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 G9: Biofluids: Microswimmers I |
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Chair: Oscar Curet, Florida Atlantic University Room: 3014/3016 |
Monday, November 24, 2014 8:00AM - 8:13AM |
G9.00001: Locomotion in a turbulent world M. Koehl When organisms swim or crawl in aquatic habitats, the water through which they travel is usually moving. Therefore, an important part of understanding how aquatic organisms locomote is determining how they interact with the fluctuating turbulent water currents through which they move. The research systems we have been using to address this question are microscopic marine animals swimming in turbulent, wavy water flow or crawling on surfaces in spatially-complex habitats exposed to such flow. Using a combination of field studies, wave-flume experiments, experiments in fluidic devices, and mathematical modeling, we have discovered that small organisms swimming or crawling in turbulent flow are not subjected to steady velocities. The shears, accelerations, and odor concentrations encountered by small swimmers and crawlers fluctuate rapidly, with peaks much higher than mean values. Although microscopic organisms swim slowly relative to ambient water flow, their locomotory behavior in response to the rapidly-fluctuating shears and odors they encounter can affect where they are transported by ambient water movement. Furthermore, the ability of small organisms to walk on surfaces without being dislodged by pulses of rapid flow constrains the microhabitats in which they can forage. [Preview Abstract] |
Monday, November 24, 2014 8:13AM - 8:26AM |
G9.00002: The Effect of Small Scale Turbulence on the Physiology of \textit{Microcystis aeruginosa} cyanobacterium Anne Wilkinson, Miki Hondzo, Michele Guala \textit{Microcystis aeruginosa} is a single-celled blue-green alga, or cyanobacterium, that is responsible for poor water quality and microcystin production, which in high concentrations can be harmful to humans and animals. These harmful effects arise during cyanobacterium blooms. Blooms occur mainly in the summer when the algae grow uncontrollably and bond together to form colonies which accumulate on the surface of freshwater ecosystems. The relationship between fluid motion generated by wind and internal waves in stratified aquatic ecosystems and \textit{Microcystis} can help explain the mechanisms of such blooms. We investigated the effect of small scale fluid motion on the physiology of \textit{Microcystis} in a reactor with two underwater speakers. Different turbulent intensities were achieved by systematically changing the input signal frequency (30-50Hz) and magnitude (0.1-0.2V) to the speakers. The role of turbulence is quantified by relating energy dissipation rates with the cell number, chlorophyll amount, dissolved oxygen production/uptake, and pH. The results suggest that turbulence mediates the physiology of \textit{Microcystis}. The findings could be instrumental in designing restoration strategies that can minimize \textit{Microcystis} blooms. [Preview Abstract] |
Monday, November 24, 2014 8:26AM - 8:39AM |
G9.00003: Response of \textit{Acartia tonsa} to Burgers' Vortex: Deconstructing Turbulence-Copepod Interactions D.L. Young, D.R. Webster, J. Yen \textit{In situ} studies suggest that in many oceanic regimes, turbulence affects the vertical position of copepods primarily by changing their behavior, and only secondarily by altering their physical position. We test the hypothesis that fine-scale turbulence alters copepod behavior, presenting as alterations in directed movement and changes in swimming kinematics. To this end, we create two Burgers' vortices, specifying the rotation rate and axial strain rate to correspond to turbulent vortices with size scale equaling the inverse wavenumber of the median viscous dissipation rate (i.e. $r \quad =$ 8.1$\eta )$ for typical turbulent conditions in the coastal or near surface region (i.e., mean turbulent dissipation rates of 0.009 and 0.096 cm$^{2}$/s$^{3})$. The vortex flow is quantified via tomo-PIV. Behavioral assays of \textit{Acartia tonsa} are conducted, generating 3D trajectories for analysis of swimming kinematics and response to hydrodynamic cues. \textit{A. tonsa} did not significantly respond to the vortex corresponding to dissipation rate of 0.009 cm$^{2}$/s$^{3}$, but drastically altered their swimming behavior in the presence of the 0.096 cm$^{2}$/s$^{3}$ vortex, including increased relative swim speed, angle of alignment with the vortex axis, net-to-gross displacement ratio, and escape acceleration, along with decreased turn frequency (relative to stagnant control conditions). Further, \textit{A. tonsa} escape location is preferentially in the core of the stronger vortex, indicating that the hydrodynamic cue triggering the distinctive escape behavior is vorticity. [Preview Abstract] |
Monday, November 24, 2014 8:39AM - 8:52AM |
G9.00004: Comparison of Turbulence-Copepod Interaction: \textit{Temora longicornis} vs. \textit{Acartia tonsa} N.H. De Jesus-Villanueva, D.L. Young, D.R. Webster, J. Yen The purpose of this study is to examine the behavioral response of the marine copepod \textit{Temora longicornis }to a Burgers' vortex intended to mimic the characteristics of a turbulent vortex that a copepod is likely to encounter in the coastal or near surface zone. Copepod behavioral assays were conducted for two turbulence levels corresponding to mean turbulent dissipation rates of 0.009 (Level 2) and 0.096 (Level 3) cm$^{2}$/s$^{3}$. The Burgers' vortex parameters (i.e., circulation and axial strain rate) are specified to match a vortex corresponding to the median viscous dissipation rate for each target turbulence level. The behavioral response of \textit{T. longicornis} compared to \textit{Acartia tonsa} is of particular interest due to differences in swim style (cruiser vs. hop-sinker, respectively) and mechanosensory array morphology (planar vs. 3D, respectively). When exposed to these vortex flow treatments, \textit{T. longicornis }exhibited a minimal behavioral response to the Level 2 vortex, but significantly altered their swimming behavior in the presence of the Level 3 vortex. Specifically, in the Level 3 vortex treatment \textit{T. longicornis} increased relative swim speed, turn frequency, and escape acceleration while decreasing angle of alignment with the vortex axis and escape frequency (relative to stagnant control conditions). Histograms of escape jump location as a function of radius reveals no preferential escape location for \textit{T. longicornis}, which contrasts the preferential escape location of \textit{A. tonsa} in the vortex core. [Preview Abstract] |
