Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session C61: Active Matter IFocus
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Sponsoring Units: GSOFT DBIO GSNP Chair: Luca Giomi, Leiden University Room: BCEC 258B |
Monday, March 4, 2019 2:30PM - 3:06PM |
C61.00001: Bend instability and topological defects in 3D active nematics Invited Speaker: Guillaume Duclos Active nematics describes a phase of matter where active particles that consume energy to produce mechanical work assemble at high density in a state with orientational order but no positional order. In this talk, I will show how the active nematic framework allows us to better understand aspects of the collective behaviors that emerge in bioinspired materials. In particular, I will present our recent efforts to describe the emergence of flows and topological defects in 3D active nematics composed of a passive colloidal liquid crystal doped with active microtubules and molecular motors. I will first describe the generic bend instability that emerges in a flow-aligned 3D active gel and show how the interplay between activity, nematic elasticity and confinement controls the wavelength of this activity driven instability. I will then present current work on the emergence of flows and topological defect in 3D. |
Monday, March 4, 2019 3:06PM - 3:18PM |
C61.00002: Defect unbinding and a motile Kosterlitz-Thouless transition in active nematics Suraj Shankar, Sriram Ramaswamy, M. Cristina Marchetti, Mark Bowick Active nematic liquid crystals formed by a collection of self-driven particles on a two-dimensional substrate exhibit complex spatio-temporal dynamics and spontaneous defect proliferation. An important consequence of the non-equilibrium drive is the spontaneous motility of strength +1/2 disclinations that drives flow in the system. Starting from the hydrodynamic equations of active nematics, we derive effective equations for the topological defects as interacting overdamped particles with pair forces and active torques. Using these equations we then show that activity lowers the defect-unbinding transition temperature driving a nonequilibrium variant of the Kosterlitz-Thouless transition into a state of defect chaos. Crucially, we find rotational noise stabilizes nematic order at low activity leading to a re-entrant transition. For large activity, orientational torques on the defects combined with many-body screening allows the spontaneous appearance of a polar defect ordered liquid, rationalizing previous work into a comprehensive phase diagram for two-dimensional active nematics. |
Monday, March 4, 2019 3:18PM - 3:30PM |
C61.00003: Activity driven defect ordering in an active nematic on an anisotropic substrate Daniel Pearce We investigate the effect of an anisotropic substrate on the turbulent dynamics of a simulated two dimensional active nematic. This results in an effectinve anisotropic friction and viscosity, with the orientation of the anisotropy being defined by the substrate. In this system we observe the emergence of global nematic order of topological defects that is controlled by the degree of anisotropy in the viscosity and the magnitude of the active stress. No such alignment is seen in passive liquid crystals with anisotropic viscosity confirming that ordering is driven by the active stress. We then closely examine the active flow generated by a single defect to show that the kinetic energy of the flow is orientation dependent, resulting in a torque on the defect to align them with the anisotropy in the substrate. |
Monday, March 4, 2019 3:30PM - 3:42PM |
C61.00004: Self-organized dynamics of confined active nematics Achini Opathalage, Michael Norton, Michael P. N. Juniper, S.Ali Aghvami, Blake Langeslay, Seth Fraden, Zvonimir Dogic We study the role of boundary conditions on a simplified experimental model of biological active matter composed of extensile filamentous bundles of microtubules driven by clusters of kinesin motors, to elucidate the structure and dynamics of active nematic liquid crystals. These bundles form a quasi-2D active nematic liquid crystals when sedimented onto a surfactant-stabilized oil-water interface. We further confine this system onto circular boundary conditions, imposing a total topological charge of +1. For diameters of 400 micrometer and larger, multiple +/- ½ defects continuously nucleate and annihilate at the boundary as well as in the confinement core and generate flows of either handedness. As the diameter is reduced, defects periodically nucleate at the boundary with slow dynamics and migrate toward the confinement core rendering a fast pairwise procession, referred to as doubly-periodic dynamics. Existing continuum theories fail to predict this phenomenon and we hypothesize what additional physics needs to be included to reconcile experiment and theory. |
Monday, March 4, 2019 3:42PM - 3:54PM |
