Bulletin of the American Physical Society
66th Annual Meeting of the APS Division of Fluid Dynamics
Volume 58, Number 18
Sunday–Tuesday, November 24–26, 2013; Pittsburgh, Pennsylvania
Session R17: Biofluids: Locomotion XI - Microswimmers and Bacteria II |
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Chair: Mahdi Mohebbi, University of Pittsburgh Room: 305 |
Tuesday, November 26, 2013 1:05PM - 1:18PM |
R17.00001: Microfluidic study of bacterial interactions and attachment to oil drops Gabriel Juarez, Roman Stocker The swimming dynamics of bacteria are strongly influenced by interfaces. Motile bacteria often accumulate near solid-liquid and liquid-liquid interfaces and eventually attach. Attachment of bacteria to these interfaces is crucial for the formation of biofilms (liquid-solid), pellicles (liquid-air), and oil-degrading communities (liquid-liquid). However, the mechanisms of attachment by microbes to these surfaces are not well understood. We have investigated the effect of external flow on the probability of attachment of motile and non-motile marine bacteria to oil droplets in straight microchannels. We find that motility enhances the encounter rate between bacteria and drops leading to an increase in attachment. Characterizing these interactions in the presence of flow will promote an understanding of oil-microbe and particulate-microbe interactions in aquatic environments where the degradation rate of organic matter depends on the attachment and colonization by bacteria. [Preview Abstract] |
Tuesday, November 26, 2013 1:18PM - 1:31PM |
R17.00002: Turbulent flow from a microscopic organism's perspective: What does it feel like to be tiny in the ocean? Rachel E. Pepper, Evan Variano, M.A.R. Koehl Microscopic organisms in the ocean live in turbulent flows. Swimming organisms navigate through the water (e.g. larvae land on substrata, predators find prey), but the mechanisms by which they do so in turbulent flow are poorly understood. Because microscopic organisms are smaller than the Kolmagorov length, they experience turbulence as a Lagrangian time series of varying linear gradients in velocity, and as a Lagrangian time series of varying accelerations. But what are these time series like? While the average gradients, accelerations, and timescales can be estimated from the dissipative scales of the flow, there are indications that organisms are disproportionally affected by intermittent events. Understanding the frequency of such events in different environments is critical to understanding how microorganisms respond to and navigate in turbulent flow. To understand the hydrodynamic cues that microscopic organisms experience in the ocean we measure velocity gradients and accelerations along Lagrangian trajectories in realistic ocean flow on the spatial and temporal scales encountered by such organisms. Here we compare results measured using PIV for hydrodynamic cues above rough biological substrata and smooth substrata, as well as cues near and far from substrata. [Preview Abstract] |
Tuesday, November 26, 2013 1:31PM - 1:44PM |
R17.00003: Squirming At Finite Reynolds Number Nicholas Chisholm, Ziyi Zhu, Aditya Khair The dynamics of swimming microorganisms at zero Reynolds number ($Re = 0$) has been the subject of extensive theoretical and experimental investigation over the past decade, and the study of locomotion at high Reynolds number ($Re \gg 1$), where inertial forces are dominant, has a venerable history. In this talk, we consider swimming between these limits, i.e. at finite Reynolds numbers, using the popular ``squirmer'' model of self-propulsion, wherein locomotion is achieved through surface distortions. We first utilize matched asymptotic expansions to derive an analytical expression for the swimming velocity of a squirmer through $O(Re^2)$, which highlights that inertia affects so-called ``pusher'' and ``puller'' swimmers in fundamentally different manners. (The equivalent of Whitehead's paradox for a self-propelled object is elucidated in the process.) Next, we employ numerical methods to compute the swimming velocity of pushers and pullers for higher Reynolds numbers. Finally, we demonstrate that inertia causes squirmers (which are non-chiral) to drift across the streamlines of an imposed shear flow. [Preview Abstract] |
Tuesday, November 26, 2013 1:44PM - 1:57PM |
R17.00004: General Squirming Motion in a Stokes Flow On Shun Pak, Eric Lauga Some microorganisms such as ciliates (\textit{Opalina}) and colonies of flagellates (\textit{Volvox}) are approximately spherical in shape and swim using beating arrays of cilia covering their surfaces. The ciliary motion over the surface may be mathematically modeled as the generation of effectively tangential velocities on the spherical surface -- known as squirming motion. Previous analyses assumed axisymmetry and hence restricted all swimming kinematics to take place along a line. Here we remove this limitation and extend the analysis to general non-axisymmetric squirming motion. We derive analytically the three-dimensional translational and rotational swimming velocities as well as the surrounding flow field of a general squirmer. The framework developed here completes the analysis of squirming motion in a Stokes flow. [Preview Abstract] |
Tuesday, November 26, 2013 1:57PM - 2:10PM |
R17.00005: An efficient framework for qualitative and quantitative analysis of magnetically actuated, rigid microswimmers Farshad Meshkati, U Kei Cheang, MinJun Kim, Henry Fu Artificial microswimmers or microrobots have been actively investigated for possible applications in microactuation, drug-delivery, in situ sensing and diagnostics, and microtransport and assembly. We describe simple achiral, rigid microswimmers actuated by rotating magnetic fields, and elucidate the the minimal conditions for propulsion. We present an efficient method for analyzing the propulsion of such swimmers, which is capable of predicting the speed and direction of swimming as well as the swimmer's rotational dynamics. The method assumes knowledge of the swimmer's geometry and magnetic dipole moment,which can be measured from its response to an impulsive change in the direction of the magnetic field. The method only requires a single calculation of the swimmer's mobility matrix using a boundary element method such as the method of regularized Stokeslets. We validate our method by finding good agreement with experiment for both qualitative and quantitative predictions. The method described can be easily applied to rigid swimmers with arbitrary geometries which are rotated by external magnetic fields. [Preview Abstract] |
