Bulletin of the American Physical Society
68th Annual Meeting of the APS Division of Fluid Dynamics
Volume 60, Number 21
Sunday–Tuesday, November 22–24, 2015; Boston, Massachusetts
Session D25: Biofluids: Cells in Flow and Flow in Cells |
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Chair: Jeffrey Guasto, Tufts University Room: 304 |
Sunday, November 22, 2015 2:10PM - 2:23PM |
D25.00001: Characterization of Intracellular Streaming and Traction Forces in Migrating Physarum Plasmodia Shun Zhang, Owen Lewis, Robert Guy, Juan Carlos del Alamo Physarum plasmodium is a model organism for cell migration that exhibits fast intracellular streaming. Single amoebae were seeded and allowed to move on polyacrilamide gels that contained 0.5-micron fluorescent beads. Joint time-lapse sequences of intracellular streaming and gel deformation were acquired respectively in the bright and fluorescent fields under microscope. These images were analyzed using particle image velocimetry (PIV) algorithms, and the traction stresses applied by the amoebae on the surface were computed by solving the elastostatic equation for the gel using the measured bead displacements as boundary conditions. These measurements provide, for the first time, a joint characterization of intracellular mass transport, the forces applied on the substrate and the signal of free intracellular calcium with high resolution in both time and space, enables a through study about the locomotive mechanism, shedding light on related biomimetic research. The results reveal a pronounced auto-oscillation character in intracellular flow, contact area, centroid speed and strain energy, all with the same periodicity about 100 seconds. Locomotion modes that were distinct in flow/ traction stress pattern as well as migration speed have been discovered and studied. [Preview Abstract] |
Sunday, November 22, 2015 2:23PM - 2:36PM |
D25.00002: Cytoplasmic flows as signatures for the mechanics of mitotic spindle positioning Ehssan Nazockdast, Abtin Rahimian, Daniel Needleman, Michael Shelley The proper positioning of the mitotic spindle is crucial for asymmetric cell division and generating cell diversity during development. We use dynamic simulations to study the cytoplasmic flows generated by three possible active forcing mechanisms involved in positioning of the mitotic spindle in the first cell division of C.elegans embryo namely cortical pulling, cortical pushing, and cytoplasmic pulling mechanisms. The numerical platform we have developed for simulating cytoskeletal assemblies is the first to incorporate the interactions between the fibers and other intracellular bodies with the cytoplasmic fluid, while also accounting for their polymerization, and interactions with motor proteins. The hydrodynamic interactions are computed using boundary integral methods in Stokes flow coupled with highly efficient fast summation techniques that reduce the computational cost to scale linearly with the number of fibers and other bodies. We show that although all three force transduction mechanisms predict proper positioning and orientation of the mitotic spindle, each model produces a different signature in its induced cytoplasmic flow and MT conformation. We suggest that cytoplasmic flows and MT conformation can be used to differentiate between these mechanisms. [Preview Abstract] |
Sunday, November 22, 2015 2:36PM - 2:49PM |
D25.00003: Cytoskeletal Dynamics and Fluid Flow in {\it Drosophila} Oocytes Gabriele De Canio, Raymond Goldstein, Eric Lauga The biological world includes a broad range of phenomena in which transport in a fluid plays a central role. Among these is the fundamental issue of cell polarity arising during development, studied historically using the model organism {\it Drosophila melanogaster}. The polarity of the oocyte is known to be induced by the translocation of mRNAs by kinesin motor proteins along a dense microtubule cytoskeleton, a process which also induces cytoplasmic streaming. Recent experimental observations have revealed the remarkable fluid-structure interactions that occur as the streaming flows back-react on the microtubules. In this work we use a combination of theory and simulations to address the interplay between the fluid flow and the configuration of cytoskeletal filaments leading to the directed motion inside the oocyte. We show in particular that the mechanical coupling between the fluid motion and the orientation of the microtubules can lead to a transition to coherent motion within the oocyte, as observed. [Preview Abstract] |
Sunday, November 22, 2015 2:49PM - 3:02PM |
D25.00004: Mirror-symmetry breakings in human sperm rheotaxis Norbert Stoop, Anton Bukatin, Igor Kukhtevich, J{\"o}rn Dunkel, Vasily Kantsler Rheotaxis, the directed response to fluid velocity gradients, has been shown to facilitate stable upstream-swimming of mammalian sperm cells along solid surfaces, suggesting a robust mechanism for long-distance navigation during fertilization. However, the dynamics by which a human sperm orients itself w.r.t ambient flows is poorly understood. Here, we combine microfluidic experiments with mathematical modeling and 3D flagellar beat reconstruction to quantify the response of individual sperm cells in time-varying flow fields. Single-cell tracking reveals two kinematically distinct swimming states that entail opposite turning behaviors under flow reversal. We constrain an effective 2D model for the turning dynamics through systematic large-scale parameter scans, and find good quantitative agreement with experiments. We present comprehensive 3D data demonstrating the rolling dynamics of freely swimming sperm cells around their longitudinal axis. Contrary to current beliefs, this analysis uncovers ambidextrous flagellar waveforms and shows that the cell’s turning direction is is not defined by the rolling direction. Instead, the different rheotactic turning behaviors are linked to a broken mirror-symmetry in the midpiece section, likely arising from a buckling instability. [Preview Abstract] |
Sunday, November 22, 2015 3:02PM - 3:15PM |