C61.00005: Fractal tree of elastic bands in living liquid crystal Andrey Sokolov, Rui Zhang, Ali Mozaffari, Juan De Pablo, Alexey Snezhko Living liquid crystal (LLC), a synthetic material combining a lyotropic liquid crystal and swimming bacteria, demonstrates a number of out-of-equilibrium phenomena ranging from active turbulence to creation and annihilation of motile topological defects. In this talk, we report a spontaneous formation of undulation bands in circularly aligned LLC. The interplay between elasticity of liquid crystal and bacteria-induced hydrodynamic forces results in the emergence of a remarkable branched pattern of radially elongated bands of a highly curved director field. The average number of such branches is increasing with the distance from the center of the pattern and leads to the emergence of a radial fractal tree or a snowflake structure. Our experimental observations show that the shape of this structure, as well as a number of bands, is controlled by the activity of bacteria and elasticity of liquid crystal and suggest a new concept for a manipulation of active nematics at a microscale. |
Monday, March 4, 2019 3:54PM - 4:06PM |
C61.00006: Control of Topological Defects and Spontaneous Flows in Engineered Active Liquid Crystals Rui Zhang, Steven Redford, Paul Ruijgrok, Nitin Kumar, Ali Mozaffari, Aaron Dinner, Vincenzo Vitelli, Zev Bryant, Margaret Gardel, Juan De Pablo Active matter gives rise to intriguing spontaneous flows and motion whose collective behavior is difficult to steer and tailor, thereby placing limits on potential applications. Here, we present a new method to control the dynamics of an active liquid crystal. Theory shows that through the interplay of local active stresses it is possible to create anchoring effects that help confine activity-induced topological defects. This prediction is confirmed by hydrodynamic simulations of active nematics. Simulations further demonstrate that at moderate-to-high activity, local active stresses can be used to create defect pairs at will and direct the motion of +1/2 defects on demand; at low activity, such stresses can induce a spontaneous flow without strong elastic distortions of the nematic. Our calculations are compared to experiments of a flexible active liquid crystal. |
Monday, March 4, 2019 4:06PM - 4:18PM |
C61.00007: Instability Driven Pattern Formation in Active Nematics Ali Mozaffari, Rui Zhang, Andrey Sokolov, Alexey Snezhko, Juan De Pablo Active nematics represent a class of active systems, operating out of |
(Author Not Attending)
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C61.00008: “Catapulting” of topological defects through elasticity bands in active nematics Nitin Kumar, Rui Zhang, Steven Redford, Juan De Pablo, Margaret Gardel Local injection of energy in active liquid crystals (LC) results in spontaneous defect generation and emerging complex flows. Elucidating the role of competition between activity and nematic elasticity is crucial to understanding this phenomenon. Here, we present our experimental results on nematic LCs composed of short actin filaments, driven by myosin motors where the elasticity is tuned by varying the filament length, l. We find that for l = 2 µm where the elasticity is high compared to an LC with l = 1 µm, elongated regions of uniform bend distortions form, which we define as elasticity bands. The bands are evocative of domain walls observed in hydrodynamic simulations of active nematics. The emergence of a shoulder in the elastic energy distribution provides an explanation – a nematic LC with high elasticity tends to minimize the total elastic deformation by localizing it to narrow regions in space. Moreover, we find that as the activity decays, the LC dissipates excess elastic energy by eliminating these bands resulting in “catapulting” of +1/2 defects at a very high speed which scales inversely with the width of the band. Our results are fully supported by hydrodynamic simulations of active nematics and advance our understanding of complex flows in active liquid crystals. |
Monday, March 4, 2019 4:30PM - 4:42PM |
C61.00009: Emergent lengthscales in confined 3D Active Incipient Nematics Minu Varghese, Yi Fan, Arvind Baskaran, Kun-Ta Wu, Zvonimir Dogic, Seth Fraden, Michael F Hagan, Kenny Breuer, Aparna Baskaran An incipient nematic is a system whose density is smaller than the critical density for isotropic-nematic transition. The stationary isotropic state, which is the stable equilibrium of a passive incipient nematic, can be destabilized by the effects of extensile activity combined with flow alignment. We study the properties of flows that arise from this instability in confined 3D systems. Calculations from hydrodynamic theory, and experimental measurements on a microtubule-based system show long-range velocity correlations, in the absence of such correlations in nematic order. Further, we show that there exists a confinement-independent lengthscale intrinsic to flows in an active incipient nematic that determines its bulk behavior. |
Monday, March 4, 2019 4:42PM - 4:54PM |
