Bulletin of the American Physical Society
2005 58th Annual Meeting of the Division of Fluid Dynamics
Sunday–Tuesday, November 20–22, 2005; Chicago, IL
Session FA: Collective Dynamics |
Hide Abstracts |
Chair: Raymond Goldstein, University of Arizona Room: Hilton Chicago Waldorf |
Monday, November 21, 2005 8:00AM - 8:13AM |
FA.00001: Transport by Collective Flagellar Beating Facilitates Evolutionary Transitions to Multicellularity Martin Short, Cristian Solari, Sujoy Ganguly, John Kessler, Raymond Goldstein, Thomas Powers A central problem underlying the evolution from single cells to multicellular organisms is the relationship between metabolic requirements and environmental metabolite exchange with increasing size. For organisms that form spherical colonies such as the volvocalean green algae, there is a bottleneck if diffusion alone governs nutrient uptake as they increase in size, for the diffusive flux is linear in the radius while the requirements of surface somatic cells grow quadratically. Using Volvox as a model organism, we examine experimentally and theoretically the role that advection of fluid by surface flagella plays in enhancing nutrient uptake. We show that the fluid flow driven by the coordinated beating of those flagella produces a boundary layer in the concentration of a diffusing solute which renders the metabolite exchange rate quadratic in the colony radius. This bypasses the diffusive bottleneck, facilitating evolutionary transitions to multicellularity which may be driven by other environmental factors. These results suggest that flagella may have evolved not only for motility, but also to enhance metabolite exchange. [Preview Abstract] |
Monday, November 21, 2005 8:13AM - 8:26AM |
FA.00002: Collective dynamics of hydrodynamically interacting self-propelled particles Juan Hernandez-Ortiz, Christopher Stoltz, Michael Graham Direct simulations of large populations of hydrodynamically interacting swimming particles confined between two infinite walls at low Reynolds number are performed. Hydrodynamic coupling between the swimmers leads to large-scale coherent vortex motions and regimes of anomalous diffusion that are consistent with experimental observations. According to the simulations the natural length scale for the collective dynamics of the suspension is the distance between walls. At low concentrations, swimmers propelled from behind (like spermatazoa) strongly migrate toward solid surfaces in agreement with simple theoretical considerations; at higher concentrations this localization is disrupted by the large-scale coherent motions. [Preview Abstract] |
Monday, November 21, 2005 8:26AM - 8:39AM |
FA.00003: The rheology of a semi-dilute suspension of swimming cells Takuji Ishikawa, T.J. Pedley In this paper, a swimming micro-organism is modelled as a squirming sphere with prescribed tangential surface velocity, in which the centre of mass of the sphere may be displaced from the geometric centre. The three-dimensional movement of 64 identical squirmers in a simple shear flow field, contained in a cube with periodic boundary conditions, is computed, for random initial positions and orientations, by the Stokesian- dynamics method. The results for non-bottom-heavy squirmers show that the squirming causes a slight decrease in the apparent viscosity. In the case of bottom-heavy squirmers, on the other hand, the suspension shows strong non-Newtonian properties. When the background simple shear flow is directed vertically, the apparent viscosity becomes smaller than that of inert spheres. When the shear flow is horizontal and varies with the vertical coordinate, however, the apparent viscosity becomes larger than that of inert spheres. In addition, significant normal stress differences appear for all relative orientations of gravity and the shear flow, in the case of bottom-heavy squirmers. [Preview Abstract] |
Monday, November 21, 2005 8:39AM - 8:52AM |
FA.00004: Flows and transverse forces of self propelled micro-swimmers John Kessler, Ricardo Cortez We employ the properties of Stokes flows, using simple model ``organisms'' possessing the features needed, nothing more. At Reynolds numbers$<<$1 self propelled swimmers exert equal forward and backward forces on the fluid. Fore/aft asymmetry =$>$ locomotion. For spheres of unequal radii R, connected by an elongating \textit{Gedanken}-rod, V(1)R(1)=V(2)R(2). Similarly other geometries, since drag is linear in velocity V. Time independence implies that an entire flow field develops ``instantaneously'': Calculating flows around an ``organism'' during an instant of self propulsion maps the entire field, while avoiding details of return strokes, or propulsion by a bundle of helices. Using the method of regularized Stokeslets, we find flows and interactions for various geometries. Transverse return flows toward the midsection of a swimmer, due to incompressibility, are associated with attraction of swimmers, to each other and boundaries, just as found in experiments with \textit{Bacillus subtilis.} This work partially funded by NSF grants DMS0094179 and DEB0075296. [Preview Abstract] |
Monday, November 21, 2005 8:52AM - 9:05AM |
