Bulletin of the American Physical Society
63rd Annual Meeting of the APS Division of Fluid Dynamics
Volume 55, Number 16
Sunday–Tuesday, November 21–23, 2010; Long Beach, California
Session QS: Biolocomotion XI: Micro-Swimming Collective Behavior |
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Chair: Saverio Spagnolie, University of California, San Diego Room: Long Beach Convention Center Grand Ballroom A |
Tuesday, November 23, 2010 12:50PM - 1:03PM |
QS.00001: Collisional low velocity phase of concentrated rod-shaped bacteria Luis Cisneros, John Kessler, Sujoy Ganguly, Raymond Goldstein Suspensions of self propelled wild-type Bacillus subtilis exhibit a transition from independent motion at low concentration to a complex collective dynamics at large concentrations, as a consequence of steric and hydrodynamic interactions. The collective phase displays domains with velocities higher than those of individual swimming cells, correlated with strong co-directionality, termed Zooming BioNematic (ZBN) phase. At intermediate concentrations we find a regime where intercellular collisions, characterized by stopping followed by reconstitution of the propulsion mechanism, produce reduction of mean swimming speeds considerably below those observed for free individual cells. This transitional phase is termed ``the jamming phase'' by analogy with concentrated automobile or pedestrian traffic. In this regime cell to cell separations are sufficiently small to produce a high frequency of collisions, but not small enough to trigger collective organization. A basic model that considers the typical acceleration of bacteria after collisions and the associated mean free time as a function of cell concentration is shown to yield the observed reduction of swimming speed in the jamming phase. [Preview Abstract] |
Tuesday, November 23, 2010 1:03PM - 1:16PM |
QS.00002: Chemotaxis affects hydrodynamics in suspensions of micro-swimmers Enkeleida Lushi, Michael Shelley Microorganisms are known to respond to a dissolved chemical substance by moving preferentially away or toward its source. We study such chemotactic responses at the population level when micro-swimmers are hydrodynamically coupled. To do this we couple a recently developed kinetic model of motile suspension dynamics with a field equation for a chemical substance that diffuses and is advected by the large-scale fluid flows created by the micro-swimmers. We also allow this substance to be produced or consumed by the swimmers themselves. Two models of chemotactic response are considered. One is a simple model for an organism smoothly turning, while moving at constant speed, to align with a chemical gradient. The second is a previously developed model of the effect of modulated run-and-tumble dynamics by individual swimmers. We investigate the linear stability of nearly isotropic suspensions for both models by considering both Pusher micro-swimmers and Pullers. An instability due to chemotaxis is shown to occur in a band of perturbation wavelengths. Nonlinear dynamics are investigated using numerical simulation in two dimensions. We observe aggregation and possible concentration divergences in suspensions of Pullers and the formation of mixing flows in suspensions of Pushers. In the latter case we observe that chemotaxis slows and modifies the mixing dynamics of the system. [Preview Abstract] |
Tuesday, November 23, 2010 1:16PM - 1:29PM |
QS.00003: Direct measurement of the flow field around swimming microorganisms Marco Polin, Knut Drescher, Raymond E. Goldstein, Nicolas Michel, Idan Tuval Swimming microorganisms create flows that influence their mutual interactions and modify the rheology of their suspensions. While extensively studied theoretically, these flows have not been measured in detail around any freely-swimming microorganism. We report such measurements for the microphytes {\it Volvox carteri} and {\it Chlamydomonas reinhardtii}. The minute ($\sim0.3\%$) density excess of {\it V. carteri} over water leads to a strongly dominant Stokeslet contribution, with the widely-assumed stresslet flow only a correction to the subleading source dipole term. This implies that suspensions of {\it V. carteri} have features similar to suspensions of sedimenting particles. The flow in the region around {\it C. reinhardtii} where significant hydrodynamic interaction is likely to occur differs qualitatively from a ``puller'' stresslet, and can be described by a simple three-Stokeslet model. [Preview Abstract] |
Tuesday, November 23, 2010 1:29PM - 1:42PM |
QS.00004: Oxygen transport and mixing dynamics in thin films of aerotactic bacteria Amir Alizadeh Pahlavan, David Saintillan We investigate the dynamics in suspensions of aerotactic bacteria using two different kinetic models: a gradient-detecting model, in which the bacteria detect the local oxygen gradients instantaneously, and a run-and-tumble model, in which the bacteria change their run-and-tumble frequency based on the recent temporal changes in the oxygen field. Using three-dimensional numerical simulations, we study the behavior of such suspensions in thin liquid films surrounded by oxygen baths on both sides. As the bacteria consume the dissolved oxygen, gradients form causing them to swim towards the free surfaces where the oxygen concentration is higher. In very thin films, a high oxygen concentration is observed in the liquid as a result of diffusion from the surfaces, but as the film thickness increases, a depletion layer forms in the center. The formation of this low-oxygen region is associated with the emergence of large-scale instabilities in the suspensions that enhance oxygen mixing into the liquid. These instabilities are accompanied by the formation of large plumes of high bacterial density. The bacterial migration towards the free surfaces is found to be slower in the run-and-tumble model. [Preview Abstract] |
Tuesday, November 23, 2010 1:42PM - 1:55PM |
