Bulletin of the American Physical Society
64th Annual Meeting of the APS Division of Fluid Dynamics
Volume 56, Number 18
Sunday–Tuesday, November 20–22, 2011; Baltimore, Maryland
Session L29: Low Reynolds Number Swimming in Shear Flow |
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Chair: Jerry Gollub, Haverford College Room: Ballroom III-IV |
Monday, November 21, 2011 3:35PM - 3:48PM |
L29.00001: Statics and dynamics of planar shearable filaments in viscous fluid: A Cosserat rod approach Hermes Gadelha, Eamonn Gaffney, Alain Goriely Cilia and flagella are ubiquitous in biology as a means of motility and constitute one of the most incredible engineering works of nature. Their inner core, namely the axoneme, consists of a remarkable phylogenetically conserved cytoskeletal structure formed by an assembly of semifexible filaments interconnected by crosslinking proteins. As a result, the flagellum or cilium is not only capable to flexure under the action of an external load, but also to shear. The latter is however a consequence from the intricate elastic crosslinking proteins which causes the elastic bending to couple with shearing deformations, modifying dramatically the effective mechanical response of these bundles of filamentous polymers. We consider deformations of nonlinearly elastic slender rods immersed in a fluid, and analyse the differences between the elastic cross-link shear response and pure material shear resistance under the action of viscous dissipation. We show that pure material shearing effects from Timoshenko's beam theory or, equivalently, Cosserat Rod Theory are fundamentally different from elastic crosslink induced shear found in filament bundles, such as the axoneme. [Preview Abstract] |
Monday, November 21, 2011 3:48PM - 4:01PM |
L29.00002: Drag reduction of a hairy disk Jun Niu, David Hu We investigate experimentally the hydrodynamics of a hairy disk immersed in a two-dimensional flowing soap film. Drag force is measured as a function of hair length, density and coating area. An optimum combination of these parameters yields a drag reduction of 17\%, which confirms previous numerical predictions of 15\% by Favier et al (2009). Flow visualization indicates the primary mechanism for drag reduction is the bending, adhesion and reinforcement of hairs trailing the disk, which reduces wake width and traps ``dead water.'' Thus, the use of hairy coatings can substantially reduce an object's drag while negligibly increasing its weight. [Preview Abstract] |
Monday, November 21, 2011 4:01PM - 4:14PM |
L29.00003: Low Reynolds number swimming in a stratified fluid Amin Doostmohammadi, Roman Stocker, Arezoo Ardekani Significant progress has been made in analyzing low-Reynolds number locomotion in homogeneous fluids. Even though many aquatic environments are influenced by vertical variations in density, the effects of stratification on the hydrodynamics of swimming of small organisms are very poorly understood. In this article, by using a squirmer model, we show that motility, energy expenditure, and nutrient uptake of small organisms in a density stratified fluid can be largely influenced due to buoyancy effects. Not only does the stratification suppress the swimming velocity, but it also enhances the nutrient uptake and the energy required for a squirmer to swim across pycnoclines. [Preview Abstract] |
Monday, November 21, 2011 4:14PM - 4:27PM |
L29.00004: Evaluating Satiated Copepod Behavioral Responses to Thin Layer Flow Structure Aaron C. True, Donald R. Webster, Marc J. Weissburg, Jeannette Yen Zooplankton exploit a variety of chemical and fluid mechanical cues in foraging, mate-seeking, and habitat partitioning contexts. To examine the influence of environmental cues on zooplankton aggregations in coastal marine thin layers, a laboratory thin layer mimic was built. The apparatus uses a laminar, planar jet (the Bickley jet) to produce ecologically-relevant layers of chemical (beneficial and harmful phytoplankton) and fluid mechanical (shear strain rate) cues for zooplankton behavioral assays. Particle image velocimetry (PIV) and laser-induced fluorescence (LIF) were employed to fully quantify the spatial structure of the chemical and fluid mechanical cues, ensuring a close match to \textit{in situ} conditions and allowing for investigations into threshold cue levels responsible for inducing behavioral responses. Evaluating the effect of hunger level on behavioral responses is particularly important for producing accurate individual-based simulations of zooplankton population dynamics. Behavioral assays with the calanoid copepod \textit{Temora longicornis} have produced digitized trajectories and, subsequently, path kinematics. Observed behaviors include increased turn frequency and decreased relative swimming speed, which result in increased residence time in the free jet shear layer. Cue-induced individual behaviors have the potential to produce population-scale aggregations. [Preview Abstract] |
Monday, November 21, 2011 4:27PM - 4:40PM |
L29.00005: Bacterial Rheotaxis Marcos Marcos, Henry Fu, Thomas Powers, Roman Stocker Rheotaxis is the directed movement of an organism resulting from fluid velocity gradients, long studied in fish, aquatic invertebrates and spermatozoa. Here we show that rheotaxis also occurs in bacteria. Using controlled microfluidic shear flows, we demonstrate and quantify rheotaxis in \textit{Bacillus subtilis}. A mathematical model of a bacterium swimming in a shear flow is in good agreement with observations and reveals that bacterial rheotaxis results from a subtle interplay between velocity gradients and the helical shape of flagella, which together generate a torque that reorients the cell, altering its swimming direction. The magnitude of the observed rheotactic velocity is comparable to typical chemotactic velocities, suggesting that rheotaxis can interfere with bacterial processes based on directed motility, such as foraging and infection. [Preview Abstract] |
