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 R39: Bio: Suspensions and Colloids |
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Chair: Sophie Ramananarivo, UC San Diego Room: Portland Ballroom 256 |
Tuesday, November 22, 2016 1:30PM - 1:43PM |
R39.00001: Manipulating colloidal assemblies with active dopants Sophie Ramananarivo, Jeremie Palacci The dynamics of a densely packed 2D layer of colloids can be significantly altered upon introducing a small amount of active microparticles. Those motile intruders drive the system out-of-equilibrium, which produces a variety of new complex phenomena such as the accentuation of density heterogeneities or the reorganization of crystalline colloidal structures. We investigate the altered dynamics of the passive spheres, as well as the behavior of micro-swimmers propelling in such crowded environment where interactions with passive obstacles or other active units become important. Ultimately, understanding and controlling such mixed systems could open new routes toward activity-assisted manipulation of colloids, potentially guiding the design of materials able to self-anneal their defects. [Preview Abstract] |
Tuesday, November 22, 2016 1:43PM - 1:56PM |
R39.00002: Effective diffusivity in active Brownian suspensions Eric Burkholder, John Brady We study the single-particle diffusion of a Brownian probe of size $R$ in a suspension comprised of a Newtonian solvent and a dilute dispersion of active Brownian particles (ABPs) with size $a$, characteristic swim velocity $U_0$, and a reorientation time $\tau_R$. These ABPs, or “swimmers,” have a run length $\ell = U_0 \tau_R$, and a mechanical activity $k_s T_s = \zeta_a U_0^2 \tau_R /6$, where $\zeta_a$ is the Stokes drag coefficient of a swimmer. When the swimmers are inactive, collisions between the probe and the swimmers sterically hinder the probe's diffusive motion. When the activity of the swimmers is greater than the Boltzmann energy, $k_s T_s > k_B T$, rather than being sterically hindered, the probe diffusivity is actually greater than its Stokes-Einstein-Sutherland diffusivity due to the mechanical energy imparted to the probe upon collisions with the swimmers. The active contribution to the effective diffusivity is a non-monotonic function of the swimmers' run length compared to the contact length between the probe and a swimmer: $\ell/(R+a)$. Comparisons are made to previous theoretical and experimental investigations of the hydrodynamic diffusion of a colloidal particle in a dilute suspension of swimming bacteria. [Preview Abstract] |
Tuesday, November 22, 2016 1:56PM - 2:09PM |
R39.00003: Shear bands in concentrated bacterial suspensions under oscillatory shear Xiang Cheng, Devranjan Samanta, Xinliang Xu Bacterial suspensions show interesting rheological behaviors such as a remarkable ``superfluidic'' state with vanishing viscosity. Although the bulk rheology of bacterial suspensions has been experimentally studied, shear profiles within bacterial suspensions have not been systematically explored so far. Here, by combining confocal rheometry with PIV, we investigated the flow behaviors of concentrated {\it E. coli} suspensions under planar oscillatory shear. We found that concentrated bacterial suspensions exhibit strong non-homogeneous flow profiles at low shear rates, where shear rates vanish away from the moving shear plate. We characterized the shape of the nonlinear shear profiles at different applied shear rates and bacterial concentrations and activities. The shear profiles follow a simple scaling relation with the applied shear rates and the enstrophy of suspensions, unexpected from the current hydrodynamic models of active fluids. We demonstrated that this scaling relation can be quantitatively understood by considering the power output of bacteria at different orientations with respect to shear flows. Our experiments reveal a profound influence of shear flows on the locomotion of bacteria and provide new insights into the dynamics of active fluids. [Preview Abstract] |
Tuesday, November 22, 2016 2:09PM - 2:22PM |
R39.00004: Interaction of localized convection cells in the bioconvection of {\it Euglena gracilis} Makoto Iima, Takayuki Yamaguchi {\it Euglena gracilis} is a unicellular flagellated photosynthetic alga. The suspension of {\it Euglena} has behavioral responses to light, which causes a macroscopic localized bioconvection pattern when illuminated from below. One of the fundamental structures of this is a pair of convection cells, and high cell density region exists in the middle of the pair[1]. Experimental studies show various types of interaction in the localized convection cells; bound state, collision, etc. We performed numerical simulation of a hydrodynamic model of this system, and show results of the interactions. Long-range interaction due to the conservation of cell number and merging process of two localized structures will be discussed. [1] "Localized Bioconvection Patterns and Their Initial State Dependency in Euglena gracilis Suspensions in an Annular Container", E. Shoji, et al. J. Phys. Soc. Jpn. 83, 043001 (2014) [Preview Abstract] |
Tuesday, November 22, 2016 2:22PM - 2:35PM |
R39.00005: Vortex lattices and defect-mediated viscosity reduction in active liquids Jonasz Slomka, Jorn Dunkel Generic pattern-formation and viscosity-reduction mechanisms in active fluids are investigated using a generalized Navier-Stokes model that captures the experimentally observed bulk vortex dynamics in microbial suspensions. We present exact analytical solutions including stress-free vortex lattices and introduce a computational framework that allows the efficient treatment of previously intractable higher-order shear boundary conditions. Large-scale parameter scans identify the conditions for spontaneous flow symmetry breaking, defect-mediated low-viscosity phases and negative-viscosity states amenable to energy harvesting in confined suspensions. The theory uses only generic assumptions about the symmetries and long-wavelength structure of active stress tensors, suggesting that inviscid phases may be achievable in a broad class of non-equilibrium fluids by tuning confinement geometry and pattern scale selection. [Preview Abstract] |
