Bulletin of the American Physical Society
72nd Annual Meeting of the APS Division of Fluid Dynamics
Volume 64, Number 13
Saturday–Tuesday, November 23–26, 2019; Seattle, Washington
Session C32: Biological Fluid Dynamics: Locomotion and Active Suspension |
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Chair: Tim Pedley, University of Cambridge Room: 614 |
Sunday, November 24, 2019 8:00AM - 8:13AM |
C32.00001: High-accuracy simulations of thousands of deformable, interacting, active droplets David Stein, Yuan-Nan Young, Michael Shelley Coarse-grained continuum models of active fluids capture important aspects of self-organizing behavior seen in experiments while providing a computationally tractable basis for their simulation. Producing accurate solutions to these models is nevertheless challenging, and numerical schemes for their solution have been largely limited to simple, stationary geometries or low-accuracy methods. In this talk, we introduce a spectrally accurate method for the simulation of active fluids in complex geometries, and show how to use this method to simulate deformable droplets of active fluids. When multiple drops are immersed in a Stokesian fluid, their interactions are captured through a robust and scalable boundary integral method, allowing for the simulation of thousands of such particles which may come into near-contact with each other. We explore some of the emergent phenomenon that occurs with such large aggregates of active droplets. [Preview Abstract] |
Sunday, November 24, 2019 8:13AM - 8:26AM |
C32.00002: Linear instability and nonlinear dynamics of a drop and thin film of active fluid Yuan-Nan Young, David Stein, Michael Shelley Active suspensions are fluids with extra stresses from the energy-consuming activity of suspended particles. Coarse-grained continuum descriptions have successfully predicted instabilities and pattern formation observed in some experimental systems. In this work we focus on the effects of surface tension on the stability and nonlinear dynamics in droplets and layers of active fluid. Specifically we study the stability of a moving boundary between a viscous fluid and droplet- or layer-bound active suspension. Linear stability analyses predict parameter regimes for various dynamics such as rotation, self-propulsion, and chaotic dynamics. Weakly nonlinear analyses predict the equilibrium drop deformation as a function of activity magnitude in the suspension. Simulations of a small system of such active drops give insight into how the activity inside the drops dictates how they communicate with each other. [Preview Abstract] |
Sunday, November 24, 2019 8:26AM - 8:39AM |
C32.00003: Towards the rheology of a concentrated array of spherical squirmers Tim Pedley, Takuji Ishikawa, Douglas Brumley Continuum models of dilute suspensions of swimming micro-organisms are well established and can incorporate external (gravitational) forces and torques as well as the particle stress generated by the swimming activity [1]. In a semi-dilute suspension of steady spherical squirmers, hydrodynamic and steric interactions between cells can be computed in a pairwise manner, and Stokesian Dynamics (SD) has been developed for higher concentrations. The stress response to externally applied simple shear has been computed for semi-dilute suspensions [2]. Recently we have examined the stability of a concentrated planar array of identical bottom-heavy squirmers, accounting for cell-cell interactions by the use of lubrication theory [3]. Here we seek to extend this theory to externally driven simple shear, in order to represent the macroscopic shear stress and normal stresses as functions of the shear-rate, the orientation of the applied shear to gravity, and the dimensionless parameters of the squirming motion. Preliminary results are compared with those of a full SD computation. References. [1] Pedley, T.J. \& Kessler, J.O. (1992) Ann. Rev. Fluid Mech. 24:313-358. [2] Ishikawa, T. \& Pedley, T.J. (2007) J.Fluid Mech. 588:399-435.[3] Brumley, D.R. \& Pedley, T.J. (2019) Phys.Rev.Fluids 4:053102 [Preview Abstract] |
Sunday, November 24, 2019 8:39AM - 8:52AM |
C32.00004: Regular and chaotic trajectories of swimmers in two-dimensional, time-independent flows Simon Berman, Kevin Mitchell We present a theoretical investigation of the motion of ellipsoidal swimmers in externally imposed, two-dimensional fluid flows. Two canonical flow geometries are considered: a hyperbolic flow and a vortex array. In the hyperbolic flow, all swimmer trajectories are regular, but we find that they are restricted by one-way barriers. These barriers are shown to be swimming invariant manifolds (SwIMs): invariant manifolds of certain fixed points of the swimmer equations of motion. In the vortex array, we find both regular and chaotic trajectories of swimmers, including trajectories that exhibit long-range transport. This stands in stark contrast to passive tracers in the same flow, which move on localized, regular orbits when the flow is time-independent. The SwIMs are shown to form leaky one-way barriers to swimmers, and we identify additional phase-space structures---namely, periodic orbits and invariant tori---which regulate the chaotic transport of swimmers throughout the vortex array. [Preview Abstract] |
Sunday, November 24, 2019 8:52AM - 9:05AM |
C32.00005: Active mixing of swimming bacteria in hyperbolic and vortex flows Casey Miller, John Buggeln, Julianna Detrick, Bree McCullough, Simon Berman, Tom Solomon We present experiments on the effects of imposed, laminar fluid flows on the motion of active (self-propelled) tracers. The active tracers are bacillus subtilis bacteria, including a wild-type strain and two variations, one with the GFP mutation and one with a smooth-swimming mutation for which the microbe doesn't tumble. The imposed flows are simple hyperbolic flows and vortex chain flows. We test theories that predict ``swimming invariant manifolds'' (SwIMs) that act as one-way barriers that impede the motion of active tracers in the flow. For the hyperbolic flows, we investigate the structure of the barriers as a function of the imposed flow magnitude. For the vortex flow, we investigate the effects of SwIMs that encircle the vortex centers. We also test predictions of chaotic trajectories of smooth-swimming tracers for time-independent, two-dimensional flows. [Preview Abstract] |
