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 A6: Biofluids: Active Fluids I |
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Chair: Arezoo Ardekani, Purdue University Room: 3010 |
Sunday, November 23, 2014 8:00AM - 8:13AM |
A6.00001: Origination of turbulence in dense suspensions of sperm cells Petr Denissenko, Jackson Kirkman-Brown, David Smith, Vasily Kantsler Motile micro-organisms with pushing flagella, such as sperm cells, can be directed by ``one way'' microchannels with ratchet teeth-like wall configuration. We use an array of such micro-channels to gradually concentrate human spermatozoa in a circular arena of 1 mm diameter and 200 micron depth. Velocities of individual cells are measured by particle tracking and velocity of cell-carrying fluid is measured using PIV. At high concentrations, fluid velocities and the velocity fluctuations of individual cells exceeding that of individual swimmers in the dilute regime by an order of magnitude have been measured. Velocity correlations are calculated to study evolution of characteristic length scales as the cell concentration increases. Results are discussed in the context of self-organisation phenomena in active fluids and cooperation of sperm cells. [Preview Abstract] |
Sunday, November 23, 2014 8:13AM - 8:26AM |
A6.00002: Inverse turbulent cascade in swarming sperm Adama Creppy, Olivier Praud, Xavier Druart, Philippa Kohnke, Franck Plouraboue Collective motion of self-sustained swarming flows has recently provided examples of small scale turbulence arising where viscosity effects are dominant. We report the first observation of an universal inverse enstrophy cascade in concentrated swarming sperm consistent with a body of evidence built from various independent measurements. We found a well-defined $k^{-3}$ power-law decay of velocity field power-spectrum and relative dispersion of small beads consistent with theoretical predictions in two-dimensional turbulence. Concentrated living sperm displays long-range, correlated whirlpool structures the size of which provides turbulence's integral scale. We propose a consistent explanation for this quasi-two-dimensional turbulence based on self-structured laminated flow forced by steric interaction and alignment, a state of active matter that we call ``swarming liquid crystal.'' We develop scaling arguments consistent with this interpretation. The implication of multi-scale collective dynamics of sperm's collective motility for fertility assessment is discussed. [Preview Abstract] |
Sunday, November 23, 2014 8:26AM - 8:39AM |
A6.00003: Bacterial turbulence in motion Roberto Rusconi, Steven Smriga, Roman Stocker, Eleonora Secchi, Stefano Buzzaccaro, Roberto Piazza Dense suspensions of motile bacteria exhibit collective dynamics akin to those observed in classic, high Reynolds number turbulence, yet this analogy has remained largely qualitative. Here we present experiments in which a dense suspension of \textit{Bacillus subtilis} bacteria was flown through narrow microchannels and the velocity statistics of the flowing suspension were accurately quantified with a recently developed velocimetry technique. This revealed a robust intermittency phenomenon, whereby the average velocity profile of the flowing suspension oscillated between a plug-like flow and a parabolic flow. This intermittency is a hallmark of classic turbulence and was associated with the presence of collective structures in the suspension. Furthermore, quantification of the Reynolds stress profile revealed a direct link between the turbulent nature of the suspension and its anomalous viscosity. [Preview Abstract] |
Sunday, November 23, 2014 8:39AM - 8:52AM |
A6.00004: Diffusion of an ellipsoid in a quasi-2D bacteria suspension Yi Peng, Xiang Cheng Enhanced translational diffusion of spherical particles induced by a suspension of bacteria has been established as a distinct feature for active fluids. Here, instead of spherical tracer particles, we study the diffusion of ellipsoidal particles of various aspect ratios in a free-standing film of bacteria. Using high-speed digital video microscopy, we measured the mean-square displacements and calculated the translational and rotational diffusion coefficients of the elliptical tracer particles. We found that both the translational and rotational diffusion of the particles are dramatically enhanced by the motion of bacteria. The probability distribution functions for linear and angular displacements become non-Gaussian at high bacterial concentrations. Moreover, we also explored the coupling between translational and rotational diffusion induced by the swimming bacteria. [Preview Abstract] |
Sunday, November 23, 2014 8:52AM - 9:05AM |
A6.00005: Enhanced diffusion self-stimulated by micro-algae in an active, aerotactic bacterial suspension Fran\c{c}ois Peaudecerf, Raymond E. Goldstein Suspensions of swimming bacteria form a new class of active fluids that generate complex phenomena. An ``active bath'' of bacteria for instance produces fluid flows which move passive colloids in a random-like walk, associated with an effective diffusion coefficient higher than for Brownian motion. The value of this enhanced diffusion coefficient depends on the local density of bacteria and their swimming behavior. However, with aerotactic, obligate aerobic bacteria such as \emph{B. subtilis}, the local oxygen concentration impacts on the distribution of cells and their swimming behavior. We consider the specific case in which non-motile photosynthetic algal cells interacting with a \emph{B. subtilis} suspension not only play the role of passive colloids, but also produce oxygen under light. We demonstrate that this new kind of active suspension, under heterogeneous illumination, can induce an effective negative phototaxis of the passive algal cells. We explain the origin of this novel phenomenon as the combination of algal oxygen production, diffusion, chemotaxis and motility switching in bacteria resulting in an heterogeneous enhanced diffusion. Finally, we present potential applications for algal cell mixing and sorting, that can inspire new methods for bioengineering. [Preview Abstract] |
