Bulletin of the American Physical Society
64th Annual Meeting of the APS Division of Fluid Dynamics
Volume 56, Number 18
Sunday–Tuesday, November 20–22, 2011; Baltimore, Maryland
Session R29: Suspension Swimming |
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Chair: Josue Sznitman, Technion Room: Ballroom III |
Tuesday, November 22, 2011 12:50PM - 1:03PM |
R29.00001: Out-of-Equilibrium effects in suspensions of light-activated artificial microwimmers Jeremie Palacci, Stefano Sacanna, David Pine, Paul Chaikin We present a new type of colloidal artificial swimmers propelled by the decomposition of hydrogen peroxide and activated by light. The effect is reversible and allows external control of the swimming/non swimming behavior of the particles. Moreover the bulk synthesis makes possible the study of very concentrated assemblies of monodisperse and identical microswimmers. These active agents are used to explore fancy effects of out-of-equilibrium systems, e.g. clustering, ``tugboat'' effect... [Preview Abstract] |
Tuesday, November 22, 2011 1:03PM - 1:16PM |
R29.00002: Laboratory studies of ocean mixing by microorganisms Monica Martinez-Ortiz, John O. Dabiri Ocean mixing plays a major role in nutrient and energy transport and is an important input to climate models. Recent studies suggest that the contribution of fluid transport by swimming microorganisms to ocean mixing may be of the same order of magnitude as winds and tides. An experimental setup has been designed in order to study the mixing efficiency of vertical migration of plankton. To this end, a stratified water column is created to model the ocean's density gradient. The vertical migration of Artemia Salina (brine shrimp) within the water column is controlled via luminescent signals on the top and bottom of the column. By fluorescently labelling portions of the water column, the stirring of the density gradient by the animals is visualized and quantified. Preliminary results show that the vertical movement of these organisms produces enhanced mixing relative to control cases in which only buoyancy forces and diffusion are present. [Preview Abstract] |
Tuesday, November 22, 2011 1:16PM - 1:29PM |
R29.00003: A kinetic model for concentrated active suspensions Barath Ezhilan, David Saintillan, Michael Shelley We study the dynamics in concentrated suspensions of active self-propelled particles using a kinetic model and three-dimensional continuum simulations.In a previous model for semi-dilute active suspensions (Saintillan and Shelley 2008), we have revealed the existence of hydrodynamic instabilities and complex correlated motions with pattern formation in suspensions of pushers, while suspensions of pullers undergo no such dynamics. Here, we modify this previous kinetic model by including an additional nematic alignment torque proportional to the local concentration in the equation for the rotational velocity of the particles, causing them to align locally with their neighbors (Doi and Edwards 1986). Linear stability analyses in the isotropic and aligned states show that in the presence of such a torque both pusher and puller suspensions are unstable to random fluctuations. Large-scale three-dimensional simulations of the kinetic equations are also performed and confirm the existence of these instabilities, which lead to the formation of nematic-like structures at high concentrations. Detailed measures are defined to quantify the degree and direction of alignment, and the effects of steric interactions on pattern formation are discussed in detail. [Preview Abstract] |
Tuesday, November 22, 2011 1:29PM - 1:42PM |
R29.00004: Instabilities and global order in concentrated suspensions of spherical microswimmers Arthur Evans, Takuji Ishikawa, Takami Yamaguchi, Eric Lauga We use numerical simulations to probe the dynamics of concentrated suspensions of spherical microswimmers interacting hydrodynamically. Previous work in the dilute limit predicted instabilities of aligned suspensions for both pusher and puller swimmers, which we confirm computationally. Unlike previous work, we show that isotropic suspensions of spherical swimmers are also always unstable. Both types of initial conditions develop long-time polar order, of a nature which depends on the hydrodynamic signature of the swimmer but very weakly on the volume fraction up to very high volume fractions. [Preview Abstract] |
Tuesday, November 22, 2011 1:42PM - 1:55PM |
R29.00005: Transverse swimming in a dilute suspension of active swimmer under an oscillating shear flow Francisca C. Guzman Lastra, Rodrigo Soto Simulations of a dilute suspension of pusher swimmers under an oscillating shear flow show that a large fraction of them swim preferentially in the vortex and flow direction, perpendicular to the gradient direction. These two directions alternate in a collective way among all the swimmers. Experiments of a suspension of E.Coli in a Hele-Shaw cell under an oscillating flow manifest the same behavior. [Preview Abstract] |
Tuesday, November 22, 2011 1:55PM - 2:08PM |
