Bulletin of the American Physical Society
65th Annual Meeting of the APS Division of Fluid Dynamics
Volume 57, Number 17
Sunday–Tuesday, November 18–20, 2012; San Diego, California
Session M15: Biofluids: Microswimmer Suspensions |
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Chair: Saverio Spagnolie, University of Wisconsin Room: 28A |
Tuesday, November 20, 2012 8:00AM - 8:13AM |
M15.00001: Concentrated active suspensions: Kinetic theory, linear stability and numerical simulations Barath Ezhilan, Michael Shelley, David Saintillan We study concentrated suspensions of self-propelled rod-like particles using a kinetic model which accounts for local hydrodynamic and steric interactions. We report a base state transition from an isotropic to a nematic orientation distribution beyond a critical effective volume fraction consistent with the Doi-Edwards theory for passive rod-like particles (Doi and Edwards 1986). We analyze the kinetic model linearized near the isotropic and nematic basestates and show that steric interactions have a destabilizing effect causing both pusher and puller suspensions to be subject to instabilities. These predictions from the linear theory are confirmed using fully nonlinear three-dimensional numerical simulations of the kinetic equations, which also demonstrate large-scale fluctuations of number density and nematic order parameter. [Preview Abstract] |
Tuesday, November 20, 2012 8:13AM - 8:26AM |
M15.00002: Simulation and continuum modelling of a non-uniform suspension of spherical squirmers Timothy Pedley, Takuji Ishikawa Stokesian dynamics simulations are performed for a non-dilute suspension of identical spherical squirmers (cells) whose initial concentration distribution $c(x,t)$ is sinusoidal in $x$. It is found that the $c$-distribution overshoots its mean, so that there are times at which the maximum values of $c$ occur at locations where initially $c$ was a minimum and vice versa. This is not consistent with a purely diffusive model. We consider continuum models in terms of the cell conservation equation, incorporating the average cell swimming velocity \mbox{\boldmath $U$} and representing random cell motion (resulting solely from hydrodynamic interaction between cells) by a diffusivity tensor $\bf{D}$. If the values of \mbox{\boldmath $U$} and $\bf{D}$ obtained from the simulation are used in the equations, the results agree well with the simulations. However, if we start from the Fokker-Planck equation for the pdf of orientation, representing hydrodynamic interactions by a constant rotational diffusivity, and truncating the sequence of moment equations at the first or second moment, agreement is not very good. We discuss what would be needed in a continuum model for it to be able to predict \mbox{\boldmath $U$} and $\bf{D}$ accurately, without doing the full simulation first. [Preview Abstract] |
Tuesday, November 20, 2012 8:26AM - 8:39AM |
M15.00003: Out-of-Equilibriumness of Light Activated Colloids Jeremie Palacci, Stefano Sacanna, Asher Preska-Steinberg, David Pine, Paul Chaikin Self-propelled micro-particles are intrinsically out-of-equilibrium. This renders their physics far richer than that of passive colloids while relaxing some thermodynamical constraints and give rise to the emergence of complex phenomena e.g. collective behavior, swarming... We will present a new form of self-assembly originating from non-equilibrium driving forces. When activated by light, a set of new self-propelled particles spontaneously assemble into living crystals which behaves as ``self-propelled colloidal carpets'' steerable with an external magnetic field. We will show that this phenomenon is intrinsically out-of-equilibrium and originates in the competition between self-propulsion, particles collisions and attractive interactions. We will also present present surprising behaviors of these particles in confined environments. [Preview Abstract] |
Tuesday, November 20, 2012 8:39AM - 8:52AM |
M15.00004: Extensive active suspension AnSheng Jhang, Micheal Shelley A suspension of rod-like growing particles, like a suspension of self-propelled particles, can exhibit complex dynamics as a result of long-ranged hydrodynamic interactions. Such suspensions can occur in bacterial colonies, liquid crystals phase transitions, or micro-tubules with kinesin. As they grow, they exert stress on the fluid which is similar to the case of swimming pushers. We use a kinetic model to study the dilute limit case. We will discuss the cases in terms of domain shapes like periodic boundary domains, simply connected domain, and annulus like domain. [Preview Abstract] |
Tuesday, November 20, 2012 8:52AM - 9:05AM |
M15.00005: Flow of active suspensions and biased swimming Salima Rafai, Philippe Peyla, Xabel Garcia, Guntars Kitenbergs, Micha\"el Garcia It is a challenge to understand the hydrodynamics associated with individual or collective motion of microswimmers through their fluid-mediated interactions in order for instance to manipulate the cells efficiently for some applications purposes. The motion of these micro-organisms can be often affected by the presence of gradients leading to a biased random walk (chemotaxis in the presence of chemicals, gyrotaxis in a gravity field, phototaxis under light exposure). In this study, we present our experimental results concerning the coupling of a Poiseuille flow with the biased random walk of Chlamydomonas Reinhardtii, a green unicellular micro-alga. This is done by illuminating the microswimmer suspension while flowing in a microchannel device. We show that one can obtain a spontaneous and reversible migration and separation of the microalgae suspension from the rest of the suspending medium under illumination and then dynamically control the concentration of the suspension with light. We present a simple model that accounts for the observed phenomenon. [Preview Abstract] |
Tuesday, November 20, 2012 9:05AM - 9:18AM |