Monday, November 24, 2014 8:52AM - 9:05AM |
G9.00005: Response of a Motile/Non-Motile Escherichia coli Front to Hydrodynamic excitations Magali Baabour, Carine Douarche, Dominique Salin In a recent study (Douarche et. al. PRL 102, 198101 (2009)), it has been shown that the motility of Escherichia coli (E. coli) is highly correlated to the oxygen level in a minimal medium: bacteria swim as long as they are provided with oxygen but reversibly transit to a non-motile state when they lack of it. Hence, when oxygen diffuses into an anaerobic sample of non-motile bacteria, a propagating front delineates a region of motile bacteria where oxygen is present from a region of non-motile ones where the oxygen is still not present. To study the response of this front to hydrodynamics excitation, we use the same fluorescent E. coli bacterial solution in rectangular cross section glass cells open to air (oxygen) at one inlet. After bacteria have consumed the oxygen of the solution, the presence of the oxygen only at the open edge of the sample leads to the formation of an analogous stationary front between motile and non-motile bacteria. We follow the response of this front to hydrodynamics flows such as an oscillating Poiseuille flow or natural convection. We analyze both the macroscopic behavior (shape and width) of the front as well as the microscopic displacements of individual bacteria. The dispersive behavior of this bacterial front is compared to the one of equivalent [Preview Abstract] |
Monday, November 24, 2014 9:05AM - 9:18AM |
G9.00006: Gyrotactic cells subject to imposed 3D flows Nicholas Hill, Scott Richardson, Andrew Baggaley We examine the effect of imposed 3-dimensional test flows, specifically a Taylor--Green Vortex flow and an ABC flow, on the patterns and mixing of suspensions of gyrotactic swimming cells. Numerically solving the deterministic swimming trajectory equations for individual cells with random starting positions, we explore how the surrounding flow and the cell shape determine the long-time patterns. For certain parameter ranges these patterns often take the form of braided ``plume-lie'' structures, even when using the chaotic ABC flow. For various pattern configurations, analysis of the governing equations of motion reveals why they are formed, as analytical solutions of the equations for the swimming cell trajectories can be obtained. These patterns persist when small random perturbations (noise) are added to individual trajectories. [Preview Abstract] |
Monday, November 24, 2014 9:18AM - 9:31AM |
G9.00007: Distribution of particle displacements due to swimming microorganisms Jean-Luc Thiffeault The experiments of Leptos et al. [\textit{Phys. Rev. Lett.} \textbf{103}, 198103 (2009)] show that the displacements of small particles affected by swimming microorganisms achieve a non-Gaussian distribution, which nevertheless scales diffusively. We use a simple model where the particles undergo repeated ``kicks'' due to the swimmers to explain the shape of the distribution as a function of the volume fraction of swimmers. The net displacement is determined by the self-convolution of the drift function caused by one swimmer, and a Poisson distribution for the frequency of interactions. The only adjustable parameter is the strength of the stresslet term in our spherical squirmer model. The effective diffusivity measured in the experiments is consistent with the model, with no further parameter adjustments. [Preview Abstract] |
Monday, November 24, 2014 9:31AM - 9:44AM |
G9.00008: Finding the best swimming sheet Tom Ives, Alexander Morozov Many microorganisms propel through fluid environments by undulating their bodies or long thin organelles (flagella). The particular waveform of the undulations can often be changed by the organism to adapt to particular environmental conditions. It has been proposed in the literature that this adaptation is driven by the desire to optimise the swimming efficiency. However, it remains an open question as to whether this is indeed the optimised quantity for microorganisms. We study propulsion in Newtonian fluids at zero inertia for a model organism, the so-called Taylor waving sheet. We develop a numerical method that allows us to calculate flow fields for sheets of arbitrary waverforms in the bulk and next to a wall. We perform optimisations of various quantities that can potentially be optimised by a swimming microorganisms (efficiency, speed, etc.) and present the optimal waveforms. We also present a simple analytical model that yields similar results. We conclude that various optimal waveforms are very similar, both in the bulk and next to a boundary, and one cannot claim that optimising the swimming efficiency is the strategy adopted by undulating microorganisms. [Preview Abstract] |
Monday, November 24, 2014 9:44AM - 9:57AM |
G9.00009: Understanding the detailed motion of a model bacterium Akanksha Thawani, Mahesh Tirumkudulu Inspired by the motion of flagellated bacteria such as \textit{Escherichia coli} and \textit{Bacillus subtilis}, we have built a macroscopic model bacterium, in order to investigate the intricate aspects of their motion which cannot be visualized under a microscope. The flagellated rod shaped cells were approximated with a spherical head attached to a rigid metal helix, via a plastic hook. The motion of model bacterium was observed in a high viscosity silicone oil to replicate the low Reynolds number flow conditions. A significant wobble was observed even in the absence of an off-axis flagellum. We suspect that the flexibility in the hook connecting the head and flagellum is the cause for wobble, since wobble was observed to increase significantly with hook-flexibility. The motion of the model bacterium was predicted using the Slender Body theory of Lighthill, and was compared with the measured trajectories. [Preview Abstract] |
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