Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session D29: Active Matter in Complex Environments IIIFocus
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Sponsoring Units: DSOFT DBIO DFD GSNP Chair: Sujit Datta, Princeton University Room: 501 |
Monday, March 2, 2020 2:30PM - 3:06PM |
D29.00001: Spatial-temporal organization of bacterial suspensions under confinement Invited Speaker: Igor Aronson Suspensions of motile bacteria or synthetic microswimmers, termed active matter, manifest a remarkable propensity for self-organization and formation of large-scale coherent structures. Most active matter research deals with almost homogeneous in space systems and little is known about the dynamics of active matter under strong confinement. We present experimental and theoretical studies on the expansion of highly concentrated bacterial droplets into an ambient bacteria-free fluid. The droplet is formed beneath a rapidly rotating solid macroscopic particle inserted in the suspension. We observed vigorous inability of the droplet reminiscent of a supernova explosion. The phenomenon is explained in terms of continuum first-principle theory based on the swim pressure concept. Furthermore, we investigated self-organization of a concentrated suspension of motile bacteria Bacillus subtilis constrained by two-dimensional (2D) periodic arrays of microscopic vertical pillars. We show that bacteria self-organize into a lattice of hydrodynamically bound vortices with a long-range antiferromagnetic order controlled by the pillars’ spacing. Our findings provide insights into the dynamics of active matter under extreme conditions and significantly expand the scope of experimental and analytic tools for the control and manipulation of active systems. |
Monday, March 2, 2020 3:06PM - 3:18PM |
D29.00002: Trapping of microswimmers in vortex flows Simon Berman, Kevin A Mitchell We theoretically investigate the trapping of rigid, ellipsoidal microswimmers in externally-driven two-dimensional vortex flows. Surprisingly, for swimmers that swim perpendicular to their elongation direction, we find that trapping depends non-monotonically on swimming speed. We identify certain stable periodic solutions of the swimmer equations of motion as the cause of trapping in individual vortices. A bifurcation analysis of these solutions explains the dependence of trapping on swimmer speed, shape, and swimming direction. We propose heteroclinic bifurcations between swimming fixed points as a general mechanism for the creation of stable swimmer trajectories. |
Monday, March 2, 2020 3:18PM - 3:30PM |
D29.00003: Sensorimotor processing and navigation in confined microswimmers Samuel Bentley, Vasileios Anagnostidis, Fabrice Gielen, Kirsty Wan All living organisms are environmentally intelligent. This is the fundamental distinction between life, and other forms of matter. Even unicellular organisms are capable of complex behaviours, for they can sense as well as respond to changes in the environment. Here, we study spontaneous and constrained motor actions in algal microswimmers, using motility as a dynamic read-out of behaviour and physiology. Previous studies have focussed on locomotion transients over short timescales ranging from milliseconds to minutes. We present a novel microfluidic platform which allowed us for the first time to monitor and analyse algal cell motility over hours, and even developmental timescales. We focus on two species, a biflagellate which exhibits a form of run-and-tumble, and an octoflagellate which exhibits a tripartite behavioural repertoire termed run-stop-shock. Excitability and stochastic transitions in swimming gait are projected onto a low-dimensional state space. We reveal how flagellar mechanosensitivity mediates repetitive boundary interactions, and discuss the discovery of a light-dependent quiescent regime. Finally, we conduct pharmacological perturbations within these microenvironments, to shed new light on the physiological origins of excitable flagellar dynamics. |
Monday, March 2, 2020 3:30PM - 3:42PM |
D29.00004: Topographical guidance of highly-motile cells using cell-sized features Joeri Wondergem, Koen K Schakenraad, Patrick Witzel, Maria Mytiliniou, David Holcman, Doris Heinrich Cells navigate complex environments where they encounter a variety of physical cues. The extracellular topography is such a cue, as a substrate of anisotropically placed, sub-micron sized pillars, was recently shown to induce directed cell migration, a process named topotaxis. Here, we focused on a larger scale, and studied the influence of cell-sized obstacles on highly-motile, persistent cell migration. Using engineered topographies, reminiscent of pores cells squeeze though in vivo, and tracking cell migration, we explored the effects of spacing and obstacle geometry on cell motility. We show that cells undergo long-range topotaxis over large distances by anisotropically placed cell-sized obstacles. Furthermore, we performed active particle simulations, a minimal model for cell motility, to investigate the physics needed to reproduce key features of cell motion in different topographies. Finally, we show that long-range topographic guidance is conserved in chemical gradients, and moreover, the cues, topotaxis and chemotaxis, abide to linear summation, guiding the cell in the direction of both cues combined. Hence, topotaxis offers an independent way to steer cells, which can be exploited as a tool for in vitro applications, like lab on chip diagnosis and tissue engineering. |
