Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session P61: Active Matter VIFocus
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Sponsoring Units: GSOFT DBIO GSNP Chair: Benjamin Loewe, Syracuse University Room: BCEC 258B |
Wednesday, March 6, 2019 2:30PM - 3:06PM |
P61.00001: Spontaneous buckling of contractile poroelastic actomyosin sheets Invited Speaker: Anne Bernheim-Groswasser Shape transitions in developing organisms can be driven by active stresses, notably, active contractility generated by myosin motors. The mechanisms generating tissue folding are typically studied in epithelia. There, the interaction between cells is also coupled to an elastic substrate, presenting a major difficulty for studying contraction induced folding. Here we study the contraction and buckling of active, initially homogeneous, thin elastic actomyosin networks isolated from bounding surfaces. The network behaves as a poroelastic material, where a flow of fluid is generated during contraction. Contraction starts at the system boundaries, proceeds into the bulk, and eventually leads to spontaneous buckling of the sheet at the periphery. The buckling instability resulted from system self-organization and from the spontaneous emergence of density gradients driven by the active contractility. The buckling wavelength increases linearly with sheet thickness. Our system offers a well-controlled way to study mechanically induced, spontaneous shape transitions in active matter (Ideses Nat. Comm. 2018). |
Wednesday, March 6, 2019 3:06PM - 3:18PM |
P61.00002: ABSTRACT WITHDRAWN
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Wednesday, March 6, 2019 3:18PM - 3:30PM |
P61.00003: Collective dynamics of microtubule-based 3D active fluids from gliding assay Edward Jarvis, Teagan E Bate, Megan Varney, Kun-Ta Wu Flows in passive fluids require temperature or pressure gradients. The gradients are not required for active fluids due to their capability of consuming local fuel to generate kinetic energy. The energy generated at a microscopic scale cascades up, resulting in macroscopic flows. However, the relation between microscopic and macroscopic dynamics remains unclear. Here we approach the problem with molecular motor-driven, microtubule (MT)-based 3D active fluids. We measure their flow mean speed at a millimeter scale, as a function of temperature, comparing with MT 2D gliding assay at a micron scale. We found that despite both systems differed in scales and dimensionality, they responded to temperature similarly. Moreover, such similarity was invariant under the change of motor processivity. Our work demonstrates collective dynamics of microtubule-based active fluids depends primarily on motor’s energy transducing rates, rather than motor’s dynamic details. Our finding expands flexibility in designing 3D active fluids using miscellaneous types of motors, as well as paves the path to outlining principles of self-organization of active fluids. |
Wednesday, March 6, 2019 3:30PM - 3:42PM |
P61.00004: ABSTRACT WITHDRAWN
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Wednesday, March 6, 2019 3:42PM - 3:54PM |
P61.00005: Active phase separation of biphasic polymer gels Nicholas Derr, Christoph Weber, L Mahadevan, Christopher Rycroft Biological systems allow for the generation of active stresses, which can lead to mechanical and chemical instabilities and the formation of patterns. Many relevant systems - e.g. bone, developing tissue, cellular interiors - can be described as multiple immiscible phases of varying rheology, introducing a rich set of couplings between active stress generation and the corresponding passive mechanical responses. One area of particular interest is the generation of contractile forces within the cytoskeleton by the binding of myosin and kinesin motor proteins onto actin and microtubule networks, respectively. In this talk, we present a phenomenological model for active stresses induced by the fuel-dependent binding dynamics of molecular motors in biphasic, incompressible, transiently cross-linked polymer gels. We show that these stresses are analogous to a spatially and temporally varying dis-affinity between polymer and solvent which drives phase separation. The system's behavior on long time scales is investigated by time-integration of the complete non-linear system of gel equations, and the results are contrasted with the case of a demixing passive gel described by classical Flory-Huggins theory. |
Wednesday, March 6, 2019 3:54PM - 4:06PM |
