Bulletin of the American Physical Society
74th Annual Meeting of the APS Division of Fluid Dynamics
Volume 66, Number 17
Sunday–Tuesday, November 21–23, 2021; Phoenix Convention Center, Phoenix, Arizona
Session T13: Biological Fluid Dynamics: Locomotion, Active Suspensions & Non-Newtonian Fluids |
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Chair: Roberto Zenit, Brown University Room: North 127 ABC |
Tuesday, November 23, 2021 12:40PM - 12:53PM |
T13.00001: Reverse osmotic propulsion (Edmond) Tingtao Zhou, Zhiwei Peng, John F Brady Modern biomedical applications such as targeted drug delivery require a delivery system capable of enhanced transport beyond that of passive Brownian diffusion. In this work an osmotic mechanism for the propulsion of vesicles is proposed. By maintaining a solute gradient inside the vesicle, a seepage flow of the solvent (water) across the membrane is generated which in turn propels the vesicle. We develop a theoretical model for this vesicle-solute system in which the fluid flow through the membrane is described by a model similar to Darcy's law. Using this model, we characterize the motility of the vesicle in relation to the concentration distribution inside the vesicle. We show by explicit calculation that for a weakly permeable membrane the interior fluid flows from the regions of high solute concentration to low—-a reverse osmotic flow. Any osmotic solute is able to propel the vesicle so long as a concentration gradient is present. To maintain such a gradient, we propose to use active Brownian particles (ABPs) with spatially varying activity as the solute. By tuning the swim speed distribution of ABPs confined inside the vesicle, a spherically asymmetric density distribution can emerge and lead to net motion of the vesicle. |
Tuesday, November 23, 2021 12:53PM - 1:06PM |
T13.00002: Two-dimensional active flow around a rotating disk Wan Luo, Robert A Pelcovits, Thomas R Powers We study the two-dimensional flow of an active isotropic nematic induced by a rotating disk. Using a time-dependent minimal hydrodynamical model for the fluid, we numerically calculate the velocity and the order parameter fields of the active flow. For needle-like particles, when the activity is less than the critical value at which the active fluid becomes unstable, activity helps the rotation of the disk and increases the relaxation time of the ordering in the extensile case, but activity resists the rotation and decreases the relaxation time in the contractile case. Additionally, increasing the size of the disk can enhance the effect of activity. We also study the effect of the external flow on the unstable. |
Tuesday, November 23, 2021 1:06PM - 1:19PM |
T13.00003: Microswimmer suspensions in 2D random porous media Akhil Varma, David Saintillan Suspensions of motile microswimmers such as bacteria and active colloids often encounter porous environments both in nature and in industrial applications, but much remains unknown about their mechanistic behaviour and transport properties in the medium. Here, we explore microswimmer dynamics in a saturated 2D porous medium, at the scale of the constituent microscopic solid inclusions. To this end, we model the microswimmers as point-sized active Brownian particles in a large doubly-periodic domain containing randomly distributed polydisperse solid inclusions. The Brownian dynamics simulations reveal the influence of the randomness of the medium on the evolution of statistical quantities of the suspension such as the number density and the polarization near the surface of the inclusions and in the fluid channels. In particular, we study how the 'activity' of the microswimmers, together with the spatial constrictions and local heterogeneities of the surrounding medium, affect these statistics. We also examine the effects of an externally-imposed pressure-driven Stokes flow through the porous matrix, where advection and local shear-induced reorientation modify the dynamics and transport of the microswimmers. |
Tuesday, November 23, 2021 1:19PM - 1:32PM |
T13.00004: HYDRODYNAMICS OF ACTIVE BACTERIA SUSPENSIONS IN A HELE-SHAW CELL. Akash Ganesh, Harold Auradou, Carine Douarche Hydrodynamic dispersion between two miscible passive fluids in a Hele-Shaw cell have been studied extensively both theoretically and experimentally over the years. However, the dispersion of active bacteria suspensions in shear flows is still poorly understood although it is a key for a wide range of applications in the fields of biology,environmental engineering, etc. The present work focuses on studying the dispersion of an active bacteria suspension subject to flow and displacing a miscible passive fluid in a Hele-Shaw cell. We monitor the spatio-temporal longitudinal concentration profiles of the fluorescent bacteria suspension for multiple initial cell concentrations, degrees of confinement and imposed average flow velocities. We observe an increased dispersivity of an active bacteria suspension when compared to that predicted by the classical Taylor dispersion model for a passive point particle suspension. We believe that this study would enhance our understanding of active dispersion of bacteria through porous media, on surfaces etc. where shear flows are ubiquitous. |