R29.00006: Effect of viscoelasticity on the collective behavior of swimming microorganisms Patrick Underhill, Yaser Bozorgi Hydrodynamic interactions of swimming microorganisms can lead to coordinated behaviors of large groups. Previous theoretical work has shown that in mean-field theories the interactions give rise to a linear instability of the uniform isotropic state if the organisms push themselves forward. However, that work is done with the organisms suspended in a Newtonian fluid, while many organisms exist in a non-Newtonian medium. Using a mean-field theory and the Oldroyd-B constitutive equation, we show how viscoelasticity of the suspending fluid alters the hydrodynamic interactions and therefore the ability of the group to coordinate. We quantify the ability to coordinate by the initial growth rate of a small disturbance from the uniform isotropic state. For small wavenumbers the response is qualitatively similar to a Newtonian fluid but the Deborah number determines the effective viscosity of the suspension. At higher wavenumber, the response of the fluid to small amplitude oscillatory shear flow leads to a maximal growth rate at a particular wavelength. This is in stark contrast to the Newtonian result. [Preview Abstract] |
Tuesday, November 22, 2011 2:08PM - 2:21PM |
R29.00007: Shaken, but not stirred: how vortical flow drives small-scale aggregations of gyrotactic phytoplankton Michael Barry, William Durham, Eric Climent, Roman Stocker Coastal ocean observations reveal that motile phytoplankton form aggregations at the Kolmogorov scale (mm-cm), whereas non-motile cells do not. We propose a new mechanism for the formation of this small-scale patchiness based on the interplay of turbulence and gyrotactic motility. Counterintuitively, turbulence does not stir a plankton suspension to homogeneity but drives aggregations instead. Through controlled laboratory experiments we show that the alga \textit{Heterosigma akashiwo} rapidly forms aggregations in a cavity-driven vortical flow that approximates Kolmogorov eddies. Gyrotactic motility is found to be the key ingredient for aggregation, as non-motile cells remain randomly distributed. Observations are in remarkable agreement with a 3D model, and the validity of this mechanism for generating patchiness has been extended to realistic turbulent flows using Direct Numerical Simulations. Because small-scale patchiness influences rates of predation, sexual reproduction, infection, and nutrient competition, this result indicates that gyrotactic motility can profoundly affect phytoplankton ecology. [Preview Abstract] |
Tuesday, November 22, 2011 2:21PM - 2:34PM |
R29.00008: Enhancement of biomixing by swimming cells in 2D films Jerry Gollub, Huseyin Kurtuldu, Jeffrey Guasto, Karl Johnson Fluid mixing in active suspensions of microorganisms is important to ecological phenomena and shows surprising statistical behavior. We investigate the mixing produced by swimming unicellular algal cells (Chlamydomonas) in quasi-2D films by tracking the motions of cells and of microscopic passive tracer particles advected by the fluid. The reduced spatial dimension of the system leads to long-range flows and a surprisingly strong dependence of tracer transport on the swimmer concentration. The mean square displacements are well described by a stochastic Langevin model, with an effective diffusion coefficient D growing as the 3/2 power of the swimmer concentration, due to the interaction of tracer particles with multiple swimmers. We also discuss the anomalous probability distributions of tracer displacements, which become Gaussian at high concentration, but show strong power-law tails at low concentration.\footnote {H. Kurtuldu et al., Proc. Nat. Acad. Sci. 108, 10391 (2011)} [Preview Abstract] |
Tuesday, November 22, 2011 2:34PM - 2:47PM |
R29.00009: Active stress driven convection in a suspension of chemotactic bacteria Kasyap T.V., Donald Koch We examine the linear stability of a suspension of swimming bacteria producing dipolar hydrodynamic disturbances confined in a channel subjected to a linear chemo-attractant gradient across the channel. At the continuum level swimming bacteria exert an ``active'' stress on the fluid which is a function of the bacterial concentration and orientation fields. In the base-state without any fluid flow, the fluxes from the chemotactic and diffusive motion of the bacteria balance to yield exponential number density and active stress profiles across the channel. We show that such a base-state is unstable to perturbations in the number density parallel to the channel walls if the bacterial concentration exceeds a critical value determined by a Peclet number measuring the strength of chemotaxis relative to diffusion. Active stress gradients resulting from the perturbation in the number density drive convective fluid flow, which transports bacteria into the regions of highest perturbed bacteria concentration reinforcing the original perturbation. [Preview Abstract] |
Tuesday, November 22, 2011 2:47PM - 3:00PM |
R29.00010: Energy Transport in a Concentrated Suspension of Bacteria Takuji Ishikawa, Naoto Yoshida, Hironori Ueno, Matthias Wiedeman, Yohsuke Imai, Takami Yamaguchi Coherent structures appear in a concentrated suspension of swimming bacteria. While transport phenomena in a suspension have been studied extensively, how energy is transported from the individual cell scale to the larger meso-scale remains unclear. In this study, we carry out the first successful measurement of the three-dimensional velocity field in a dense suspension of bacteria. The results show that most of the energy generated by individual bacteria dissipates on the cellular scale. Only a small amount of energy is transported to the meso-scale, but the gain in swimming velocity and mass transport due to meso-scale coherent structures is enormous. These results indicate that collective swimming of bacteria is efficient in terms of energy. This study sheds light on how energy can be transported toward smaller wave numbers in the Stokes flow regime. [Preview Abstract] |
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