FA.00005: Emergent large-scale behavor in collonies of swimming bacteria Andrey Sokolov, Igor Aranson, Raymond Goldstein, John Kessler {\it Bacillus subtilis} are flagellated, rod-shaped micro-organisms, 5-10 microns long and capable of swimming up to 20 microns/second. The hydrodynamic and chemical interactions between individual cells results in remarkably rich collective behavior; self-concentration due to gradients of dissolved oxygen or pH level; phase transitions and self-organization in confined geometries. The self-organization often takes the form of coherent structures with typical sizes that are many times larger than those of the individual bacteria. We conducted experimental investigation of emergent collective behavior in dense bacterial colonies. The studies were performed in thin liquid film with controlled thickness. We presented a new way of controlling the density of bacteria and separation of living and dead cells by transmitting electric current. We explored experimentally the dependence of the scales of emergent dynamic structures on the concentration of cells. [Preview Abstract] |
Monday, November 21, 2005 9:05AM - 9:18AM |
FA.00006: Tracking bacterial dynamics in three dimensions Mingming Wu, John Roberts, Qian Liao, Matthew P. DeLisa As we enter an era of quantitative biology, there is a clear need for innovative quantitative experimental tools to probe cellular dynamics at the micron-scale. We have developed a novel 3D micro DPTV~(defocused particle tracking velocimetry) that is able to track multiple micron-scale particles in fluid flow simultaneously. Using this technique, we tracked multiple swimming \textit{Escherichia coli} cells simultaneously and in three dimensions~for the first time. Using the tracking data, we obtained~a wealth of information about the motion~of each individual~cell as well as its group behavior. We identified~different types of locomotion of swimming \textit{E. coli} as a function of its genetic make-up using well-characterized mutant strains.~ The diffusion coefficient of the \textit{E. coli} suspension was computed from the tracking data, and was found to be $\sim $200 times larger than that of a non-motile bacterial suspension.~ The average motor power of each bacteria is estimated to be $\sim $ 10$^{-18}$ Watts. Finally, the role of cell-cell interactions was also explored via the evaluation of a pairwise correlation function.~ [Preview Abstract] |
Monday, November 21, 2005 9:18AM - 9:31AM |
FA.00007: Wall climbing in B. subtilis biofilms Michael P. Brenner, Marcus Roper, Panadda Dechadilok, Steve Branda, Roberto Kolter B. subtilis produces surfactin which aids its spreading on the buffer, increasing the wetted area of a nutrient substrate. Synthesis of the surfactin polar group is regulated by a so called ``quorum sensing'' pathway so that effective quantities are produced only when the population density is high. We describe the implications for the simple case of wall-climbing swarms of bacteria, in which Marangoni stresses drive the swarm against gravity up an angled substrate, prior to the synthesis of an exopolysaccharide matrix that leads to the formation of a floating pellicle. Wrinkles observed on the mature pellicle are related to the familiar `tears of wine' instability. [Preview Abstract] |
Monday, November 21, 2005 9:31AM - 9:44AM |
FA.00008: Ant colonies and foraging line dynamics: Modeling, experiments and computations Louis Rossi Ants are one of several types of insects that form robust and complex societies, and as such, provide rich theoretical ground for the exploration and understanding of collective dynamics and the behaviorial parameters that drive the dynamics. Many species of ants are nearly or completely blind, so they interact locally through behaviorial cues with nearby ants, and through pheromone trails left by other ants. Consistent with biological observation, two populations of ants are modeled, those seeking food and those returning to the nest with food. A simple constitutive model relating ant densities to pheromone concentrations yields a system of equations describing two interacting fluids and predicts left- and right-moving traveling waves. All the model parameters can be reduced to two Froude numbers describing the ratio between a chemical potential and the kinetic energy of the traveling ants. Laboratory experiments on {\em Tetramorium caespitum (L)} clearly indicate left and right-moving traveling density waves in agreement with the mathematical model. We focus on understanding the evolutionary utility of the traveling waves, and the optimality of the Froude numbers and other parameters. [Preview Abstract] |
Monday, November 21, 2005 9:44AM - 9:57AM |
FA.00009: Liquid Crystal Pre-Patterning for Cell Division Nicholas Hill, Dmitri Miroshnychenko, Nigel Mottram, John Lydon We are examining the hypothesis that the overall geometry of mitosis is determined by liquid-crystal pre-patterning of the cytoplasm. The identification of mitosis with liquid crystalline (LX) phases is at least 50 years old but no attempt has been made to propose a detailed theory, presumably because of the difficulties in applying a theory of liquid crystals (LCs) in a 3D geometry. In this work, we use a mathematical model ($Q$-tensor theory) of a nematic LC for the cytoplasm of the cell and solve this numerically to show that the geometry of the prophase and metaphase can be explained using LX phases. The pre-patterning for the spindle is regarded as a bipolar LX assembly with the centrosomes acting as LC poles (centres of LX defects). The centrosomes and the nuclear envelope are both treated as bodies submerged in the LC medium between two spherical shells (the nuclear and cell membranes). The geometries considered are novel and 3D. [Preview Abstract] |
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