active attraction based on the forcing imposed by the motile rods on the crystalline medium of beads, and the action of the medium on the rod orientation. Our theoretically calculated the displacement field around a single rod compares favourably with measurements in simulations. We also discuss the segregation instability for the case of many rods. |
Tuesday, March 3, 2020 9:12AM - 9:24AM |
F29.00007: Light-regulated swimming motility induces cell aggregation in confinement Alexandros Fragkopoulos, Jérémy Vachier, Johannes Frey, Flora-Maud Le Menn, Michael Wilczek, Marco G. Mazza, Oliver Baeumchen A highly concentrated suspension of self-propelled particles can form large-scale concentration patterns, separating into regions of high and low particle concentrations. This phenomenon is induced by the activity of the particles and mediated by their mutual interactions. We observe that a suspension of freely swimming Chlamydomonas reinhardtii cells, a unicellular soil-dwelling microalgae and a model organism of puller-type microswimmers, forms such large-scale aggregations under confinement in specific light conditions. We find that the cell's motility changes under different light conditions. Through active Brownian particle simulations, we show that the change of the motility is sufficient to regulate cell aggregation. Finally, we show evidence that the extent of the photosynthetic activity controls the cell's motility, and consequentially, the aggregation formation. |
Tuesday, March 3, 2020 9:24AM - 9:36AM |
F29.00008: Undulatory intruder dynamics in water-saturated granular beds Arshad Kudrolli, Trinh Huynh We will report an investigation of active intruder dynamics through complex media with {\texit Lumbriculus variegatus} moving through water saturated granular beds which are refractive index matched to enable observation of their motion inside the bed. These common marsh and pond dwelling worms display dual longitudinal peristaltic and transverse undulatory strokes. We will demonstrate that only the undulatory stroke plays a role in locomotion while swimming in water and in very shallow sediment layers which do not permit anchoring. The greater drag anisotropy of the rod like body in the sediments is shown to enable faster locomotion speeds compared to water because of drag-assisted propulsion. By constructing a magnetically driven soft filament robot, we will then describe the observed speeds as a function of undulatory strokes under prescribed conditions which include the undulation frequency and amplitude and the rheology of the medium. |
Tuesday, March 3, 2020 9:36AM - 9:48AM |
F29.00009: Pressure of bacterial suspensions on confining walls and freely moving chains Xiaolei Ma, Shuo Guo, Xinliang Xu, Xiang Cheng An active matter consists of a large number of self-propelling units capable of taking in, employing and dissipating energies. Different from equilibrium systems with detailed balance, active matter exhibits many unusual properties, highlighting the nonequilibrium nature of the system. In particular, mechanical pressure, a state variable in equilibrium systems, shows a nontrivial dependence on system boundary as well as specific measurement methods. Here, we experimentally study the mechanical pressure of bacterial suspensions by exploring the interactions between swimming Escherichia coli and confining walls of different shapes, as well as flexible chains of colloids linked by DNA in a quasi-two-dimensional space. We find that the pressure exerted by E. coli depends nonmonotonically on the apex angle of the V-shaped confining walls. We further characterize the interactions between E. coli and the freely-moving particle chains via imaging the fluctuation of the configuration of the particles and explore their dependence on the rigidity and the length of the chains. A hydrodynamic model is finally proposed to explain our results. Our study sheds light onto the mechanical properties of active matter and provides new understanding on the pressure exerted by swimming microorganisms. |
Tuesday, March 3, 2020 9:48AM - 10:00AM |
F29.00010: Active surfers on a vibrating bath: self-propulsion and interactions Anand Oza, Ian Ho, Giuseppe Pucci, Daniel M Harris We present a combined experimental and theoretical investigation of a newly discovered active matter system consisting of objects floating on a vertically vibrating fluid bath. Recent experiments have demonstrated that such "surfers" oscillate vertically while self-propelling at constant horizontal velocity on the fluid interface. Multiple surfers may self-organize through the collective interfacial deformation in the form of propagating capillary waves. The experiments are complemented by a theoretical model for the surfer’s positional and orientational dynamics. The model predictions exhibit good agreement with experimentally observed interaction modes between two surfers. Generally, this work shows that self-propelling objects coupled by interfacial flows constitute a tunable platform for expanding our understanding of active systems. |
Tuesday, March 3, 2020 10:00AM - 10:12AM |
F29.00011: Nutrient depletion thresholds in 3D printed cell assemblies Cameron Morley, Thomas Angelini Energy consumption is an essential part of active matter; in biological systems energy consumption is often limited by the delivery of oxygen and glucose. Within tissues, these molecules are delivered to cells by a combination of flow through vasculature and intra-tissue diffusion. For dense tissues, the rates of oxygen and glucose diffusion compete with the rates at which they are metabolized, creating a threshold distance from a blood vessel, beyond which cells will become starved. It is almost universally accepted that in tissues of living organisms and in engineered tissues, all cells must be within 200 microns of vasculature because of this competition between diffusion and consumption. However, this rule cannot apply generally to all tissues because cellular metabolic rates are highly variable across cell types and cell densities. In this talk, we will describe experiments that test the 200 micron rule in which we 3D print controlled distributions of hepatocytes into 3D culture medium made from jammed granular microgels. By monitoring cell viability and the consumption of oxygen and glucose, we identify the diffusion-limited crossover to nutrient or oxygen deprived conditions. We vary the size of 3D printed structures and the cell density within them. |
Tuesday, March 3, 2020 10:12AM - 10:24AM |
F29.00012: Control of Active Nematics through Friction Kristian Thijssen, Luuk Metselaar, Tyler N Shendruk, Amin Doostmohammadi, Julia Yeomans Using continuum simulations, we study the impact of anisotropic friction and of position-dependent friction on the emergent flows of active nematics. We show that, depending on whether the active particles align with or tumble in their collectively self-induced flows, anisotropic friction can result in markedly different patterns of motion. For a flow-aligning system with high anisotropic friction, the otherwise chaotic flows are streamlined into flow lanes, reproducing the experimental laning state, which has been obtained by interfacing microtubule-motor protein mixtures with smectic liquid crystals. However, this state is not possible in the flow-tumbling regime, emphasising the role of the flow-tumbling parameter in the dynamics of active nematics. Furthermore, we show that position-dependent friction in active nematic layers allows for the control of the active flows by creating effective hydrodynamic boundaries. Our work demonstrates novel methods to control active matter without the introduction of invasive solid bounding walls. |
Tuesday, March 3, 2020 10:24AM - 10:36AM |
F29.00013: Inertial effects on trapped active matter Mario Sandoval-Espinoza, Lorenzo Gutierrez
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Tuesday, March 3, 2020 10:36AM - 10:48AM |
F29.00014: Wiggling arthropods induce flow in granular materials Melia Kendall, Shih-Yuan Chen, Karen Daniels
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