Monday, March 2, 2020 3:42PM - 3:54PM |
D29.00005: Quenching active matter using light: Light mediated motility in swarming Serratia marcescens Arvind Gopinath, Junyi Yang, Alison Patteson, Paulo Arratia Understanding the connection between external stimuli such as light and emergent response manifested in multicellular, collective and long-range motility of cells and cell-like active systems remains a challenge. Swarming, a mode of bacterial surface migration is characterized by emergent flows, persistent structures and collaborative collective motility. Here we present the effects of light on the collective motility in a dense and far from equilibrium active system – swarms of the bacteria Serratia marcescens. Using a experimental light-based technique, we induce and generate localized condensed domains differing in motility. We then study immobilization and quenching of flow inside these domains and map the response to light in terms of the light intensity and duration of exposure. Together these parameters determine the reversibility of the response, domain size and extent of the mobility impaired region as well as its temporal evolution. Complementing our experimental results, we propose and analyze a minimal Brownian dynamics model to study the escape of bacteria from the exposed region before they are completely immobilized and trapped. |
Monday, March 2, 2020 3:54PM - 4:06PM |
D29.00006: Submerged micro-strucutures generate a soft boundary effect on active nematic flows Dimitrius Khaladj, Amanda Tan, Linda S. Hirst Actively driven bundled microtubule networks have become a useful framework to study energy driven defects in structured fluids. Moreover, the implementation of soft boundaries instead of hard side walls can be a useful strategy to control defect flow and dynamics. We study the behavior of an active nematic microtubule system confined by submerged complex geometries. From our preliminary work, we have observed that submerged 3D structures can influence defect dynamics. We also demonstrate that the soft barrier generated by the submerged structures form stagnation points near the boundary reminiscent of those seen for hard boundaries. With the assistance of micro fabrication, we investigate the spontaneous flows of this novel system under confined conditions i.e. in proximity to the boundary of the submerged structures. For this investigation, we used cylindrical pillars and rectangular trenches. We are also interested to learn if the submerged surfaces will impact chaotic mixing dynamics. |
Monday, March 2, 2020 4:06PM - 4:18PM |
D29.00007: Lattices by design: exploring long-ranged interactions with gradient sensing droplets Anton Molina, Manu Prakash, Shailabh Kumar Many body interactions are ubiquitous and occur across length scales, giving rise to a vast diversity of phenomena ranging from frustration to pattern formation with important implications for the design of new materials. However, their behavior can be challenging to predict since the number of relevant configurations scales exponentially with the number of particles. Two-component Marangoni-contracted droplets have been shown to interact via long-ranged vapor mediated attractions, resulting in a dynamic behavior that resembles chemotaxis and a capacity to self-organize. Meanwhile, hydrophobic barriers can create well-defined potential energy wells that are impassable to droplets but transparent to a vapor gradient landscape. Here, we develop exotic lattice systems that exploit the tools of photolithography to fabricate arbitrary patterns of hydrophobic boundaries, enabling us to experimentally manipulate the geometry in which long-ranged degrees of freedom interact. The macroscopic nature of this system provides real time access to microstate configurations, providing the opportunity for mechanistic insights and the development of control strategies in complex, interacting systems. |
Monday, March 2, 2020 4:18PM - 4:30PM |
D29.00008: Arresting Active Spinning Particle Coarsening in Passive Media via Actuation Protocols Joshua Steimel, Ryan Tollefsen, Alfredo Alexander-Katz Active matter systems are unique in the rich non-equilibrium dynamical behavior that emerges such as flocking, lanning, swarming, and vortexting. Such emergent behavior has been previously observed in complex hybrid active-inactive systems with in-plane rotating magnetic particles; where active particle attraction is induced by the activity of the particles and the mechanical properties of the passive media. When continuously actuated these active particles will aggregate and coarsen. We present a novel approach which allows the control of coarsening behavior and resultant characteristic domain size via modulation of the activity actuation protocol. By tuning the frequency at which the rotational direction of the active spinning magnetic particles is changed, either clockwise or counter-clockwise, the system can exhibit microphase separation behavior distinct from the macrophase separation of active and passive particles. Changing the rotational direction is effectively equivalent to reversing the torque and unloading the stress in the system. This limits the ability of active particle clusters to attract, can arrest the coarsening behavior, and decrease the characteristic domain size. Thus, allowing control of the structure of these hybrid active-inactive systems. |