M15.00006: Spontaneous Circulation of Confined Active Suspensions Francis Woodhouse, Raymond Goldstein Many active fluid systems encountered in biology are set in total geometric confinement; cytoplasmic streaming is a prominent and ubiquitous example. Using the simple paradigm of a dilute dipolar swimmer suspension, we demonstrate that the two key constraints of circular confinement and fluid incompressibility yield qualitatively new dynamics, effectively quantizing the behaviour regimes. We show analytically that there is an activity threshold for spontaneous auto-circulation and verify this numerically. Long-time non-linear behaviour is investigated via simulations, which reveal steady states displaying nematic defect separation and a high-activity bifurcation to an oscillatory regime. [Preview Abstract] |
Tuesday, November 20, 2012 9:18AM - 9:31AM |
M15.00007: Simulations of bacterial chemotaxis in the turbulent ocean Romain Watteaux, John Taylor Nearly half of the global primary production occurs in the oceans. Between 30 and 50\% of the newly generated carbon is released into the surrounding water as dissolved organic matter (DOM), and is almost exclusively accessible to bacteria. By consuming DOM and returning the carbon to the marine food web, bacteria act as recyclers. Some bacteria are motile and have the ability to modify their otherwise random motility in response to a chemical cue, a process known as chemotaxis. It has been recently shown that motile bacteria can benefit from turbulence by clustering around thin DOM filaments, thereby increasing their uptake (Taylor and Stocker, Science, \textit{accepted}). Here, we extend the previous analysis by considering weakly diffusive DOM patches (with a Schmidt number, $Sc=\nu/\kappa_C$ up to 1000), and examine the counteracting effects of chemotaxis and random motility. Using direct numerical simulations (DNS), we find that the uptake enhancement depends on characteristic length scales of DOM and bacteria filaments, which in turn depend on three parameters: the turbulent dissipation rate, the bacteria swimming speed, and the DOM diffusivity. By exploring a range of realistic parameter values, we are able to characterize the advantage afforded by motility. [Preview Abstract] |
Tuesday, November 20, 2012 9:31AM - 9:44AM |
M15.00008: Meso-scale turbulence in living fluids Jorn Dunkel, Rik Wensink, Sebastian Heidenreich, Knut Drescher, Ray Goldstein, Hartmut Loewen, Julia Yeomans The mathematical characterization of turbulence phenomena in active non-equilibrium fluids proves even more difficult than for conventional liquids or gases. It is not known which features of turbulent phases in living matter are universal or system-specific, or which generalizations of the Navier-Stokes equations are able to describe them adequately. We combine experiments, particle simulations, and continuum theory to identify the statistical properties of self-sustained meso-scale turbulence in active systems. To study how dimensionality and boundary conditions affect collective bacterial dynamics, we measured energy spectra and structure functions in dense Bacillus subtilis suspensions in quasi-2D and 3D geometries. Our experimental results for the bacterial flow statistics agree well with predictions from a minimal model for self-propelled rods, suggesting that at high concentrations the collective motion of the bacteria is dominated by short-range interactions. To provide a basis for future theoretical studies, we propose a minimal continuum model for incompressible bacterial flow. A detailed numerical analysis of the 2D case shows that this theory can reproduce many of the experimentally observed features of self-sustained active turbulence. [Preview Abstract] |
Tuesday, November 20, 2012 9:44AM - 9:57AM |
M15.00009: Discrete population dynamics in flows Prasad Perlekar, Roberto Benzi, Luca Biferale, Herman Clercx, Simone Pigolotti, Mogens Jensen, David Nelson, Federico Toschi Bacteria and plankton populations living in oceans and lakes reproduce and die under the influence of turbulent currents. Fluid transport interacts in a complex way with the dynamics of populations because the typical reproduction time of microorganism is comparable with the time scale of the flows. We review recent results on the population dynamics for off-lattice models. We then investigate the role of chaotic/turbulent flows on the dynamic of populations. The populations are modeled as discrete entities (particles) that reproduce, die, and compete with each other. Furthermore, to mimic various seggregation mechanisms like gyrotaxis, chemotaxis, and/or food variability we associate an inertia with the entities. We show that the presence of advecting flows with same ``inertial'' entities leads to a dramatic reduction in the population sizes and fixation times. We also discuss the interesting case of species with slightly different inertial properties where a long coexistence of species is possible. [Preview Abstract] |
Tuesday, November 20, 2012 9:57AM - 10:10AM |
M15.00010: Mixing by individual swimmers Dmitri Pushkin, Henry Shum, Julia Yeomans Despite their evolutionary and technological importance, different biomixing mechanisms, their effectiveness and universality remain poorly understood. In this talk we focus on the Lagrangian transport of the surrounding fluid by swimmers. Low Re passive tracers advected by swimmers move in loops that are, in general, almost closed. We analyze the reasons for this behavior and, as non-closedness of the loops is a natural requirement for an efficient mixing, propose a classification of possible mechanisms for biogenic mixing. Next, we discuss the universal (common to all swimmers) and the swimmer-dependent features of the resulting tracer displacements and analyze the Darwin drift, the total fluid volume displaced by a swimmer passing from and to infinity. We show that the Darwin drift is finite for force-free swimmers and can be decomposed into a universal and a swimmer-dependent part. We illustrate our consideration with examples for model swimmers and biological data. [Preview Abstract] |
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