Bulletin of the American Physical Society
69th Annual Meeting of the APS Division of Fluid Dynamics
Volume 61, Number 20
Sunday–Tuesday, November 20–22, 2016; Portland, Oregon
Session E19: Bio: Motion in Non-Newtonian Fluids |
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Chair: Derek Tretheway, Portland State University Room: D136 |
Sunday, November 20, 2016 5:37PM - 5:50PM |
E19.00001: Role of passive body dynamics in micro-organism swimming in complex fluids Becca Thomases, Robert Guy We investigate the role of passive body dynamics in the kinematics of swimming micro-organisms in complex fluids. Asymptotic analysis and linear theory are used to predict shape changes that result as body elasticity and fluid elasticity are varied. The analysis is compared with a computational model of a finite length swimmer in a Stokes-Oldroyd-B fluid. Simulations and theory agree quantitatively for small amplitude motions with low fluid elasticity (Deborah number). This may not be surprising as the theory is expected hold in these two regimes. What is more remarkable is that the predicted shape changes match the computational shape changes quantitatively for large amplitudes, even for large Deborah numbers. Shape changes only tell part of the story. Swimming speed depends on other effects as well. We see that shape changes can predict swimming speed well when either the amplitude is small (including large Deborah number) or when the Deborah number is small (including large amplitudes). It is only in the large De AND large amplitude regime where the theory breaks down and swimming speed can no longer be inferred from shape changes alone. [Preview Abstract] |
Sunday, November 20, 2016 5:50PM - 6:03PM |
E19.00002: An active particle in a complex fluid Charu Datt, Giovanniantonio Natale, Savvas G. Hatzikiriakos, Gwynn J. Elfring Active particles are self-driven units capable of converting stored or ambient free-energy into systematic movement. We discuss here the case when such particles move through non-Newtonian fluids. Neglecting inertial forces, we employ the reciprocal theorem to calculate the propulsion velocity of a single swimmer in a weakly non-Newtonian fluid with background flow. We also derive a general expression for the velocity of an active particle modelled as a squirmer in a second-order fluid. We then discuss how active colloids are affected by the medium rheology, namely viscoelasticity and shear-thinning. [Preview Abstract] |
Sunday, November 20, 2016 6:03PM - 6:16PM |
E19.00003: Swimming in mud Neil Balmforth, Duncan Hewitt We extend G.I. Taylor's classic problem of the swimming of a flexible sheet in a viscous fluid driven by waves propagating down its length. In particular, we add a yield stress to the problem and calculate how the swimming speed is modified for waves of both low and high amplitude. We examine the flow patterns created around the swimmer as it locomotes and comment on designing strategies for optimal progress. [Preview Abstract] |
Sunday, November 20, 2016 6:16PM - 6:29PM |
E19.00004: Undulatory Swimming in Shear-Thinning Fluids: Flow Fields & Power Consumption David Gagnon, Thomas Montenegro-Johnson, Eric Lauga, Paulo Arratia In this talk, we investigate the flow and dynamics of the undulatory swimmer Caenorhabditis elegans in shear-thinning fluids. Recent theoretical and numerical studies have shown that the cost of swimming, or mechanical power, for a 2D waving sheet is reduced in shear-thinning fluids. Here, we use velocimetry and tracking techniques to experimentally investigate this hypothesis using two methods: (i) an estimate of the mechanical power of the swimmer and (ii) the viscous dissipation rate of the flow field. We find the cost of swimming for C. elegans in shear-thinning fluids is reduced when compared to the cost of swimming in Newtonian fluids, scales with a fluid’s effective viscosity, and can be predicted using fluid rheology and simple swimming kinematics. These results, however, have a caveat: only a planar (2D) slice of the 3D flow field around swimmer is accessible for analysis. In order to better interpret our flow measurements, we compare our planar velocimetry to a full 3D boundary element method simulation. We find that nearly all deviations between experiments and simulations can be accounted for by a simple correction factor involving the out-of-plane velocity gradient, which can be computed directly from planar experimental data using incompressibility. [Preview Abstract] |
Sunday, November 20, 2016 6:29PM - 6:42PM |
E19.00005: Micro-scale undulatory locomotion in heterogeneous viscoelastic environments Arshad Kamal, Eric Keaveny While many microorganisms swim in viscoelastic fluids, there are notable examples where the suspended microstructure that makes the fluid viscoelastic is at the same length scale as the swimmers. Here, the swimming cells experience the surrounding medium as a set of obstacles suspended in a viscous fluid, rather than a viscoelastic continuum. Using simulations based on the force-coupling method, we explore this situation for a simple undulatory swimmer as it moves through an environment of obstacles that are tethered to random points in space via linear springs. We examine how swimming behaviour is altered by mechanical interactions with the obstacles by varying obstacle density and tether stiffness. We find that the mechanical interactions can either enhance or hinder locomotion, and often for fixed tether stiffness, there is an obstacle density for which the average speed is maximized. We also find cases where the swimmer is completely trapped by the environment. In addition, we find that the velocity fluctuations, and consequently the effective swimmer diffusion, are also highly dependent on environment composition and a non-monotonic dependence on the relevant parameters can be found here as well. [Preview Abstract] |
Sunday, November 20, 2016 6:42PM - 6:55PM |
E19.00006: Helicobacter pylori Couples Motility and Diffusion to Actively Create a Heterogeneous Complex Medium in Gastric Mucus Henry Fu, Seyed Amir Mirbagheri Helicobacter pylori swims through mucus gel by generating ammonia that locally neutralizes the acidic gastric environment, turning nearby gel into a fluid pocket. The size of the fluid zone is important for determining the physics of the motility: in a large zone swimming occurs as in a fluid through hydrodynamic principles, while in a very small zone the motility could be strongly influenced by nonhydrodynamic cell-mucus interactions including chemistry and adhesion. We calculate the size of the fluid pocket. We model how swimming depends on the de-gelation range using a Taylor sheet swimming through a layer of Newtonian fluid bounded by a Brinkman fluid. Then, we model how the de-gelation range depends on the swimming speed by considering the advection-diffusion of ammonia exuded from a translating sphere. Self-consistency between both models determines the values of the swimming speed and the de-gelation range. We find that \textit{H. pylori} swims through mucus as if unconfined, in a large pocket of Newtonian fluid. [Preview Abstract] |
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