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 G6: Biofluids: Active Fluids III |
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Chair: Eva Kanso, University of Southern California Room: 3010 |
Monday, November 24, 2014 8:00AM - 8:13AM |
G6.00001: Copepod Behavior in ``Cryptic Blooms'' of Toxic Algae A.C. True, D.R. Webster, M.J. Weissburg, J. Yen Copepods,\textit{ Acartia tonsa} and \textit{Temora longicornis}, were exposed to thin layers of exudates from the toxic dinoflagellate \textit{Karenia brevis} (1 - 10,000 cells/mL) (i.e. models of ``cryptic blooms'' of toxic phytoplankton). Planar laser-induced fluorescence (PLIF) was used to quantify the spatiotemporal structure of the layer allowing for correlation of behavioral responses with toxin levels. Both species explicitly avoided the exudate layer and the vicinity of the layer. Measures of path kinematics (swimming speed, turn frequency) by location (in-layer vs. out-of-layer) and exposure (pre-contact vs. post-contact) revealed some similarities, but also significant differences, in trends for each species. \textit{A. tonsa} significantly increases swimming speed and swimming speed variability in the exudate layer and post-contact, whereas \textit{T. longicornis} slightly increases both in-layer and slightly reduces both post-contact. Both species increase turn frequency in-layer and post-contact with increasing \textit{K. brevis} exudate concentration. Path fracticality indicates that \textit{A. tonsa }trajectories became more diffuse/sinuous and \textit{T. longicornis} trajectories became more linear/ballistic (trending effects). Regression analyses revealed that the rate of change of behavior with increasing exudate concentration for \textit{A. tonsa} was thrice to fifty times that of \textit{T. longicornis}. Toxic \textit{K. brevis} can essentially eliminate top-down grazer control$, $another sinister means by which it gains a competitive advantage over the local phytoplankton taxa. [Preview Abstract] |
Monday, November 24, 2014 8:13AM - 8:26AM |
G6.00002: Collective fluid mechanics of honeybee nest ventilation Nick Gravish, Stacey Combes, Robert J. Wood, Jacob Peters Honeybees thermoregulate their brood in the warm summer months by collectively fanning their wings and creating air flow through the nest. During nest ventilation workers flap their wings in close proximity in which wings continuously operate in unsteady oncoming flows (i.e. the wake of neighboring worker bees) and near the ground. The fluid mechanics of this collective aerodynamic phenomena are unstudied and may play an important role in the physiology of colony life. We have performed field and laboratory observations of the nest ventilation wing kinematics and air flow generated by individuals and groups of honeybee workers. Inspired from these field observations we describe here a robotic model system to study collective flapping wing aerodynamics. We microfabricate arrays of 1.4 cm long flapping wings and observe the air flow generated by arrays of two or more fanning robotic wings. We vary phase, frequency, and separation distance among wings and find that net output flow is enhanced when wings operate at the appropriate phase-distance relationship to catch shed vortices from neighboring wings. These results suggest that by varying position within the fanning array honeybee workers may benefit from collective aerodynamic interactions during nest ventilation. [Preview Abstract] |
Monday, November 24, 2014 8:26AM - 8:39AM |
G6.00003: Collective interaction of microscale matters in natural analogy: human cancer cells vs. microspheres Sungsook Ahn, Sang Joon Lee Collective behaviors have been considered both in living and lifeless things as a natural phenomenon. During the ordering process, a sudden and spontaneous transition is typically generated between an order and a disorder according to the population density of interacting elements. In a cellular level collective behavior, the cells are distributed in the characteristic patterns according to the population density and the mutual interaction of the individual cells undergo density-dependent diffusive motion. On the other hand, density-controlled surface-modified hollow microsphere suspension induces an overpopulation via buoyancy which provides a driving force to induce an assembly. The collective behaviors of the cells and microspheres in a designed liquid medium are explained in terms of the deviation from the interparticle distance distribution and the induced strength to organize the particle position in a specific distance range. as a result, microscale particulate matters exhibit high resemblance in their pair correlation and dynamical heterogeneity in the intermediate range between a single individual and an agglomerate. Therefore, it is suggested that biological systems are analogically explained to be dominated by physically interactive aspects. [Preview Abstract] |