Monday, March 2, 2020 4:30PM - 4:42PM |
D29.00009: Flocking Transition in a Self-propelled Particle Model Using Experimental Motility Conditions Jelani Lyles, Paul Yanka, Chih Kuan Tung, Daniel Sussman, M. Lisa Manning Flocking transition has been studied using self-propelled particle models for decades. In these models, the initial angular distribution is random, the step-by-step angular fluctuation is either a bounded flat noise or a Gaussian noise, and the magnitude of the velocity of each moving particle is thought to be a constant. Experimental study of sperm flocking show that an aligned initial condition promotes sperm to form large flocks, angular fluctuation follows an exponential decay, and the velocity distribution follows a Gamma distribution. Our research has focused on using a computational model to understand the effects from those differences between experimental observation and the traditional model conditions. We found that aligned initial condition does help sperm to form larger flocks when the system is at the transition, but not much effect elsewhere. No major difference was seen between exponential and Gaussian angular noise. The Gamma velocity distribution was found to lower the density of the flocks. Our results provide evidence to rethink adapting the conclusion from active matter models to experimental systems. |
Monday, March 2, 2020 4:42PM - 4:54PM |
D29.00010: Re-entrant self pumping in confined active fluids Minu Varghese, Arvind Baskaran, Michael Hagan, Aparna Baskaran Spontaneous flows in the form of swirls is ubiquitous in biological systems. Such active swirling/turbulent flows self organize into net pumping flows when confined appropriately. Studies on 2D systems suggest that self pumping results when the confinement length scale is smaller than the size of swirls that form spontaneously in the bulk. However, recent experiments have shown that when confined in 3D channels, the emergence of self pumping can be more sensitive to the aspect ratio of the channel than to the size of the channel itself. This is a phenomenon that is unique to 3D and to active fluids. In this talk, I will discuss the origin of this phenomenon using an active hydrodynamic theory. |
Monday, March 2, 2020 4:54PM - 5:06PM |
D29.00011: Tunable self-organization of swimming magnetic bacterial suspensions Hiran Wijesinghe, Christopher J Pierce, Eric Mumper, Brian Lower, Steven Lower, Ratnasingham Sooryakumar When units of active matter are brought together, the properties of the new construct are not always equal to the sum of the properties of the components. Instead, at each level, new behavior and rules (i.e., emergent properties) appear. For instance, the emergence of spontaneous order in collections of disordered biological components, from suspensions of biofilaments to bird flocks, is widespread in nature. We present population-level self-organization in a flagellated magnetic bacterial suspension of Magnetospirillum magneticum (AMB-1) that is amenable to experimental control using programmable fields. A major hindrance to the understanding of the emergent properties in a typical biological system is its complexity due to competing inter-component interactions, biological and external constraints, non-equilibrium dynamics, as well as stochasticity. Detailed computational modeling of hydrodynamics and magneto-aerotaxis in these motile AMB-1 suspensions reveals the underlying mechanisms leading to their self-organization, by producing experimentally verifiable population-level dynamics. This approach can be generalized to other species and active matter systems in general. |
Monday, March 2, 2020 5:06PM - 5:18PM |
D29.00012: Dynamics of 2D Active Nematics Confined in an Annulus Chaitanya Joshi, Zahra Zarei, Michael M Norton, Seth Fraden, Aparna Baskaran, Michael Hagan Active nematics are a class of non-equilibrium systems with constituents that consume energy |
Monday, March 2, 2020 5:18PM - 5:30PM |
D29.00013: Self-organization of bacterial active matter in space and time Song Liu, Yilin Wu Simultaneous control of spatial and temporal organization of active matter is challenging and generally requires complex interactions. Here we found that tuning the rheological properties of bacterial active fluids enables large-scale spatial and temporal self-organization. Combining experiments with an active matter model, we explain the phenomenon 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. |
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