Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session P6: Fluid Dynamics and Biology |
Hide Abstracts |
Sponsoring Units: DFD Chair: Silas Alben, Georgia Institute of Technology Room: Morial Convention Center RO4 |
Wednesday, March 12, 2008 8:00AM - 8:36AM |
P6.00001: Depolymerization-driven flow and the crawling of nematode sperm Invited Speaker: Cell crawling motility is integral in many biological and biomedical processes, such as wound healing, cancer metastasis, and morphogenesis. A complete understanding of the mechanisms by which cells crawl is still lacking, but it is known to entail at least three separate physical processes: (i) cytoskeletal extension at the front of the cell; (ii) adhesion to the substrate at the cell front and release at the rear; and (iii) advance of the cell body. In most cells, the cytoskeletal network is composed of actin. The mechanism by which force is generated to drive translocation of the cell body is still debated. Originally, this force was attributed to an actomyosin system similar to muscle. However, nematode sperm utilize a cytoskeleton composed of a network of Major Sperm Protein (MSP) that forms non-polar filaments for which molecular motors have not been identified. The motility of these cells still exhibits all three fundamental processes required for standard crawling motility. Experiments suggest that depolymerization of the cytoskeletal network is the force-producing mechanism for pulling up the rear. In this talk I will present a mechanical model that describes how depolymerization of the cytoskeleton can drive motility. This model accounts for both cytoskeletal displacements and cytsolic (the fluid component of the cell) flow. The model accurately fits in vitro data using nematode sperm extracts where depolymerization induces contraction of MSP polymer bundles. Application of this model to cell crawling produces testable predictions about how the size and shape of a cell affect crawling speed. Experiments using {\it Caenorhabditis elegans} sperm show good agreement with the model predictions. Interestingly, the model requires that cells are anisotropically elastic, being more stiff in the direction of motion than perpendicular to it. A simple physical picture can account for this anisotropy. The model also predicts that cell speed increases with anisotropy and with depolymerization rate. [Preview Abstract] |
Wednesday, March 12, 2008 8:36AM - 9:12AM |
P6.00002: Optimizing Low Reynolds Number Locomotion Invited Speaker: In this talk I will discuss optimal stroke patterns for low Reynolds number linked swimmers. We begin by optimizing stroke patterns for Purcell's 3-link swimmer modeled as a jointed chain of three slender links moving in an inertialess flow. The swimmer is optimized for efficiency and speed and we are able to attain significant increases is efficiency over those previously suggested by authors who only consider geometric design rather than kinematic criteria. We then go on to investigate uniflagellate and biflagellate organisms and compare the optimized results to biological data from spermatozoa and chlamydomonas. [Preview Abstract] |
Wednesday, March 12, 2008 9:12AM - 9:48AM |
P6.00003: Instabilities and pattern formation in active particle suspensions Invited Speaker: Suspensions of swimming microorganisms are characterized by complex dynamics involving strong fluctuations and large-scale correlated motions. These motions, which result from the many-body interactions between particles, are biologically relevant as they impact mean particle transport, mixing and diffusion, with possible consequences for nutrient uptake. Using direct numerical simulations, I first investigate aspects of the dynamics and microstructure in suspensions of interacting self-propelled rods at low Reynolds number. A detailed model is developed that accounts for hydrodynamic interactions based on slender-body theory. It is first shown that aligned suspensions of swimming particles are unstable as a result of hydrodynamic fluctuations. In spite of this instability, a local nematic order persists in the suspensions over short length scales and has a significant impact on the mean swimming speed. Consequences of the large-scale orientational disorder for particle dispersion are discussed and explained in the context of generalized Taylor dispersion theory. Dynamics in thin liquid films are also presented, and are characterized by a strong particle migration towards the interfaces. The results from direct numerical simulations are then complemented by a kinetic model, in which the dynamics are captured using a continuity equation for the particle configurations, coupled to a mean-field description of the flow arising from the active stress exerted by the particles on the fluid. Based on this model, the linear stability of both aligned and isotropic suspensions is revisited. In aligned suspensions, the instability observed in the simulations is predicted to occur at all wavelengths, a result that generalizes previous predictions by Ramaswamy et al. (2002). In isotropic suspensions, an instability for the active particle stress is also found to exist, in which shear stresses are eigenmodes and grow exponentially at long scales. Non-linear effects are also investigated using numerical simulations in two-dimensions. The results of the stability analysis are confirmed, and the long-time non-linear behavior is shown to be characterized by strong density fluctuations, which appear to be driven by the active stress instability. [Preview Abstract] |
Wednesday, March 12, 2008 9:48AM - 10:24AM |
P6.00004: Life in a drop of Ocean: microfluidic insights into microbial ecology Invited Speaker: Bacteria are the most abundant and successful form of life on Earth. Their physico-chemical interactions with their fluid environment are surprisingly complex and have enormous implications, which we can only hope to grasp if we learn to study microorganisms within realistic microenvironments. Microfluidics for the first time enables us to create microhabitats, including chemical and fluid mechanical landscapes, while visualizing bacterial behavior at a single-cell resolution. Here I focus on the application of microfluidics to gain insight in the life of marine bacteria. In their quest for nturients, marine bacteria often experience the Ocean as a desert, where rare and ephemeral nutrient patches represent transient resource oases. In this patchy seascape, swimming and chemotaxis represent critical assets, but effective patch utilization is constrained by energetic requirements. And then there are predators and viruses... These interactions form the basis of the 'microbial loop', the ensemble of microbial processes known to directly impact the productivity of marine ecosystems and the rates of carbon turnover in the Ocean. I will show how fundamental new insight on selected aspects of microbial life in a drop of Ocean can be achieved by a combination of microfluidic experiments and theoretical modeling. [Preview Abstract] |
Wednesday, March 12, 2008 10:24AM - 11:00AM |
P6.00005: Optimal flexibility in flapping appendages Invited Speaker: When oscillated in a fluid, appendages such as insect wings and fish fins can produce large thrust forces while undergoing considerable bending. Can we understand these bending patterns by comparing them with the patterns which produce maximum thrust, or a given thrust at maximum efficiency? We present a general model for how flexible surfaces produce vorticity and bend actively and passively in a fluid. We solve the model numerically, and discuss results for moderate deflections (relevant for large thrust), and for small deflections (relevant for high efficiency). We'll then consider how a fish-fin-like structure might be designed for optimal performance. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700