P61.00006: Chaotic mixing in a biological active fluid Amanda Tan, Kevin Mitchell, Linda S. Hirst We study an active self-mixing fluid composed of biopolymers (microtubules) and molecular motors (kinesin). The kinesin motors are clustered together and crosslink bundles of microtubules. As the motors hydrolyze ATP, they walk along the microtubule bundles, forcing the bundles to extend away from each other. When confined in 2D at an oil-water interface, the network forms an active nematic with defects that are continuously created and annihilated. We consider these defects to be virtual stirring rods and the microtubule/kinesin system to be the the fluid. We use fluid dynamic concepts to characterize this new self-mixing fluid by measuring the mixing efficiency, or topological entropy, by coupling beads directly onto the microtubule bundles and tracking their motion as they are mixed. The separation between beads in the material is exponential. We use these trajectories to measure the rate of separation in the material and thereby to calculate the topological entropy. In addition, we change the local stretching rate by varying the ATP concentration to study how changing the energy input on the microscale changes the global mixing efficiency. |
Wednesday, March 6, 2019 4:06PM - 4:18PM |
P61.00007: Topological mixing in 2D active nematic liquid crystals Kevin Mitchell, Amanda Tan, Eric Roberts, Spencer Smith, Linda S. Hirst Recent years have seen a surge of interest in active materials, in which energy injected at the microscale gives rise to larger-scale coherent motion. One prominent example is an active 2D liquid crystal composed of microtubules in the nematic phase. The activity is generated by molecular motors that consume ATP to generate local shearing between the microtubules. The resulting 2D fluid flow exhibits self-generated mesoscale chaotic dynamics with a characteristic folding and stretching pattern. We analyse this dynamics in the context of chaotic advection, in which the fluid can be viewed as "stirred" by the topological defects in the nematic order parameter. We compute the topological entropy from the braiding of these defects and show that all of the entropy arises from the positive one-half defects; the negative one-half defects, which are also present, contribute nothing to the entropy. We also show that the topological entropy generated by this stirring can be understood as a direct consequence of the micro-scale stretching quantified by the Lyapunov exponent, which is computed from PIV data. Our work is based on experimental fluorescence images of the microtubule structure. |
Wednesday, March 6, 2019 4:18PM - 4:30PM |
P61.00008: Active pressure of bacterial suspensions Shuo Guo, Xinliang Xu, Xiang Cheng For an equilibrium system, thermodynamic pressure is considered as a fundamental state variable of the system, which always equals to mechanical pressure, i.e., force per unit area acting on confining walls. In contrast, for an active system out-of-equilibrium such as bacterial suspensions, mechanical pressure is no longer a state variable, which may depend on the stiffness and shape of confining walls. Here, by combining optical tweezers with biochemical engineering technique, we create quasi-two-dimensional bacterial suspensions and systematically study active pressure exerted by swimming E. coli on confining walls of different geometries. In particular, we measure the pressure of bacterial suspensions on V-shaped walls of different angles. We find that the active pressure is a function of the angle of the walls: a sharper angle leads to a stronger pressure. In addition, the fluctuation of pressure increases with decreasing angle. We construct a simple model based on the wall-induced alignment of bacteria to quantitatively explain our observations. Our study provides benchmark experiments for characterizing the mechanical pressure of bacterial suspensions and sheds new light on the nonequilibrium statistical principle of active fluids. |
Wednesday, March 6, 2019 4:30PM - 4:42PM |
P61.00009: Shape-directed rotation of homogeneous micromotors via catalytic self-electrophoresis Allan Brooks, Mykola Tasinkevych, Syeda Sabrina, Darrell Velegol, Ayusman Sen, Kyle J. M. Bishop The purpose of this work is to demonstrate that platinum microparticles with asymmetric geometries move spontaneously in hydrogen peroxide solutions. We can rationally design these motions by controlling particle shape using nanofabrication techniques. We design particles with n-fold rotational symmetry that rotate about their axis at rates specified by their extent of shape asymmetry. Experiments support a self-electrophoretic propulsion mechanism, where anodic oxidation and cathodic reduction of hydrogen peroxide occur at different rates at different locations across the particle surface. We develop a model to explain how the transport-limited electrochemical decomposition of hydrogen peroxide across an asymmetric particle surface leads to electro-osmotic flows that drive particle motion. Our results suggest that geometric control is an effective method to encode micromotor dynamics at the individual particle level. Insights from our proposed mechanism should be useful to design catalytic micromachines with complex dynamics and functions. |
Wednesday, March 6, 2019 4:42PM - 4:54PM |