Tuesday, November 22, 2016 2:35PM - 2:48PM |
R39.00006: Flagellum synchronization inhibits large-scale hydrodynamic instabilities in sperm suspensions Simon F. Sch\"oller, Eric E. Keaveny Sperm in suspension can exhibit large-scale collective motion and form coherent structures. Our picture of such coherent motion is largely based on reduced models that treat the swimmers as self-locomoting rigid bodies that interact via steady dipolar flow fields. Swimming sperm, however, have many more degrees of freedom due to elasticity, have a more exotic shape, and generate spatially-complex, time-dependent flow fields. While these complexities are known to lead to phenomena such as flagellum synchronization and attraction, how these effects impact the overall suspension behaviour and coherent structure formation is largely unknown. Using a computational model that captures both flagellum beating and elasticity, we simulate suspensions on the order of $10^3$ individual swimming sperm cells whose motion is coupled through the surrounding Stokesian fluid. We find that the tendency for flagella to synchronize and sperm to aggregate inhibits the emergence of the large-scale hydrodynamic instabilities often associated with active suspensions. However, when synchronization is repressed by adding noise in the flagellum actuation mechanism, the picture changes and the structures that resemble large-scale vortices appear to re-emerge. [Preview Abstract] |
Tuesday, November 22, 2016 2:48PM - 3:01PM |
R39.00007: Self-Driven Droplet Powered By Active Nematics Tong Gao, Zhaorui Li, Michael Shelley Active matter defines a class of emerging bio-inspired materials composed of self-driven micro-particles and far away from equilibrium. Their anormalous physical properties and the means to control them, suggest novel methods in mixing/separation, micro-pumps and motors, self-healing materials etc. The possibility of realizing these applications hinges on a through understanding of the physical mechanisms as well as developing means to manipulate various active systems. By using of a coarse-grained active liquid crystal model, we design and investigate self-driven droplets encapsulating a dense suspension of active particles. We show that a single droplet can be set into motion due to the internal collective motions that are featured by active flows and motile disclination defects. We illustrate that the interplays between the induced directional flows, liquid crystalline structures, and the deformable interface with surface tension can result in tunable mobilities of motile droplets that undergo novel locomotion and rotation. [Preview Abstract] |
Tuesday, November 22, 2016 3:01PM - 3:14PM |
R39.00008: Theory of meiotic spindle assembly Sebastian Furthauer, Peter Foster, Daniel Needleman, Michael Shelley The meiotic spindle is a biological structure that self assembles from the intracellular medium to separate chromosomes during meiosis. It consists of filamentous microtubule (MT) proteins that interact through the fluid in which they are suspended and via the associated molecules that orchestrate their behavior. We aim to understand how the interplay between fluid medium, MTs, and regulatory proteins allows this material to self-organize into the spindle's highly stereotyped shape. To this end we develop a continuum model that treats the spindle as an active liquid crystal with MT turnover. In this active material, molecular motors, such as dyneins which collect MT minus ends and kinesins which slide MTs past each other, generate active fluid and material stresses. Moreover nucleator proteins that are advected with and transported along MTs control the nucleation and depolymerization of MTs. This theory captures the growth process of meiotic spindles, their shapes, and the essential features of many perturbation experiments. It thus provides a framework to think about the physics of this complex biological suspension. [Preview Abstract] |
Tuesday, November 22, 2016 3:14PM - 3:27PM |
R39.00009: Transient aggregation and long-time diffusion of bacterial suspensions in time periodic flows Boyang Qin, Rebecca Winter, Madhura Gurjar, David Gagnon, Alison Patteson, Paulo Arratia In this talk, the transport dynamics of swimming bacteria in time-periodic flows is investigated in experiments and simulations. Experiments are performed by introducing swimming bacteria (\textit{Vibrio cholerae}) in a low Reynolds number, two-dimensional flow driven electromagnetically. We observe two distinct transport regimes: (i) entrapment of bacteria inside vortex and near elliptic points and (ii) aggregation and subsequent transport along the flow manifolds. These time-dependent behaviors are set by the interaction between swimmer kinematics (e.g. speed, tumbling frequency, etc) and flow properties. Numerical simulation using a stochastic Langevin model are able to capture the main experimental results including the entrapment of bacteria near elliptic points and the rapid spreading along manifolds. Results show a significant reduction in long-time effective diffusion of the swimmer as vortex strength is increased. The conditions for bacterial entrapment in vortex flows are discussed. [Preview Abstract] |
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