QS.00005: Turbulent unmixing: the sorting of motile phytoplankton by flow William Durham, Eric Climent, Michael Barry, Roman Stocker Motile phytoplankton in the Ocean are heterogeneously distributed at mm to cm scales, corresponding to the size of the smallest turbulent fluctuations. We demonstrate that this patchiness may originate from a coupling between turbulent shear and gyrotactic motility, a defining feature of many phytoplankton species. By tracking individual cells within a direct numerical simulation (DNS) of turbulence, we observed gyrotactic phytoplankton aggregate in tightly packed clusters. The fate of a species is characterized by two dimensionless parameters, measuring cell stability and swimming speed. We find that turbulent flow separates different species into spatially distinct patches and rationalize these predictions with a simple model of vortical flow. Preliminary experiments support model results. By reducing the mean distance between organisms, this previously unconsidered mechanism can markedly increase encounter rates, which shape all ecological interactions in the Ocean. [Preview Abstract] |
Tuesday, November 23, 2010 1:55PM - 2:08PM |
QS.00006: Modelization and numerical simulations of a microswimmer suspension. The impact on rheology. Philippe Peyla, Levan Jibuti, Salima Rafai Measuring quantitative and macroscopic parameters to estimate the global motility of a large population of swimming cells is a challenge. The rheology of suspensions containing such cells is a good solution to achieve such measurements. As a matter of fact, recent rheological measurements on suspensions of bacteria [1] or algae [2] have been performed very recently. These experiments showed the strong impact of microscopic swimming on macroscopic effective viscosity. Because their flagellae are located at the rear and push the bacteria forward, the chosen bacteria (Bacillus subtilis) are called pushers . The algae (Chlamydomonas Reinhardtii), though, are pullers as they use two front flagellae to pull on the fluid in a breast stroke motion. We discuss the models that have already predicted the rheology of such suspensions. We also show numerical simulations for alga suspensions. We use these simulations in order to discriminate the relevant ingredients of the modelization of the alga puller-like suspensions. \\[4pt] [1] Andrey Sokolov and Igor S. Aranson, Phys. Rev. Lett. 103, 148101 (2009)\\[0pt] [2] Salima Rafai, Levan Jibuti and Philippe Peyla, Phys. Rev. Lett. 104, 098102 (2010) [Preview Abstract] |
Tuesday, November 23, 2010 2:08PM - 2:21PM |
QS.00007: Random flow induced by swimming algae Vasily Kantsler, Ilia Rushkin, Raymond Goldstein In this work we studied the random flow induced in a fluid by the motion of a dilute suspension of the swimming algae {\it Volvox carteri}. The fluid velocity in the suspension is a superposition of the flow fields set up by the individual organisms, which in turn have multipole contributions that decay as inverse powers of distance from the organism. Here we show that the conditions under which the central limit theorem guarantees a Gaussian probability distribution function of velocity fluctuations are satisfied when the leading force singularity is a Stokeslet. Deviations from Gaussianity are shown to arise from near-field effects. Comparison is made with the statistical properties of abiotic sedimenting suspensions. The experimental results are supplemented by extensive numerical studies. [Preview Abstract] |
Tuesday, November 23, 2010 2:21PM - 2:34PM |
QS.00008: Effective viscosity of actively swimming algae suspensions Randy Ewoldt, Lucas Caretta, Anwar Chengala, Jian Sheng Suspensions of actively swimming microorganisms exhibit an effective viscosity which may depend on volume fraction, cell shape, and the nature of locomotion (e.g. ``pushers'' vs. ``pullers''). Here we report experimental measurements of shear viscosity for suspensions of unicellular green algae (\textit{Dunaliella primolecta}, a biflagellated ``puller''). We use a cone-and-plate rheometer to measure the dynamic shear viscosity for both motile and non-motile suspensions of \textit{D. primolecta}. Viscosity increases with concentration for both cases, but the active suspensions of ``pullers'' have a comparatively lower effective viscosity than passive suspensions. This observation contrasts recently proposed theories which predict that ``pullers'' should instead have a higher viscosity than non-motile suspensions. Additionally, we observe shear-induced migration of active suspensions and consider its impact on the resulting effective shear viscosity. [Preview Abstract] |
Tuesday, November 23, 2010 2:34PM - 2:47PM |
QS.00009: Taylor-Aris dispersion of swimming algae in pipe flow: predictions and experimental tests Ottavio Croze, Martin Bees, Rachel Bearon Classical Taylor-Aris dispersion theory is extended to describe the transport of suspensions of biased swimming cells in a vertical pipe flow. These suspensions differ from those of molecular or colloidal solutes: e.g. algae or bacteria in suspension respond to directional stimuli (e.g. chemotaxis, phototaxis or gyrotaxis). Gyrotactic instabilities focus bottom-heavy swimming cells into beautiful plumes. Solving for axial moments, we have derived general exact expressions for the mean drift and effective diffusivity of cells along such plumes, and apply these to predict the dispersion of a ``dyed slug'' of gyrotactic algae in a down-welling flow in a tube. We present predictions for the effective axial drift and diffusivity of the slug using consitutive relations from: (a) generalised Taylor dispersion theory and (b) Fokker-Planck models of dispersion. We then discuss experimental measurements to test our predictions using dyed cells and the relevance to bioreactor design. [Preview Abstract] |
Tuesday, November 23, 2010 2:47PM - 3:00PM |
QS.00010: Noise constricts the Hydrodynamic Horizon of Bacteria Knut Drescher, Jorn Dunkel, Luis Cisneros, Sujoy Ganguly, Raymond E. Goldstein Bacterial processes ranging from gene expression to motility and biofilm formation are constantly challenged by internal and external noise. While the importance of stochastic fluctuations has been appreciated for chemotaxis, fluid dynamics is currently believed to determine cell-cell scattering -- the elementary event that leads to swarming and collective swimming in active suspensions. Here we present the first direct measurement of the bacterial flow field generated by individual Escherichia coli, and infer that Brownian noise and intrinsic variability of the swimming mechanism drown the effects of fluid dynamics. This implies that steric collisions are the dominant physical interactions between bacteria. [Preview Abstract] |
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