C61.00010: Avalanches and Clogging in Active Matter Systems Cynthia Reichhardt, Charles Reichhardt Jamming and clogging have been extensively studied in passive soft matter systems such as granular matter or colloids moving through disordered environments such as obstacle arrays. Here we consider a system of disks moving through a random obstacle array and examine the transition to a clogged state as function of obstacle density as we go from the passive particle limit to an active matter limit. In the passive case, for a fixed disk density there is a well defined obstacle density above which the system reaches a clogged state. As the activity is increased, the disks can become mobile again when the activity breaks up the clogs; however, for large activity, which corresponds to long run times in a run-or-tumble system, the disks undergo intermittent clogging and we find that above a certain activity level, the unclogging events exhibit a power law distribution. We argue that the activity induced clustering brings the system into a critical state associated with local regions that have reached the jamming density known as point J. For infinite run times or the ballistic active matter limit, the system can reach a completely clogged state with no avalanches for disk densities that are much lower than those at which clogging is observed for passive particles. |
Monday, March 4, 2019 4:54PM - 5:06PM |
C61.00011: Collective ordering of bacterial vortices controlled by geometry and frustration Kazusa Beppu, Ziane IZRI, Yusuke T. Maeda Controlling collective patterns has attracted interest due to their potential in exploiting their hidden ordered phase of active bacterial suspension. Here, by imposing a geometric boundary condition, we study controlled collective motion of Escherichia coli bacteria inside designed microwells. In a doublet circles, two vortices emerge but their spinning directions show two distinct phases of either parallel pattern (ferromagnetic vortices, FMV) or anti-parallel one (anti-ferromagnetic vortices, AFMV). The transition from FMV to AFMV occurs when the ratio of vortex size to vortex distance is sqrt2. Analytical solution with mean-field approximation accounts for this geometric rule [1]. By using this relation, we can control quadruplet pairing of bacterial vortices. Moreover, in a triplet circles, coexistence of FMV and AFMV pairings is emerged despite frustration, and the transition point from FMV pattern to coexisted phase is shifted because frustration stabilizes FMV pattern. Our result proposes simple design of boundary as promising mean in order to understand collective ordering of bacterial vortices. |
Monday, March 4, 2019 5:06PM - 5:18PM |
C61.00012: Swimming bacteria swirl around nematic attractors Runa Koizumi, Taras Turiv, Robert J. Lastowski, Hao Yu, Qi-Huo Wei, O D Lavrentovich Microswimmers such as bacteria exhibit collective behavior that can be controlled when placed in a nematic liquid crystal (NLC) with long-range orientational order [1]. We explore the collective motion of motile Bacillus subtilis dispersed in an aqueous solution of DSCG, a lyotropic chromonic NLC. The director field, imposed through photoalignment, contains defects of topological charge +1 which serve as attractors for bacteria. We vary the azimuthal angle of the director from 0 to 90 degrees to have predominant director distortions of either splay, bend, or a mixture of the two. The bacteria exhibit collective circular motion around +1 defects, with the radius of maximum concentration and velocity increasing as bend distortion dominates over splay. The experiment presents an example of how microswimmers interact with attractor type singular defects with different degrees of splay or bend deformation. The ability to control the collective motion of microswimmers can be used as a source of energy to power microscopic mechanical systems. |
Monday, March 4, 2019 5:18PM - 5:30PM |
C61.00013: Kinetics and Order-Disorder Transitions in Hydrodynamically Self-Assembled Magnetotactic Bacteria Christopher Pierce, Hiran Wijesinghe, Eric Mumper, Brian Lower, Steven Lower, R Sooryakumar Magnetotactic bacteria are inherently magnetic motile micro-organisms. Hence, their swimming orientations are readily controlled by external magnetic fields making them a valuable model active matter system. When oriented perpendicular to a surface, they experience attractive hydrodynamic interactions that result in spontaneous self-organization (clustering). By tuning the cell density and magnetic field strength, this ordering may be systematically removed and restored, permitting construction of a phase boundary defining the onset of ordering and experimental exploration of cluster kinetics. The self-organization process is quantified using an order-parameter independent approach based on lossless compression algorithms (Lempel-Ziv), as well as a more conventional method employing the radial distribution function. These analyses reveal that the clusters scale logarithmically in time, representing a non-equilibrium analog of the “self-focusing” regime of charged colloids, and implying that the interplay between hydrodynamic attraction and magnetic repulsion control the kinetics. Furthermore connections between experimentally determined pair-wise interactions and the many-body dynamics, as well as the role viscous dissipation plays in stabilizing the structures will be discussed. |
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