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 A30: Biological Fluid Dynamics : Micro-Swimmer General I |
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Chair: Henry Fu, University of Utah Room: 612 |
Saturday, November 23, 2019 3:00PM - 3:13PM |
A30.00001: 3D Confinement Effects on \textit{Helicobacter pylori} Swimming Suraj Kumar Kamarapu, Henry Fu H. pylori bacterium has evolved to swim through highly acidic gastric mucus layer by diffusing ammonia from its body, neutralizing the surrounding medium and forming a pocket of Newtonian fluid around itself. The shape of this pocket, which depends on the Peclet number, determines the overall swimming behavior of the bacterium. We previously used a 2D Taylor sheet to model the swimming bacterium nearby a Brinkman medium that represented the mucus gel and found that the swimming speed monotonically increases as the distance between the swimmer and the gel decreases. However, swimming in such situations can also be highly dependent on body geometry, diffusion, swimming flows around the swimmer and requires a complete 3D model factoring in the above possible influences. Here we model the mucus gel with a random spatial distribution of regularized stokelets placed outside the Newtonian fluid pocket, and quantify its influence on the swimming speeds for a constant stroke. Advection-diffusion of ammonia is treated numerically allowing us to access large Peclet numbers. We find that for small Peclet numbers, the bacterium swims faster than predicted by a 2D model, but beyond a certain Peclet number, the bacterium can swim with reduced speed as it faces 3D confinement upstream. [Preview Abstract] |
Saturday, November 23, 2019 3:13PM - 3:26PM |
A30.00002: Motility of flagellated bacteria in colloidal media Shashank Kamdar, Lorraine F. Francis, Xiang Cheng Recent years have seen increasing interests in understanding the mechanism and motility of microswimmers in non-Newtonian fluids due to their relevance in biological and biomedical applications. Nevertheless, despite extensive study on the locomotion of microswimmers in polymeric fluids, their motion in a colloidal suspension remains largely unexplored. Here, we study the motility of \textit{E. coli}, a flagellated bacterium in colloidal media. We systematically vary the size of colloidal particles from 50 nm to 1 $\mu $m and the volume fraction up to 20{\%}. The motion of fluorescent-labeled bacteria is imaged using confocal microscopy and speeds of bacteria are extracted using a robust in-house tracking algorithm. Our results show that bacterial mobility decreases with increasing volume fractions at low volume fractions, but remains constant beyond a critical volume fraction. In addition, we find that the motility depends on the size of passive colloid. Finally, we construct a simple model that qualitatively explains our experimental observation. This work enriches the current understanding of microswimmers' locomotion in complex fluids. [Preview Abstract] |
Saturday, November 23, 2019 3:26PM - 3:39PM |
A30.00003: How Azimuthal Swirl Impacts Swimming Kinematics in a Viscoelastic Fluid Jeremy P. Binagia, Ardella Phoa, Eric S. G. Shaqfeh Microorganisms are often found moving through viscoelastic environments such as mucus layers or biofilms. In 2014, Zhu \& Lauga simulated the steady motion of a spherical ``squirmer'' in a viscoelastic fluid to understand how fluid elasticity impacts the organisms's speed. The squirmer model considers a spherical swimmer that includes a specified slip velocity at its surface. This model has been used extensively to study the motion of ciliates like \textit{Paramecium}, colonies of the green algae \textit{Volvox}, or as a simplified model for general swimmers like \textit{E. coli}. In all cases, they found that a squirmer swims slower than it does in a Newtonian fluid. In that study and many others that use the squirmer model, only the first two axisymmetric swimming modes are considered. Only very recently have authors considered the addition of other modes, such as those that involve azimuthal surface velocities. Recently, we have conducted simulations showing that particular combinations of the axisymmetric swirling modes can actually lead to a speed increase in an elastic fluid. In this talk, we will describe how the inclusion of this azimuthal swirl affects swimming kinematics in elastic fluids, with a focus on how polymer deformation leads to changes in speed. [Preview Abstract] |
Saturday, November 23, 2019 3:39PM - 3:52PM |
A30.00004: Effective diffusivity of microswimmers in a crowded environment Marvin Brun-Cosme-Bruny The effect of crowded environments on micro-swimmers is studied using the micro-alga Chlamydomonas Reinhardtii (CR) as a model system. Performing a Run-and-Tumble motion in bulk, its swimming describes a persistent random walk characterized by an effective diffusion coefficient for the large-time dynamics. This swimming is experimentally observed in a complex medium made of series of pillars designed in a regular lattice, using soft lithography microfabrication. Their trajectories are tracked and analyzed. The measure of relevant statistical observables provides insight into the bias induced by the obstacles. Particularly, the mean correlation time of direction and the effective diffusion coefficient are shown to decrease when increasing the density of pillars. This provides some bases of understanding for active matter in complex environments. [Preview Abstract] |
Saturday, November 23, 2019 3:52PM - 4:05PM |
A30.00005: Modeling helical swimming in shear-thinning fluids Noah Lordi, Ebru Demir, Yang Ding, On Shun Pak Swimming bacteria, such as Escherichia coli, propel by rotating their helical flagella with rotary motors in the cell membrane. Helical microswimmers rotated by external magnetic fields have been fabricated to mimic the helical propulsion of bacteria. While helical propulsion has already been extensively studied with the Newtonian fluid assumption, the performance of this propulsion mechanism in non-Newtonian fluids has attracted considerable attention recently. Biological and synthetic microswimmers move through complex fluids that often display shear-thinning viscosity. In this talk, we will discuss a theoretical model to investigate the effect of shear-thinning rheology on helical propulsion. We will highlight the similarities and differences in the propulsion performance in contrast to the results in the Newtonian limit, and compare model predictions with recent experiments on helical propulsion in shear-thinning fluids. [Preview Abstract] |
Saturday, November 23, 2019 4:05PM - 4:18PM |
A30.00006: Active particles in viscosity gradients Gwynn Elfring, Charu Datt Microswimmers in nature often experience spatial gradients of viscosity. In this work we develop theoretical results for the dynamics of active particles, biological or otherwise, swimming through viscosity gradients. We model the active particles (or microswimmers) using the squirmer model, and show how the effects of viscosity gradients depend on the swimming gait of the swimmers and how viscosity gradients lead to viscotaxis for squirmers. We also show how such gradients in viscosity may be used to sort and control swimmers based on their swimming style. [Preview Abstract] |
Saturday, November 23, 2019 4:18PM - 4:31PM |
A30.00007: The Dispersal of Swimming Microalgae in Viscosity Gradients Michael R. Stehnach, Nicolas Waisbord, Jeffrey S. Guasto Swimming cells often live in fluid environments characterized by spatial gradients of rheological properties, including biofilms and mucus layers. However, our understanding of cell transport in such environments is lacking. In this work, we use microfluidic devices to generate a spatial concentration gradient of a Newtonian polymer suspension -- thus creating a viscosity gradient. Video microscopy is used to quantify the viscosity landscape and the cell motility. We demonstrate experimentally that swimming biflagellates (wild-type $Chlamydomonas \quad reinhardtii)$ accumulate in high viscosity regions (viscotactic response), stemming from a local reduction in cell swimming speed. A statistical analysis of the cell motility reveals that the viscous slowdown of the microalgae is due to their approximately constant flagellar thrust force in different ambient viscosities. We further demonstrate that this local viscous slowing of cell motility, leading to accumulation, is generalized in highly nonlinear viscosity gradients. [Preview Abstract] |
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