Sunday, November 24, 2019 9:05AM - 9:18AM |
C32.00006: Swimming near Tensorial Slip Surfaces Christopher DuPre, Saverio Spagnolie Surfaces with anisotropic mobilities present an intriguing opportunity for passively influencing the motion of microorganisms or active particles in their presence. We study the self-propulsion of a classical model of microorganism locomotion, Taylor's swimming sheet, in the presence of one or two tensorial-slip surfaces. The swimming speed generically includes a lateral motion relative to the direction of undulatory waves on the body, and body rotation, and depends critically upon the distance to the wall(s). The theory also speaks to swimming and crawling near surfaces with rough boundaries. [Preview Abstract] |
Sunday, November 24, 2019 9:18AM - 9:31AM |
C32.00007: Upstream swimming of active Brownian particles in pressure-driven flow Zhiwei Peng, John Brady Active Brownian particles (ABPs) accumulate at boundaries that confine them owing to their persistent self-propulsion. In the presence of a pressure-driven flow, the fluid vorticity tends to rotate ABPs toward the upstream direction. As a result, ABPs in pressure-driven flow can exhibit a net upstream motion. In this work, we use continuum theory and Brownian dynamics simulation to study the effects of pressure-driven flow on the dynamics of ABPs. In particular, we quantify the net mean speed of ABPs in a channel, which results from a competition between downstream fluid advection and upstream swimming. We characterize the transition between net upstream and downstream motion as a function of the flow speed, Brownian diffusion and the intrinsic swimming speed of ABPs. Our results show that the interplay between self-propulsion, fluid vorticity and confinement provides a robust mechanism for upstream motility. [Preview Abstract] |
Sunday, November 24, 2019 9:31AM - 9:44AM |
C32.00008: Rheology of active polar emulsions: from linear to unidirectional and unviscid flow, and intermittent viscosity Giuseppe Negro, Livio Nicola Carenza, Antonio Lamura, Giuseppe Gonnella, Adriano Tiribocchi Active fluids are systems where active components present in the fluid (microtubules with molecular motors such as kinesin or actomyosin bundles) display interesting collective ordering properties. Active fluids also exhibit peculiar rheological properties. Depending on the characteristic of the active stress, activity is capable to heighten viscosity, enough to develop shear-thickening properties in contractile systems or induce in extensile suspensions a superfluid regime under suitable condition. We study, by lattice Boltzmann methods, the rheological behavior of an emulsion made of an active polar component and an isotropic passive fluid. Different flow regimes are found by varying the values of shear rate and extensile activity (occurring, e.g., in microtubule-motor suspensions). By increasing activity, a first transition occurs from linear flow regime to spontaneous persistent unidirectional macro-scale flow, followed by another transition either to (low shear) intermittent flow regime with coexistence of states with positive, negative, and vanishing apparent viscosity, or to (high shear) symmetric shear thinning regime. [Preview Abstract] |
Sunday, November 24, 2019 9:44AM - 9:57AM |
C32.00009: Generalized rheotaxis of active particles in confined Stokes flow William Uspal We consider spherical active particles exposed to shear flow near a planar surface. The swimming activity of a particle gives rise to interactions (e.g., hydrodynamic or chemical interactions) with the surface that couple back to the motion of the particle. Via a dynamical systems approach, we show that a robust directional response can emerge from the interplay of external flow and near-surface swimming activity. For instance, depending on the external flow strength and the character of the swimming activity of the particle, the particle can align against the flow direction (“upstream rheotaxis”) or nearly perpendicular to it (“cross-stream rheotaxis”). As an instructive and analytically tractable example, we apply our findings to the “squirmer” model of a mechanically actuated microswimmer. Using far-field approximations, lubrication theory, and exact numerical calculations, we identify the conditions on the squirming mode amplitudes to obtain the various steady states. Finally, we discuss collective rheotaxis of a group of squirmers. Overall, our findings demonstrate that microswimmers can exhibit surprisingly rich behavior when operating in confined flows, which occur in many of their envisioned applications. [Preview Abstract] |
Sunday, November 24, 2019 9:57AM - 10:10AM |
C32.00010: Spatiotemporal Control of an Extensile Active Nematic Suspension Michael Norton, Michael Hagan, Seth Fraden Active nematic suspensions are self-driven fluids that exhibit rich spatiotemporal dynamics characterized by director field buckling, defect nucleation/annihilation and chaotic trajectories of those defects. Towards developing experimental methods for controlling these dynamics, we consider an optimal control problem which seeks to find the spatiotemporal pattern of active stress strength required to drive the system towards a desired director field configuration. As an exemplar, we consider an extensile active nematic fluid confined to a disk which, in the absence of control, produces two topological defects that perpetually circulate, and seek the time-varying active stress field that drives the system to circulate in the opposite direction. [Preview Abstract] |
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