Monday, November 21, 2011 4:40PM - 4:53PM |
L29.00006: Bacterial motility and chemotaxis in shear Roberto Rusconi, Jeffrey S. Guasto, Kwangmin Son, Roman Stocker Bacteria often exhibit directed motility (``taxis'') in response to gradients of dissolved resources, like nutrients or oxygen. While we have a detailed understanding of chemotaxis in quiescent environments, it has been largely overlooked how this behavior is affected by fluid flow, despite the ubiquity of flow in bacterial habitats. Here we present experiments on aerotaxis (attraction to dissolved oxygen) of \textit{Bacillus subtilis} in controlled shear flows. Using novel microfluidic devices we expose bacterial suspensions to steady oxygen gradients, with independent control over shear rates. From single-cell trajectories and the spatial distribution of bacteria, we show that the cell rotation induced by shear reduces the aerotactic performance, demonstrating that hydrodynamic conditions affect bacterial fitness. [Preview Abstract] |
Monday, November 21, 2011 4:53PM - 5:06PM |
L29.00007: Chemotactic Motility of Sperm in Shear Jeffrey S. Guasto, Jeffrey A. Riffell, Richard K. Zimmer, Roman Stocker Chemical gradients are utilized by plants and animals in sexual reproduction to guide swimming sperm cells toward the egg. This process (``chemotaxis''), which can greatly increase the success of fertilization, is subject to interference by fluid flow, both in the bodily conduits of internal fertilizers (e.g. mammals) and in the aquatic environment of external fertilizers (e.g. benthic invertebrates). We studied the biomechanics of chemotaxing sea urchin spermatozoa using microfluidic devices, which allow for the precise and independent control of attractant gradients and fluid shear. We captured swimming trajectories and flagellar beat patterns using high-speed video-microscopy, to detect chemotactic responses and measure the effect of fluid forces on swimming. This work will ultimately help us to understand how swimming sperm cells actively navigate natural chemoattractant gradients for successful fertilization. [Preview Abstract] |
Monday, November 21, 2011 5:06PM - 5:19PM |
L29.00008: Enhanced vertical mixing by grass shrimp (Palaemenets) in a linearly stratified fluid Claudia Cenedese, Colm-cille P. Caulfield, William K. Dewar, Peter H. Wiebe, Nancy Copley Laboratory experiments investigated the vertical mixing generated by grass shrimp (Palaemenets) in a linearly stratified tank (50x50x70 cm). The linear stratification ($N$~=~0.19~s$^{-1})$ becomes homogeneous in approximately two days, much faster than if only molecular diffusion was at play. The density evolution in time agrees well with the solution of the diffusion equation for values of enhanced $k_{s}$ (2.0 10$^{-2 }$cm$^{2}$/s when 100 shrimp are present and $k_{s}$~=~1.0 10$^{-2 }$cm$^{2}$/s with 50 shrimp) when compared to the molecular diffusivity of salt $k_{s}$~=~1.3 10$^{-5}$ cm$^{2}$/s. Hence, the laboratory experiments suggest that the enhanced vertical mixing generated by the presence of grass shrimp in a linearly stratified ambient fluid has a diffusive behavior with a diffusivity enhanced by 3 orders of magnitude when compared to the molecular diffusivity of salt. [Preview Abstract] |
Monday, November 21, 2011 5:19PM - 5:32PM |
L29.00009: Bacterial locomotion in a wall bounded shear flow Jian Sheng, Roman Stocker, Mehdi Moalei Statistically robust experimental observations on swimming characteristics of bacteria in a wall bounded shear flow are crucial for understanding cell attachment and detachment, interfacial rheology during the initial formation of a biofilm. We combined microfluidics and holography to measure 3-D trajectories of \textit{E. coli} (AW405), subjecting to a carefully controlled shear flow. Experiments are conducted in a straight micro-channel of 40x3x0.2 mm, latter being the depth, with the maximum shear rates up to 200 s$^{-1}$. Holographic microscopic movies recorded at 40X magnification and 15 fps are streamed in real time to a data acquisition computer for an extended period of time ($>$20 min) that allows us to examine long term shear responses. Three-dimensional locations and orientations of bacterial are extracted with a resolution of 0.185 $\mu $m in lateral directions and 0.5 $\mu $m in the wall normal direction. The 3-D trajectories are tracked by an in-house developed particle tracking algorithm. Over thousand 3-D trajectories over a sample volume of 380$\times $380$\times $200 $\mu $m have been obtained. On-going analysis focuses on the effects of flow on cell migration and attachment near a sheared surface. [Preview Abstract] |
Monday, November 21, 2011 5:32PM - 5:45PM |
L29.00010: Bacteria Aggregation in a Steady Vortical Flow Shahrzad Yazdi, Sixing Li, Tony Jun Huang, Arezoo Ardekani The interaction between microorganisms and flow field is an important, yet complicated topic that affects the design of biological reactors, marine ecological processes, and biofilm formation in porous media. Vortical structures and secondary flows are inherently present in porous media despite small Reynolds numbers. Our experimental results show that bacteria in a steady vortical flow aggregate and subsequently form biofilm streamers in a microfluidic system. The combined effects of shape, motility and the vortical background flow contribute to this fast bacteria aggregation. [Preview Abstract] |
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