Sunday, November 23, 2014 9:05AM - 9:18AM |
A6.00006: Hydrodynamic dispersion of microswimmers in suspension Matthieu Martin, Salima Rafa\"I, Philippe Peyla In our laboratory, we study hydrodynamics of suspensions of micro-swimmers. These micro-organisms are unicellular algae Chlamydomonas Rheinhardii which are able to swim by using their flagella. The swimming dynamics of these micro-swimmers can be seen as a random walk, in absence of any kind of interaction. In addition, these algae have the property of being phototactic, i.e. they swim towards the light. Combining this property with a hydrodynamic flow, we were able to reversibly separate algae from the rest of the fluid. But for sufficiently high volume fraction, these active particles interact with each other. We are now interested in how the coupling of hydrodynamic interactions between swimmers and phototaxis can modify the swimming dynamics at the scale of the suspension. To this aim, we conduct experiments in microfluidic devices to study the dispersion of the micro-organisms in a the liquid phase as a function of the volume fraction. We show that the dispersion of an assembly of puller type microswimmers is quantitatively affected by hydrodynamics interactions. [Preview Abstract] |
Sunday, November 23, 2014 9:18AM - 9:31AM |
A6.00007: Photomixing of chlamydomonas rheinhardtii suspensions Julien Dervaux, Marina Capellazzi Resta, B\'ereng\`ere Abou, Philippe Brunet Chlamydomonas rheinhardtii is a fast swimming unicellular alga able to bias its swimming direction in gradients of light intensity, an ability know as phototaxis. We have investigated experimentally both the swimming behavior of individual cells and the macroscopic response of shallow suspensions of these micro-organisms in response to a localized light source. At low light intensity, algae exhibit positive phototaxis and accumulate beneath the excitation light. In weakly concentrated thin layers, the balance between phototaxis and cell motility results in steady symmetrical patterns compatible with a purely diffusive model using effective diffusion coefficients extracted from the analysis of individual cell trajectories. However, at higher cell density and layer depth, collective effects induce convective flows around the light source. These flows disturb the cell concentration patterns which spread and may then becomes unstable. Using large passive tracer particles, we have characterized the velocity fields associated with this forced bioconvection and their dependence on the cell density and layer depth. By tuning the light distribution, this mechanism of photo-bioconvection allows a fine control over the local fluid flows, and thus the mixing efficiency, in algal suspensions. [Preview Abstract] |
Sunday, November 23, 2014 9:31AM - 9:44AM |
A6.00008: Phototactic number-density flux in the localized bioconvection of Euglena gracilis Erika Shoji, Nobuhiko Suematsu, Hiraku Nishimori, Akinori Awazu, Shunsuke Izumi, Makoto Iima Euglena gracilis is a unicellular phototactic flagellate; it escapes from light sources if the light intensity is higher than 200W/m$^{2}$ (negative phototaxis). When the suspension of E.gracilis is illuminated from the bottom by strong light, bioconvection patterns are generated. In the case of E.gracilis, the patterns can be spatially localized. The localization mechanism has not been clarified. We report experimental results related to the localization mechanism. In particular, we experimentally measured the strength of the phototaxis in the lateral direction as well as vertical direction. We prepared a thin container in which the suspension is included, and gave the linearly-changing light intensity. We found the number density gets a peak at a particular light intensity, which never happens if the suspension has the vertical phototaxis only. Further, we succeeded in getting the function representing lateral phototaxis. The relationship between the measured functions and the localized convection cells will be also reported.\\[4pt] [1] Localized bioconvection patterns and their initial state dependency in Euglena suspensions in an annular container, E. Shoji, H. Nishimori, A. Awazu, S. Izumi, and M. Iima, J. Phys. Soc. Jpn. 83(2014)04300 [Preview Abstract] |
Sunday, November 23, 2014 9:44AM - 9:57AM |
A6.00009: Suppression of resistance to flow in suspensions of bacteria Hector Lopez, J\'er\'emie Gachelin, Carine Douarche, Eric Cl\'ement, Harold Auradou It is usually believed that the influence of small amounts of bacteria on the rheological properties of a fluid is negligible. However, recent theoretical studies predict that the activity results in a decrease of the viscosity at values lower than the suspending fluid viscosity. We present experimental measurements of the viscosity of suspensions of \textit{Escherichia coli} (volume fractions $\phi $\textless 1{\%}) in a simple Couette flow over a broad range of shear rates. For shear rates larger than 1.5 s$^{-1}$, the viscosity is constant and slightly above the viscosity of the suspending fluid. This behavior is similar to the one expected for non-active particles. For lower shear rates the fluid exhibits a non-Newtonian behavior: the viscosity decreases and finally reaches a second Newtonian plateau for shear rates below 0.1 s$^{-1}$. For $\phi $ \textless 0.6{\%}, the decrease is proportional to the bacteria concentration, as predicted by the theories, suggesting that it is a result of the energy input of each individual microswimmer. For $\phi $ \textgreater 0.6{\%}, we evidence for the first time the existence of a super-lubrication regime where the viscous resistance to shear vanishes. We will demonstrate that this regime holds up over a large window of concentration. [Preview Abstract] |
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