Tuesday, November 23, 2021 1:32PM - 1:45PM |
T13.00005: Self-similar clustering of beads in a bacterial suspension Frederic Moisy, Julien Bouvard, Harold Auradou Passive particles suspended in a bath of active swimmers are known to produce cooperative phenomena such as dynamic clustering or phase separation. However, such clusters have never been reported experimentally for particles in a bacterial suspension. Here we report experimental observations of two-dimensional clustering of micron-sized beads in a bath of Burkholderia contaminans bacteria. This clustering is induced by the short-range hydrodynamic attraction produced by the swimming of bacteria between neighboring beads. The characteristic cluster size, computed from the correlation length of the fluorescence intensity pattern, shows a self-similar growth in time. We explore this coarsening dynamics by varying the radius and surface fraction of the beads and the bacterial concentration. |
Tuesday, November 23, 2021 1:45PM - 1:58PM |
T13.00006: Tractionless self-propulsion of an active drop: a mechanism for cell motion without adhesion Aurore Loisy, Jens G Eggers, Tanniemola B Liverpool The autonomous motion exhibited by living cells, called motility, is fundamental to many physiological and pathological processes. Because it is easily observable, most studies of cell motility have focused on crawling, a mode of motion exhibited by cells on surfaces. Crawling crucially relies on focal adhesions with the surface, which allow a cell to exert traction and thereby propel itself forward. This implies that crawling is not suited for motion on nonadhesive surfaces or for moving rapidly through three dimensional environments such as tissues. How cells achieve self-propulsion in such environments remains a debated question. In this talk, we will report on a new mode of motility whose signature is the absence of traction exerted on the surrounding environment. We show, both analytically and by numerical simulations, that the equations of motion for a thin drop of active nematic (a minimal model of the cytoskeleton) on a substrate possess a simple self-propelling solution with no traction imparted on the solid surface. This mode of self-propusion is driven in the bulk (rather than at the boundaries in crawling) and provides a robust physical mechanism for adhesion-independent cell migration in crowded environments like tissues. |
Tuesday, November 23, 2021 1:58PM - 2:11PM |
T13.00007: Self-propulsion of a freely-suspended, rotationally-symmetric swimmer enabled by viscoelastic normal stresses, Part I: Theory and simulation Jeremy P Binagia, Laurel A. A Kroo, Manu Prakash, Eric S G Shaqfeh Recently there has been interest in developing novel propulsion mechanisms that leverage the rheology of the surrounding fluid. Such artificial microswimmers could be used in a variety of applications, e.g. to infer the properties of the fluid they are immersed in or as a model for studying motility in complex fluids. To this end, we've designed a simple force- and torque-free swimmer consisting of two counter-rotating bodies of revolution(e.g. unequal sized spheres). Using a combination of microhydrodynamic theory and numerical simulations, we demonstrate that such a swimmer displays zero net translation in a Newtonian fluid at zero Reynolds numbers but translates in the direction of the larger sphere when placed in a viscoelastic fluid. We find that the swimming speed is nearly linear in the Deborah number (De), the appropriate measure of the elasticity of the fluid, and is nearly linear in the concentration of polymer in the fluid at low De. By considering a variety of relative sizes and shapes for the two halves of the swimmer, we can determine the specific geometry that maximizes the swimmer's speed. Part II of this talk will focus on recent experimental work we've conducted to realize this model swimmer in the lab, with comparison to theoretical and numerical predictions. |
Tuesday, November 23, 2021 2:11PM - 2:24PM |
