Bulletin of the American Physical Society
60th Annual Meeting of the Divison of Fluid Dynamics
Volume 52, Number 12
Sunday–Tuesday, November 18–20, 2007; Salt Lake City, Utah
Session KH: Biofluids XII: Propulsion |
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Chair: John Kessler, University of Arizona Room: Salt Palace Convention Center 250 B |
Tuesday, November 20, 2007 8:00AM - 8:13AM |
KH.00001: Measure of coherence on the collective phase of concentrated swimming bacteria Luis Cisneros, Ricardo Cortez, Christopher Dombrowski, Raymond Goldstein, John Kessler Nearly close-packed populations of the swimming bacterium B. subtilis form a collective phase, the ``Zooming BioNematic'' (ZBN). This state exhibits turbulence-like dynamics. It also develops large-scale orientational coherence, analogous to the molecular alignment of nematic liquid crystals. The alignment of the bacterial bodies is polar. The resulting dynamics exhibit remarkable spatial and temporal correlations of velocity and vorticity, measured by standard applications of particle imaging velocimetry. We introduce a new method of analysis which consists of defining a scalar field to measure the level of coherent directional motion in the velocity field. This novel method shows the global continuity of angular and polar correlations -- information that is otherwise hidden in the standard correlation function analysis. By means of this new tool, we define an order parameter to characterize the collective phase. Some new statistical analyses on the distribution of coherent regions and velocities have been developed. [Preview Abstract] |
Tuesday, November 20, 2007 8:13AM - 8:26AM |
KH.00002: Propulsion of a bacterial phalanx John O. Kessler, Ricardo Cortez, Luis Cisneros, Raymond E. Goldstein, Christopher Dombrowski Concentrated populations of swimming rod-shaped bacteria spontaneously form domains. Within a domain the bacteria are nearly close-packed; they all swim in the same direction. A propagating domain is a phalanx. The forward members of the domain push fluid forward, the flagella of the rear rows push fluid backwards, yielding propulsion. In the transverse direction fluid flows both in and out, but there is little net flow within a phalanx. The bacterial cell bodies and the flagella that surround them (emerging from preceding cell bodies) exert opposite stresses on the interstitial fluid. Using slender body theory, in the Reynolds number $<<$1 regime, a cylindrical phalanx is propelled by the flagella emerging from its rear. Dimensional analysis yields a collective propagation velocity of the same order as that of individual swimmers, even though a large population of swimmers is propelled by only a small number of bacteria, proportional to volume fraction to the 2/3 power. [Preview Abstract] |
Tuesday, November 20, 2007 8:26AM - 8:39AM |
KH.00003: Prey-Induced Swimming Dynamics Changes among Predatory Algae J. Katz, J. Sheng, E. Malkiel, J. Adolf, A. Place, R. Belas High speed, cinematic digital holographic microscopy allows us to track thousands of microorganisms over a volume with substantial depth without loss of resolution. This technique enables us, for the first time, to examine, measure and characterize the swimming dynamics of microorganisms located within dense suspensions. The present experiments examine dense populations of predatory algae, \textit{K. veneficum} and \textit{P. piscicida}, prior to and after introducing prey. Swimming dynamics are characterized by radius and pitch of helical swimming trajectories, by translational and angular velocity, and their velocity spectra. \textit{K. veneficum} moves in both left and right hand helices, while\textit{ P. piscicida} swims only in right hand helices. The radii increase with increasing velocity for both cases. Presented with its prey, \textit{K. veneficum} reduces its velocity, radius and pitch, but increases its angular velocity. Conversely, \textit{P. piscicida} increases its speed, radius and angular velocity. Power spectra of velocity reveal differences between scales of vertical velocity and those of horizontal components. Power spectra of velocity component aligned with the helix centerline reveals a shift in \textit{K. veneficum's }swimming strategy from almost random-walk to a levy-walk as prey is introduced. \textit{P. piscicida} always displays clear preference towards levy-walk, but spectral slope increases as prey is introduced. [Preview Abstract] |
Tuesday, November 20, 2007 8:39AM - 8:52AM |
KH.00004: Long-range correlations in suspensions of swimming micro-organisms Patrick Underhill, Juan Hernandez-Ortiz, Michael Graham Recently large collections of swimming microorganisms have been observed producing collective motions on a scale much larger than the scale of a single organism. To better understand the cause of these motions, computer simulations of large populations of hydrodynamically interacting swimming particles have been performed at low Reynolds number in periodic and confined geometries. The interactions of the particles lead to long-range spatial correlations in the fluid at scales larger than the size of a single organism. These long-range correlations lead to a large enhancement in the fluid transport properties. The diffusivity of passive, non-Brownian tracer particles diverges in the periodic geometry with increasing the simulation box size. However, simple scaling arguments can still capture much of the physics of both the swimmer and tracer motions. [Preview Abstract] |
Tuesday, November 20, 2007 8:52AM - 9:05AM |