Monday, November 24, 2014 8:39AM - 8:52AM |
G6.00004: Responding to flow: How phytoplankton adapt migration strategies to tackle turbulence Anupam Sengupta, Francesco Carrara, Roman Stocker Phytoplankton are among the ocean's most important organisms and it has long been recognized that turbulence is a primary determinant of their fitness. Yet, mechanisms by which phytoplankton may adapt to turbulence have remained unknown. We present experiments that demonstrate how phytoplankton are capable of rapid adaptive behavior in response to fluid flow disturbances that mimic turbulence. Our study organism was the toxic marine alga \textit{Heterosigma akashiwo}, known to exhibit ``negative gravitaxis,'' $i.e.$, to frequently migrate upwards against gravity. To mimic the effect of Kolmogorov-scale turbulent eddies, which expose cells to repeated reorientations, we observed \textit{H. akashiwo} in a ``flip chamber,'' whose orientation was periodically flipped. Tracking of single cells revealed a striking, robust behavioral adaptation, whereby within tens of minutes half of the population reversed its direction of migration to swim downwards, demonstrating an active response to fluid flow. Using confocal microscopy, we provide a physiological rationalization of this behavior in terms of the redistribution of internal organelles, and speculate on the motives for this bet-hedging-type strategy. This work suggests that the effects of fluid flow -- not just passive but also active -- on plankton represents a rich area of investigation with considerable implications for some of earth's most important organisms. [Preview Abstract] |
Monday, November 24, 2014 8:52AM - 9:05AM |
G6.00005: A hybrid numerical-experimental study of fluid transport by migrating zooplankton aggregations Monica Martinez, John Dabiri, Janna Nawroth, Brad Gemmell, Samantha Collins Zooplankton aggregations that undergo diel vertical migrations have been hypothesized to play an important role in local nutrient transport and global ocean dynamics. The degree of the contributions of these naturally occurring events ultimately relies on how efficiently fluid is transported and eventually mixed within the water column. By implementing solutions to the Stokes equations, numerical models have successfully captured the time-averaged far-field flow of self-propelled swimmers. However, discrepancies between numerical fluid transport estimates and field measurements of individual jellyfish suggest the need to include near-field effects to assess the impact of biomixing in oceanic processes. Here, we bypass the inherent difficulty of modeling the unsteady flow of active swimmers while including near-field effects by integrating experimental velocity data of zooplankton into our numerical model. Fluid transport is investigated by tracking a sheet of artificial fluid particles during vertical motion of zooplankton. Collective effects are addressed by studying different swimmer configurations within an aggregation from the gathered data for a single swimmer. Moreover, the dependence of animal swimming mode is estimated by using data for different species of zooplankton. [Preview Abstract] |
Monday, November 24, 2014 9:05AM - 9:18AM |
G6.00006: Population dynamics in non-homogeneous environments Kim M.J. Alards, Francesca Tesser, Federico Toschi For organisms living in aquatic ecosystems the presence of fluid transport can have a strong influence on the dynamics of populations and on evolution of species. In particular, displacements due to self-propulsion, summed up with turbulent dispersion at larger scales, strongly influence the local densities and thus population and genetic dynamics. Real marine environments are furthermore characterized by a high degree of non-homogeneities. In the case of population fronts propagating in ``fast'' turbulence, with respect to the population duplication time, the flow effect can be studied by replacing the microscopic diffusivity with an effective turbulent diffusivity. In the opposite case of ``slow'' turbulence the advection by the flow has to be considered locally. Here we employ numerical simulations to study the influence of non-homogeneities in the diffusion coefficient of reacting individuals of different species expanding in a 2 dimensional space. Moreover, to explore the influence of advection, we consider a population expanding in the presence of simple velocity fields like cellular flows. The output is analyzed in terms of front roughness, front shape, propagation speed and, concerning the genetics, by means of heterozygosity and local and global extinction probabilities. [Preview Abstract] |
Monday, November 24, 2014 9:18AM - 9:31AM |