T13.00008: Self-propulsion of a freely-suspended, rotationally-symmetric swimmer enabled by viscoelastic normal stresses, Part II: Experiment Laurel A. A Kroo, Jeremy P Binagia, Manu Prakash, Eric S G Shaqfeh Building upon theoretical results describing a novel swimmer consisting of counter-rotating symmetric objects (Part I), here we present a physical implementation as an untethered robot capable of propulsion at low Reynolds number, only when submerged in a non-Newtonian fluid. This optically controlled, battery-powered robot propels itself in the direction of the larger “head”. We directly compare the experimental performance of this swimmer to theoretical and numerical predictions. Variations in the geometry of the robot are explored (tail sizes, shapes) to optimize the forward propulsion speed. By controlling the relative rotation rate while recording motility, we propose a novel application of this robot to function as a rheological probe of its local surrounding fluid. We assess the accuracy and limitations of such an approach in several fluids, by comparing the measured primary normal elastic stress to measurements taken with a standard benchtop rheometer. This proof-of-concept device experimentally demonstrates that a unique propulsion mechanism exists for elastic, non-Newtonian fluids. |
Tuesday, November 23, 2021 2:24PM - 2:37PM |
T13.00009: Swimming speed enhancement of a helix in viscoelastic fluids Yunxing Su, Veronica Angeles, Roberto Zenit We report on the viscoelastic effects on the swimming speed of a model helix. In this study, we experimentally measure the thrust and drag forces on a rotating and translating helix in viscous fluids using a novel technique. The swimming speed of the helix is then inferred from the balance between the measured thrust and drag forces on the helix. Three helices of different pitch angles are tested in both Newtonian fluids and viscoelastic Boger fluids. Compared with the results from Newtonian fluids, we found that the swimming speed can be larger in viscoelastic fluids, depending on the Deborah number (De) and the helix geometry (pitch angle). When the Deborah number is close to 1 (De ~ 1), the swimming speed enhancement reaches its maximum. When the Deborah number is De < 1 or De > 1, the swimming speed enhancement decreases and can actually fall below the Newtonian swimming speed. In addition, the viscoelastic swimming speed enhancement also depends strongly on the helix pitch angle . Particle Image Velocimetry measurements are also performed to understand this swimming speed enhancement via the fluid pumping induced by the helix. |
Tuesday, November 23, 2021 2:37PM - 2:50PM |
T13.00010: Mixing and stretching in microfluidics with active matter Fan Yang, David Larios, Matt Thomson Microtubules and motor proteins have attracted great scientific interests in the past two decades for their rich non-equilibrium dynamics and importance in cell division. In this talk, we propose an application using microtubules and light-activatable motors to design flow fields in microfluidic devices. Motor proteins can bind and walk on microtubules and generate active stresses. Active fluids with such living matter can be chaotic without a control mechanism, limiting their utility for bioengineering applications. Here we use the recently-developed light-activatable motors, which only crosslink the microtubules under illumination, to control the fluid flows through the spatiotemporal design of light patterns. In particular, we present how to generate local extensional and rotational flows in this system, with no requirements on the geometry of channels or control of inlet and outlet flows. A continuum model for both the active microtubules and emergent flows has been formulated and shown good agreement with experiments. In the end, we present the experiments where different types of flows are generated simultaneously at different locations in a single channel. Our results may be used for developing programmable lab-on-a-chip devices in the future. |
Tuesday, November 23, 2021 2:50PM - 3:03PM Not Participating |
T13.00011: The effect of particle geometry on swimming in a shear-thinning fluid Brandon van Gogh, Ebru Demir, Devanayagam Palaniappan, On Shun Pak Biological and artificial microscopic swimmers often find themselves immersed in fluids with |
Tuesday, November 23, 2021 3:03PM - 3:16PM |
T13.00012: Helical Swimming in Suspensions Andres Zambrano, Roberto Zenit, Albane Thery, Eric Lauga To understand the effect of media heterogeneity in the swimming dynamics of bacteria, we study the motion of synthetic helical swimmers in suspensions. Low Reynolds number swimmers with helical tails are rotated by an external magnetic field acting on a magnet embedded within the swimmer. In addition to the swimming speed, both the drag force and the thrust force are measured directly using a force transducer. For the fluid, several suspensions of neutrally buoyant particles with varying volume concentrations were made and tested. Our experiments suggest that the swimming speed increases systematically with the concentration of particles in the fluid. Our experimental data are then compared with a theoretical model that accounts for the stress induced by moving spheres in the suspension around the helical tail and computes the changes in the values of the drag coefficients. The theoretical predictions are in good agreement with the experiments. |
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