D25.00005: Tumbling and quasi-tumbling motions of E.coli over a solid surface under shear flows Mehdi Molaei, Jian Sheng Flow shear is known to alter bacterial motility by inducing Jeffery Orbit, rheotaxis, and trapping cells in the high shear region. Over a solid surface flow shear Interferes with hydrodynamic interaction of cells with solid surface. Our previous study shows that in the quiescent condition the tumbles of wild \textit{E.coli} are suppressed and tumbling reorientation of cells is restricted to the surface parallel direction. In the current study, we exposed bacteria to the well controlled shear flows inside a microchannel and applying Digital Holography Microscopy to track them over time. The results show that flow shear promotes tumbling of \textit{E.coli} and preserve reorientation of the cells during tumbles. Our hydrodynamic model indicates that in the low shear levels the tumble enhancement is due to shear induced flagella unbundling, while in the high shear flow regime, Jeffery Orbit causes rapid cell re-orientation which causes quasi-tumbles with similar angular displacement one would expect during a tumbling. [Preview Abstract] |
Sunday, November 22, 2015 3:15PM - 3:28PM |
D25.00006: Effect of cell size and shear stress on bacterium growth rate Hadi Fadlallah, Mojtaba Jarrahi, \'Eric Herbert, Hassan Peerhossaini Effect of shear stress on the growth rate of \textit{Synechocystis} and \textit{Chlamydomonas} cells is studied. An experimental setup was prepared to monitor the growth rate of the microorganisms versus the shear rate inside a clean room, under atmospheric pressure and 20 $^{\circ}$C temperature. Digital magnetic agitators are placed inside a closed chamber provided with airflow, under a continuous uniform light intensity over 4 weeks. In order to study the effect of shear stress on the growth rate, different frequencies of agitation are tested, 2 vessels filled with 150 ml of each specie were placed on different agitating system at the desired frequency. The growth rate is monitored daily by measuring the optical density and then correlate it to the cellular concentration. The PH was adjusted to 7 in order to maintain the photosynthetic activity. Furthermore, to measure the shear stress distribution, the flow velocity field was measured using PIV. Zones of high and low shear stress were identified. Results show that the growth rate is independent of the shear stress magnitude, mostly for \textit{Synechocystis}, and with lower independency for \textit{Chlamydomonas} depending on the cell size for each species. [Preview Abstract] |
Sunday, November 22, 2015 3:28PM - 3:41PM |
D25.00007: Laboratory and Field Observations of \textit{Microcystis aeruginosa} in nearly homogeneous turbulent flows Anne Wilkinson, Miki Hondzo, Michele Guala \textit{Microcystis aeruginosa} is a single-celled cyanobacterium, forming large colonies on the surface of freshwater ecosystems during summer, and producing a toxin (microcystin) that in high concentration can be harmful to humans and animals. In addition to water temperature, light and nutrient abundance, fluid motion is also an abiotic environmental factor affecting the growth and metabolism of \textit{Microcystis}. Systematic investigations in a laboratory bioreactor are paired with field measurements in the lacustrine photic zone from two sites in Lake Minnetonka (MN) to ensure that dissipation levels, water temperature, dissolved oxygen and pH are correctly reproduced under laboratory conditions. Laboratory results for biomass accrual and photosynthetic activity suggest that turbulence levels within the range observed in the field, mediates the metabolism, rather than the cell population growth, of \textit{Microcystis aeruginosa}. [Preview Abstract] |
Sunday, November 22, 2015 3:41PM - 3:54PM |
D25.00008: Rheotaxy induced localisation of E-coli in Poiseuille flow Harold AURADOU, Matias Lopez, Carine Douarche, Eric Clément The transport of bacteria in Poiseuille flow is crucial in many situations in particular those involving flow in porous media. By counting the number of bacteria as function of the distance from the center of a capillary tube, we show that the bacteria are not equally spread over the section. Using different strains of E-coli bacteria and bacteria with different swimming velocity, we bright evidence that bacteria depletes (at low shear rate) or accumulates (at high shear rate) in the vicinity of the wall. We finally show that this phenomena comes from rheotaxy. [Preview Abstract] |
Sunday, November 22, 2015 3:54PM - 4:07PM |
D25.00009: Invariant manifolds as barriers to the motion of bacteria in vortex flows Katie Lilienthal, Doan Minh, Tom Solomon We present experiments that study the motion of swimming bacteria (bacillus subtilis) in a time-independent vortex flow. The flow is a pair of vortices generated in a microfluidic cell composed of either a cross or an H-shaped channel. Experiments are done with both wild-type and a genetically-mutated ``smooth swimming'' \footnote{R. Rusconi, J.S. Guasto and R. Stocker, Nature Physics {\bf 10}, 212 (2014).} bacillus subtilis. We analyze the trajectories of these bacteria in terms of invisible barriers, based on a theory of ``burning invariant manifolds'' \footnote{J. Mahoney, D. Bargteil, M. Kingsbury, K. Mitchell and T. Solomon, Europhys. Lett. {\bf 98}, 44005 (2012).} that act as one-way barriers that impede the motion of reaction fronts in a fluid flow. We explore whether similar one-way barriers impede the motion of bacteria. [Preview Abstract] |
Sunday, November 22, 2015 4:07PM - 4:20PM |
D25.00010: Imaging of microscale mixing in biological suspensions Kwangmin Son, Roman Stocker In many biological processes, reaction rates are set by the degree of mixing. A prime example is virus-host infection. Protocols and approaches in the study of these processes often ignore fundamental principles on stirring and mixing, which show how difficult or lengthy it can be to truly mix biological scalars, such a microorganisms. Such results date back to the classical works of Purcell (JFM 1978) and Batchelor (JFM 1979), yet were mostly limited to theoretical predictions, which have awaited accurate experimental testing and have not made their way into biological applications to date. Here we investigate the stirring and mixing of suspensions of motile and nonmotile microorganisms by real-time imaging with optical microscopy, testing theoretical predictions and demonstrating that fundamental protocols in biology often vastly underestimate the heterogeneity in biological suspensions arising from incomplete mixing. [Preview Abstract] |
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