KH.00005: The role of near field interactions in the collective behaviour of model swimming micro-organisms Tobias Locsei, Takuji Ishikawa, Tim Pedley Experimentalists have reported that high concentration suspensions of swimming microorganisms exhibit collective behaviour, in the sense that each organism aligns its swimming direction with those of its near neighbours. Here, we present Stokesian dynamics simulations of swimming model micro-organisms (spherical `squirmers'), which exhibit similar collective behaviour to what others have observed experimentally. Analysis of the simulations results reveals the following: (i) the collective behaviour is due to very close range (lubrication-regime) interactions between squirmers, (ii) the outcome of a close-range interaction between two squirmers is effectively deterministic, in that it is only weakly affected by the far-fields of the other squirmers, and (iii) the beginnings and ends of close-range interactions may be understood in terms of a `suction' effect, in that two adjacent squirmers attract one another if the sum of the surface divergences of their surface velocities is positive at the point of nearest contact, and repel one another otherwise. [Preview Abstract] |
Tuesday, November 20, 2007 9:05AM - 9:18AM |
KH.00006: Theory of swimming filaments in viscoelastic media Henry Fu, Thomas Powers, Charles Wolgemuth Motivated by the swimming of sperm in the non-Newtonian fluids of the female mammalian reproductive tract, we examine the swimming of filaments in the nonlinear viscoelastic Upper Convected Maxwell model. We obtain the swimming velocity and hydrodynamic force exerted on an infinitely long cylinder with prescribed beating pattern. We use these results to examine the swimming of a simplified sliding-filament model for a sperm flagellum. Viscoelasticity tends to decrease swimming speed, and changes in the beating patterns due to viscoelasticity can reverse swimming direction. [Preview Abstract] |
Tuesday, November 20, 2007 9:18AM - 9:31AM |
KH.00007: On the propulsive efficiency of rotating elastica. Marc Fermigier, Nicolas Champagne, Eric Laik, Joel Marthelot, Olivia du Roure A majority of microorganisms propel themselves with long flexible cilia or flagella. Understanding in detail the hydrodynamics of such propulsion mechanisms is important both from biological and engineering point of views, in particular to design artificial microswimmers. We report an experimental investigation of the propulsive force delivered by a rotating elastic filament. Macroscopic filaments made of an elastomer (Young's modulus $E$) loaded with solid particles to match the density of the suspending liquid are rotated at constant velocity $\omega$ in a bath of glycerin. Their three dimensional shape is time independent but varies with $\omega$ and aspect ratio $L/a$. The force on the filament is computed from the experimental shape using a slender body approximation (ratio of perpendicular and parallel friction coefficients : $\zeta_{\perp} = 2 \zeta_{\parallel}$). The evolution of axial force is captured by a single dimensionless parameter comparing viscous and elastic stresses: $Sp = (\eta \omega/E) (L/a)^4$. As for a planar oscillating flexible tail a maximum force is found at $Sp \approx 1$. [Preview Abstract] |
Tuesday, November 20, 2007 9:31AM - 9:44AM |
KH.00008: Tethered fleximags: a physical model for ciliary propulsion. Olivia du Roure, Avin Babataheri, Patrice Jenffer, Marc Fermigier, Cecile Goubault Fleximags are linear colloidal structures made of micron-sized superparamagnetic particles. Permanent links between colloids are established through molecules grafted on the particles. The elasticity of the linker bestows a flexibility to the filament. The fleximags have already been used to make one of the first artificial microswimmers (Dreyfus et al. Nature 2005) resembling a spermatozoon. They can also be anchored to a glass substrate isolated or as arrays. Those arrays build up experimental models of the array of cilia on paramecium for studying physical aspects of the propulsion. Here we'll show our first studies concerning anchored flexiamgs submitted to time-dependent field. The actuation is controlled by three electromagnets and allowing all types of 3-D movements: (a)symmetric beating in a plane, rotation... We first study one single anchored fleximag when the field is rotating on a cone. Only a part of the filament is moving reflecting the competition between magnetic interactions, elasticity and viscosity. The length of this mobile fraction decreases with frequency. We also study the induced flow by PIV. [Preview Abstract] |
Tuesday, November 20, 2007 9:44AM - 9:57AM |
KH.00009: Modeling of the motion of the actin filament on the myosin motility assays Yuan Young, Mike Shelley In motility assays, cytoskeletal actin filaments (actin filaments) glide over a surface coated with motor proteins, and the different modes of motion provide a simple measure of the force exerted by the motor proteins (Bourdieu, 1995). Motivated by these experiments, we consider the actin filament as a slender, elastic filament immersed in Stokesian flow, driven by a tangential forcing that mimics the force by the motor proteins. We find qualitative agreement on several points between our analysis and simulations and experimental observations. Furthermore, we study the correlation between filament transport and the characteristics of motion with the spatial pattern of motor protein density. [Preview Abstract] |
Tuesday, November 20, 2007 9:57AM - 10:10AM |
KH.00010: The hydrodynamics of interactions between two swimming bacteria Takuji Ishikawa, Go Sekiya, Yohsuke Imai, Takami Yamaguchi This study evaluates the hydrodynamic interactions between two swimming bacteria precisely. We assume that each bacterium is force-free and torque-free, with a Stokes flow field around it. The geometry of each bacterium is modeled as a spherical or spheroidal body with a single helical flagellum. The movements of two interacting bacteria in an infinite fluid otherwise at rest are computed using a boundary element method and the trajectories of the two interacting bacteria and the stresslet are investigated. The results show that as the two bacteria approach each other, they change their orientations considerably in the near field. The bacteria always avoided each other; no stable pairwise swimming motion was observed in this study. The effects of the hydrodynamic interactions between two bacteria on the rheology and diffusivity of a semi--dilute bacterial suspension are discussed. [Preview Abstract] |
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