P61.00010: Bacteria Motility in Porous Media: Not a Random Walk Tapomoy Bhattacharjee, Sujit Datta While bacterial motility is well-studied for motion on flat surfaces or in unconfined liquid media, most bacteria are found in heterogeneous porous media, such as biological gels and tissues, soils, sediments, and subsurface formations. Understanding how confinement alters bacterial motility is therefore critical to model the progression of infections, apply beneficial bacteria for drug delivery, and bioremediation. Unconfined bacteria move via runs and tumbles, leading to random walk-like motion; in a porous medium, previous research has assumed random walk-like motion for bacteria, with a reduced diffusivity due to collisions with obstacles. However, this assumption has never been directly tested due to the inability to visualize processes in opaque 3D media. Here, we directly visualize the motion of single E. coli cells inside a model 3D porous medium, having controlled pore structure. By analyzing the individual cell trajectories, we find that the bacteria do not move via a random walk process. We will present how bacterial motility depends sensitively on pore-scale confinement. Our findings overturn standard assumptions made in the field and provide guidance for the development of more accurate macroscopic models of bacterial motion. |
Wednesday, March 6, 2019 4:54PM - 5:06PM |
P61.00011: Controlling Morphology of Aerotactic Bacterial Bands Hiran Wijesinghe, Eric Mumper, Zachery Oestreicher, Steven Lower, Brian Lower, Ratnasingham Sooryakumar Microorganisms have perfected numerous evolutionary adaptations to improve their odds of survival and proliferation. This study exploits some of these adaptations such as flagellated swimming, aerotaxis and magnetotaxis in magnetic bacteria (MTB) to reshape entire swarms of them. These microorganisms have specialized strategies to navigate oxygen landscapes, of which perhaps the most intriguing is the biomineralization of magnetic nanocrystals that are thought to help them align with the geomagnetic field (magnetotaxis) and swim along natural oxygen gradients. Resulting migration to oxic-anoxic transition zones (OATZ) leads to aerotactic band formation. Many bacteria achieve motility through spinning helical flagellar appendages that produce flow fields that are well described in the far field by analytical models. However near-field dynamics remain elusive. We utilize lattice Boltzmann-based numerical models to simulate near field hydrodynamic interactions, metabolism-dependent oxygen diffusion in the medium, aerotactic response of the cells, as well as field controlled magnetotaxis. Strategies to achieve swarm morphologies beyond simple MTB aerotactic bands will be discussed. |
Wednesday, March 6, 2019 5:06PM - 5:18PM |
P61.00012: Three-body Interactions Drive the Transition to Polar Order in a Simple Flocking Model Purba Chatterjee, Nigel David Goldenfeld Active systems are characterized by a discontinuous flocking transition from a disordered isotropic state to a polar ordered state with increasing density and decreasing noise. A large class of mesoscopic or macroscopic theories for flocking are coarse grained from microscopic models that feature binary interactions as the chief aligning mechanism. However, at the high densities at which the system flocks, binary interactions are too weak to account for the ordering transition. Here we introduce a solvable one-dimensional model of flocking, and derive a series of approximations for the stochastic hydrodynamics. We show that three-body interactions are not only necessary but also sufficient to capture the full phenomenology of flocking. |
Wednesday, March 6, 2019 5:18PM - 5:30PM |
P61.00013: Active reorientation in an active granular system powered by toy vibrobots Kyle Welch, Bo-kai Zhang, Xiang Cheng, Xinliang Xu While emergent collective motion in systems of self-propelled objects is remarkable, one proclivity of active agents, particularly animals, is often overlooked when building models: collision avoidance. We present here an active granular experiment of disks propelled by toy vibrobots, which integrates an “active reorientation” behavior in analogy to collision avoidance in animals. The system demonstrates local flocking, wherein particle velocities locally align. Inspired by this experiment, we develop a computational model of self-propelled disks with an active reorientation mechanism. This simple numerical model exhibits rich phase behaviors: a disordered state, a flocking state and a clustering state under different parametric conditions. We find a notable suppression of aggregation in regions of parameter space corresponding to strong collective motion. Clusters develop quickly in this region, but are metastable, and collapse once the flocking state is achieved. Our experiment and model demonstrate the importance of active reorientation on emergent behaviors in systems of self-propelled agents, and illustrate the profound interplay between different emergent phases of active matter. |
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