Bulletin of the American Physical Society
67th Annual Meeting of the APS Division of Fluid Dynamics
Volume 59, Number 20
Sunday–Tuesday, November 23–25, 2014; San Francisco, California
Session D6: Biofluids: Active Fluids II |
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Chair: David Saintillan, University of California, San Diego Room: 3010 |
Sunday, November 23, 2014 2:15PM - 2:28PM |
D6.00001: Swim pressure of active matter Sho Takatori, Wen Yan, John Brady Through their self-motion, all active matter systems generate a unique ``swim pressure'' that is entirely athermal in origin. This new source for the active stress exists at all scales in both living and nonliving active systems, and also applies to larger organisms where inertia is important (i.e., the Stokes number is not small). Here we explain the origin of the swim stress and develop a simple thermodynamic model to study the self-assembly and phase separation in active soft matter. Our new swim stress perspective can help analyze and exploit a wide class of active soft matter, from swimming bacteria and catalytic nanobots, schools of fish and birds, and molecular motors that activate the cellular cytoskeleton. [Preview Abstract] |
Sunday, November 23, 2014 2:28PM - 2:41PM |
D6.00002: Hydrodynamic interactions in dilute suspensions of microswimmers Joakim Stenhammar, Rupert Nash, Davide Marenduzzo, Alexander Morozov We present a numerical method based on a Lattice-Boltzmann algorithm to simulate hydrodynamic interactions between a large number of model swimmers (order 10$^{5})$, modelled as extended force dipoles. Similar to previous studies of this problem, both experimental and theoretical, we observe that, depending on the concentration of microswimmers, there exists a transition to large-scale structures, often referred to as bacterial turbulence. We introduce a simple theory to characterize the onset of this transition and compare it to our observations. We will also present results on the influence of the large-scale structures on the enhanced diffusion of tracer particles suspended in a solution of microswimmers. [Preview Abstract] |
Sunday, November 23, 2014 2:41PM - 2:54PM |
D6.00003: A numerical model of localized convection cells of Euglena suspensions Makoto Iima, Erika Shoji, Takayuki Yamaguchi Suspension of Euglena gracilis shows localized convection cells when it is illuminated form below with strong light intensity. Experiments in an annular container shows that there are two elementary localized structures. One consists of a pair of convection cells and a single region where number density of Euglena is high. The other consists a localized traveling wave [1]. Based on the measurements of the flux of number density, we propose a model of bioconvection incorporating lateral phototaxis effect proportional to the light intensity gradient. Using pseudo spectral method, we performed numerical simulation of this model. We succeed in reproducing one of the localized structures, a convection pair with single region of high number density. Also, when the aspect ratio is large, there are a parameter region where the localized structure and conductive state are both stable, which is suggested by experiments [1]. Spatial distribution of the number density implies that the accumulation of microorganism due to the convective flow causes such bistability. \\[4pt] [1] Localized bioconvection patterns and their initial state dependency in Euglena suspensions in an annular container, E. Shoji, H. Nishimori, A. Awazu, S. Izumi, and M. Iima, J. Phys. Soc. Jpn. 83(2014)04300 [Preview Abstract] |
Sunday, November 23, 2014 2:54PM - 3:07PM |
D6.00004: Effect of hydrodynamic interactions in confined active suspensions Barath Ezhilan, David Saintillan The dynamics of biologically active suspensions in confined geometries is investigated by incorporating accurate boundary conditions within the kinetic theory framework [Saintillan and Shelley, Phys. Fluids. (2008)]. Even in the absence of wall hydrodynamic interactions or imposed flow, swimming microorganisms have a tendency to accumulate at confining boundaries due to self-propulsion. Satisfying a zero wall-normal translational flux condition on the active particle probability distribution function captures this effect. Using a moment-closure approximation, analytical expressions for the equilibrium concentration/polarization profiles are derived in the dilute limit. As particle density increases, we expect particle-particle hydrodynamic interactions to become significant and to destabilize these equilibrium distributions. Using a linear stability analysis and 3D finite volume simulation of the equations for the orientational moments, we study in detail the effect of fluid coupling on the stability properties of the equilibrium states in confined active suspensions. [Preview Abstract] |
Sunday, November 23, 2014 3:07PM - 3:20PM |
D6.00005: Phase transitions in dense active suspensions Sam Matthew, Pallab Simhamahapatra, Srikanth Vedantam, Mahesh Panchagnula We study dense suspensions of active particles embedded in a Newtonian fluid medium using discrete element computations. The particles are modeled as soft spheres capable of generating a thrust oriented in the direction of its instantaneous velocity. The embedding fluid provides viscous drag in the Stokes regime. The dynamics of the active suspension are investigated in a square cavity. Simulations of dilute suspensions show classical clustering and collective motion. In dense suspensions, the ratio of the thrust to drag force (denoted by $\lambda )$ is found to be an important dimensionless parameter governing the system dynamics. Phase transitions in this material are investigated in this parameter space. It was observed that for low values of $\lambda $, the material arranges itself an oscillatory modes. At intermediate values of $\lambda $, the oscillatory modes transition to a single steady vortex. At higher $\lambda $, multiple vortices are observed in the computational domain. At very high $\lambda $, diffusive effects dominate and a gas-like phase is observed. All the transitions occur over small changes in $\lambda $ indicating sharp transitions between the phases. This model system shows multiple phase transitions driven by a single parameter. [Preview Abstract] |