Tuesday, November 26, 2013 2:10PM - 2:23PM |
R17.00006: Copepod Trajectory Characteristics in Thin Layers of Toxic Algal Exudates D.R. Webster, A.C. True, M.J. Weissburg, J. Yen Recently documented thin layers of toxic phytoplankton (``cryptic blooms'') are modeled in a custom flume system for copepod behavioral assays. Planar laser-induced fluorescence (LIF) measurements quantify the spatiotemporal structure of the chemical layers ensuring a close match to \textit{in situ} bloom conditions and allowing for quantification of threshold dissolved toxin levels that induce behavioral responses. Assays with the copepods \textit{Acartia tonsa} (hop-sinker) and \textit{Temora longicornis}(cruiser) in thin layers of toxic exudates from the common dinoflagellate \textit{Karenia brevis }(cell equivalent $\sim$ 1 - 10,000 cells/mL) examine the effects of dissolved toxic compounds and copepod species on swimming trajectory characteristics. Computation of parameters such as swimming speed and the fractal dimension of the two-dimensional trajectory (F$_{2D}$) allows for statistical evaluation of copepod behavioral responses to dissolved toxic compounds associated with harmful algal blooms (HABs). Changes in copepod swimming behavior caused by toxic compounds can significantly influence predator, prey, and mate encounter rates by altering the fracticality (``diffuseness'' or ``volume-fillingness'') of a copepod's trajectory. As trophic mediators linking primary producers and higher trophic levels, copepods can significantly influence HAB dynamics and modulate large scale ecological effects through their behavioral interactions with toxic blooms. [Preview Abstract] |
Tuesday, November 26, 2013 2:23PM - 2:36PM |
R17.00007: Capturing stealthy microswimmers into sphere-bound orbits Daisuke Takagi, Jeremie Palacci, Adam Braunschweig, Michael Shelley, Jun Zhang In potential applications ranging from microfluidic mixing to cargo transport, microscopic swimmers must propel themselves through complex environments. However the interaction of swimmers and obstacles is not well understood. Here we study the autonomous movement of catallytically-driven Au-Pt nanorods through a suspension of solid spheres resting on a horizontal plane. Though the spheres exert no net force or torque on the rods, the rods experience a short-range attraction and orbit around the spheres with essentially no decrease in their speed. We propose that the apparent attraction and speed conservation are a consequence of lubrication effects and the phoretic propulsion mechanism of nanorods. This suggests strategies to capture various self-propelled bodies and motile cells in confined spaces. [Preview Abstract] |
Tuesday, November 26, 2013 2:36PM - 2:49PM |
R17.00008: ABSTRACT WITHDRAWN |
Tuesday, November 26, 2013 2:49PM - 3:02PM |
R17.00009: The sperm flagellum counterbend phenomenon Hermes Gadelha, Eamonn Gaffney, Alain Goriely Recent observations of flagellar counterbend in sperm show that the mechanical induction of curvature in one part of a passive flagellum induces a compensatory countercurvature elsewhere. This apparent paradoxical effect cannot be explained using the standard elastic rod theory of Euler and Bernoulli, or even the more general Cosserat theory of rods. Here, we develop a mechanical model capable of predicting the curvature reversal events observed in eukaryotic flagella. This is achieved by allowing the interaction of deformations in different material directions, by not only accounting for structural bending, but also the elastic forces originating from the cross-linking mechanics. Large amplitude configurations can be described analytically and an excellent match between the model and the observed counterbend deformation was found. This allowed a simultaneous estimation of multiple sperm flagellum material parameters, namely the cross-linking sliding resistance, the bending stiffness and the sperm head junction compliance ratio. Our analysis demonstrates that the counterbend emerges as a fundamental property of sliding resistance, which also suggests that cross-linking proteins may contribute to the regulation of the flagellar waveform in swimming sperm via counterbend mechanics. Finally, we investigate how the counterbend-type dynamics in sperm flagella is affected by viscous dissipation. [Preview Abstract] |
Tuesday, November 26, 2013 3:02PM - 3:15PM |
R17.00010: Stability of a downflowing gyrotactic microorganism suspension: formation of blips Yongyun Hwang, Tim Pedley Hydrodynamic focusing of cells is a robust feature in downflowing suspensions of swimming gyrotatic microorganisms. In the early experiments with a downward pipe flow, Kessler (1986, \emph{J. Fluid Mech}, 173:191-205) observed that the focussed beam-like structure of cells in the region of most rapid downflow exhibits regular-spaced axisymmetric blips, but the mechanism by which the blips are formed has not been well understood yet. For this purpose, we perform a linear stability analysis of a downflowing suspension of randomly swimming gyrotactic cells in a two-dimensional vertical channel. For relatively small flow rates, the focussed beam in the channel exhibits a varicose instability strikingly similar to the blips in the pipe flow, and this becomes gradually damped out as the flow rate increases. It is found that the varicose instability essentially originates from the interaction of cell-concentration fluctuations with the horizontal gradient of the cell-orientation vector field, which does not appear in unform suspensions. A comparison is finally made with recent experimental results by Croze \& Bees (2013, In preparation), showing qualitatively good agreement. [Preview Abstract] |
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