G6.00007: Confining collective motion Denis Bartolo, Antoine Bricard, Jean-Baptiste Caussin, Charles Savoie, Debasish Das, Oleskar Chepizhko, Fernando Peruani, David Saintillan It is well established that geometrical confinement have a significant impact on the structure and the flow properties of complex fluids. Prominent examples include the formation of topological defects in liquid crystals, and the flow instabilities of viscoelastic fluids in curved geometries. In striking contrast very little is known about the macroscopic behavior of confined active fluids. In this talk we show how to motorize plastic colloidal beads and turn them into self-propelled particles. Using microfluidic geometries we demonstrate how confinement impacts their collective motion. Combining quantitative experiments, analytical theory and numerical simulations we show how a population of motile bodies interacting via alignement and repulsive interactions self-organizes into a single heterogeneous macroscopic vortex that lives on the verge of a phase separation. [Preview Abstract] |
Monday, November 24, 2014 9:31AM - 9:44AM |
G6.00008: Density shocks in confined microswimmers Alan Cheng Hou Tsang, Eva Kanso Motile microorganisms are often subject to different types of boundary confinement in their natural environment, but the effects of confinement on their dynamics are poorly understood. We consider an idealized model of confined microswimmers restricted to move in a two-dimensional Hele-Shaw cell. We then impose two different types of boundary confinement: circular and sidewalls confinement. We study how boundaries trigger the emergence of global modes. In the case of circular confinement, the microswimmers can spontaneously organize themselves into a single vortex state when the radius of the circular boundary is below a certain critical value, reminiscent to what have been observed in recent experiments of bacterial suspensions. In the case of sidewalls confinement in a rectangular channel, the microswimmers form density shock, via interaction with the sidewalls and background flow. We show that, through controlling the strength of background flow, we can manipulate the density shock to form at the back or front of the swimmer clusters or the suppression of the shock which gives rise to a uniform traveling wave of swimmers. [Preview Abstract] |
Monday, November 24, 2014 9:44AM - 9:57AM |
G6.00009: Rotating bacteria aggregate into active crystals Alexander Petroff, Xiao-lun Wu, Albert Libchaber The dynamics of many microbial ecosystems are determined not only by the response of individual bacteria to their chemical and physical environments but also the dynamics that emerge from interactions between cells. Here we investigate the collective dynamics displayed by communities of Thiovulum majus, one of the fastest known bacteria. We observe that when these bacteria swim close to a microscope cover slip, the cells spontaneously aggregate into a visually-striking two-dimensional hexagonal lattice of rotating cells. Each cell in an aggregate rotates its flagella, exerting a force that pushes the cell into the cover slip and a torque that causes the cell to rotate. As cells rotate against their neighbors, they exert forces and torques on the aggregate that cause the crystal to move and cells to hop to new positions in the lattice. We show how these dynamics arises from hydrodynamic and surface forces between cells. We derive the equations of motion for an aggregate, show that this model reproduces many aspects of the observed dynamics, and discuss the stability of these and similar active crystals. Finally, we discuss the ecological significance of this behavior to understand how the ability to aggregate into these communities may have evolved. [Preview Abstract] |
Monday, November 24, 2014 9:57AM - 10:10AM |
G6.00010: Bacterial populations growth under co- and counter-flow condition Francesca Tesser, Jos C.H. Zeegers, Herman J.H. Clercx, Federico Toschi For organisms living in a liquid ecosystem, flow and flow gradients play a major role on the population level: the flow has a dual role as it transports the nutrient while dispersing the individuals. In absence of flow and under homogeneous conditions, the growth of a population towards an empty region is usually described by a reaction diffusion equation. The solution predicts the expansion as a wave front (Fisher wave) proceeding at constant speed, till the carrying capacity is reached everywhere. The effect of fluid flow, however, is not well understood and the interplay between transport of individuals and nutrient opens a wide scenario of possible behaviors. In this work, we experimentally observe non-motile E. coli bacteria spreading inside rectangular channels in a PDMS microfluidic device. By use of a fluorescent microscope we analyze the dynamics of the population density subjected to different co- and counter-flow conditions and shear rates. [Preview Abstract] |
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