Sunday, November 23, 2014 3:20PM - 3:33PM |
D6.00006: Density fluctuations and topological structures in collective surface motion of microswimmers Tong Gao, Michael Shelley Active matter that consists of self-propelled particles, such as bacterial suspensions and assays of self-driven biofilaments, can exhibit collective motions with large-scale complex flows and topological defect dynamics. Using a Doi-Onsager kinetic theory, we study suspensions of microswimmers confined to an air/liquid interface, and identify correlations between particle density fluctuations, defect structures, nematic order, and surface flows. When considering a free-standing liquid film where the microswimmers are distributed on the air/liquid interfaces, we capture hydrodynamic coupling of the two active surface, characterized by synchronization of motile disclination defects. We estimate the effective ``penetration distance'' between the two coupled surfaces through a linear stability analysis. [Preview Abstract] |
Sunday, November 23, 2014 3:33PM - 3:46PM |
D6.00007: Continuum Level Results from Particle Simulations of Active Suspensions Blaise Delmotte, Eric Climent, Franck Plouraboue, Eric Keaveny Accurately simulating active suspensions on the lab scale is a technical challenge. It requires considering large numbers of interacting swimmers with well described hydrodynamics in order to obtain representative and reliable statistics of suspension properties. We have developed a computationally scalable model based on an extension of the Force Coupling Method (FCM) to active particles. This tool can handle the many-body hydrodynamic interactions between $O(10^{5})$ swimmers while also accounting for finite-size effects, steady or time-dependent strokes, or variable swimmer aspect ratio. Results from our simulations of steady-stroke microswimmer suspensions coincide with those given by continuum models, but, in certain cases, we observe collective dynamics that these models do not predict. We provide robust statistics of resulting distributions and accurately characterize the growth rates of these instabilities. In addition, we explore the effect of the time-dependent stroke on the suspension properties, comparing with those from the steady-stroke simulations. [Preview Abstract] |
Sunday, November 23, 2014 3:46PM - 3:59PM |
D6.00008: Instability and spinodal decomposition of chemically active suspensions Wen Yan, John Brady Chemically active particles can self-propel by diffusiophoresis with velocity $\mathbf{U}=-M\nabla c$ by changing the local solute concentration $c$ via a surface catalytic reaction. Here, $M$ is the particle dffusiophoretic mobility. The particle and fluid motion is such that the convection of solute can be ignored and the concentration field $c$ is governed by Laplace's equation. We explore the collective dynamics of active particles by both continuum theory and particle-tracking simulation. In simulation the solute concentration field is accurately resolved simultaneously with the particles' motion by a multipole scattering method allowing the simulation of thousands of active particles. Active suspensions exhibit a Brinkman-like screening of long-range interactions which predicts an instability in the collective dynamics that scales with the volume fraction of active particles to the $1/2$ power. For weak phoretic motion (small $M$), the instability theory is verified by the simulations. For strong phoretic motion (large $M$), the active particles show a spinodal decomposition. Transient fractal structures are identified in 3D, while individual clusters are observed in a particle monolayer. [Preview Abstract] |
Sunday, November 23, 2014 3:59PM - 4:12PM |
D6.00009: Pair Interaction of Catalytically Active Colloidal Particles Nima Sharifi-Mood, Sergey Shklyaev, Ubaldo C\'ordova-Figueroa An increasing number of experiments on catalytically-driven (active) colloidal particles have shown that the interaction of chemically active particles is more complicated than usual interaction of two nonreactive (passive) particles. Indeed, each chemically active particle changes the distribution of reactants which, in turn, generates an overall force on other particles. First, we consider a pair of spherically symmetric catalytic particles, which are far from each other, in a colloidal dispersion of reactants and products. In this case there appears a force which can be either attractive or repulsive depending on the stoichiometry factor of the reaction. In fact, the interaction force can be thought of as a force between two charged particles which can bear charges of either the same or opposite signs depending on the stoichiometry factor. Next, we deal with interaction between catalytic and passive (cargo) particles. It is demonstrated that the force on a cargo is exactly the same as the force imposed by a catalytic particle on another one. On the other hand, the force on a catalytic particle imposed by the cargo is much smaller. Within the above-mentioned electrostatic analogy, the cargo particle is equivalent to a particle of vanishing permittivity. [Preview Abstract] |
Sunday, November 23, 2014 4:12PM - 4:25PM |
D6.00010: Dynamics of micro-swimmers inside a peristaltic pump Adam Stinchcombe, Charles Peskin, Enkeleida Lushi Peristaltic pumping is a form of fluid transport along the length of a tube containing liquid when the tube undergoes a contraction wave. While much is known about the peristalsis of Newtonian liquids, complex ones have received limited attention. There are many examples in nature where motile micro-particles or micro-swimmers (such as bacteria or spermatozoa) are suspended in the fluid inside a peristaltic micro-pump. We present a simulation method that couples the dynamics of many micro-swimmers to each-other, the pump and the fluid flow. The pump and the fluid flow it pushes can affect the swimmer dynamics in interesting ways. Moreover, the presence of the swimmers and their collective motion can affect the net transport and mixing in the pump. The efficiency of mixing abilities of the suspension for a variety of parameters will be discussed. [Preview Abstract] |
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