Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session P23: Focus Session: Biological Hydrodynamics II |
Hide Abstracts |
Sponsoring Units: DFD DBP GSNP Chair: Peter Lenz, University of Marburg Room: LACC 410 |
Wednesday, March 23, 2005 11:15AM - 11:27AM |
P23.00001: Self-organization of hydrodynamically entrained sperm cells into an array of vortices Ingmar Riedel, Karsten Kruse, Jonathon Howard The emergence of spatiotemporal patterns is of great interest in many scientific disciplines. Here we report a new dynamically self-organized pattern formed by hydrodynamically entrained sperm cells at planar surfaces. The sperm cells form vortices resembling quantized rotating waves. These vortices form an array with local hexagonal order. Using a novel order parameter, we show that the array is only formed above a critical sperm density. Supported by numerical simulation we suggest a mechanism for the appearance of the array and we estimate the strength of the hydrodynamic coupling between the cells. The vortex array represents a new chiral active gel and may serve as an experimentally accessible model for the metachronal wave of ciliated epithelia and other non-equilibrium phenomena in general. Finally we discuss the biological implications of our work. [Preview Abstract] |
Wednesday, March 23, 2005 11:27AM - 11:39AM |
P23.00002: Individual and collective dynamics of gyrotactic algae Sujoy Ganguly, Cristian Solari, John Kessler, Raymond Goldstein When a swimming microorganism has an anistropic distribution of mass, rotational drag due to shear can strongly affect the orientation of its swimming trajectories. {\it Gyrotaxis} is motility guided by this combination of torques. An important example is provided by upward swimming of slightly negatively buoyant organisms, which results in unstable density stratification in the fluid, then descending curtains that become plumes and bioconvection patterns. The details of these self-concentrative dynamics are determined by gyrotaxis and the associated flows. We report novel experiments on the development of these instabilities, their eventual settling into steady states or, occasionally, secondary dynamical instabilities. Details of the collectively generated flow fields were obtained using PIV, Particle Imaging Velocimetry. The implications of self-generated flow fields with large Peclet numbers are discussed. [Preview Abstract] |
Wednesday, March 23, 2005 11:39AM - 11:51AM |
P23.00003: Tracking the bacterial dynamics in three dimensions Mingming Wu, John W. Roberts We present experimental results on the tracking of swimming {\it Escherichia coli} cells using a novel 3D particle tracking method. {\it E.coli} is a single cell organism, it is about 1-3 $\mu$m in size. Under favorable condition, it can run at a speed of ~30 times of its body length in one second. Swimming {\it E.coli} cells provide us an unique opportunity to probe the transport properties of a nonequilibrium system, where it consists of self propelled objects. In our experiments, a wildtype {\it E.coli} (RP437) is used for its known motile behavior. The cells are made fluorescent by the insertion of a plasmid that expresses GFP (Green Fluorescent Proteins) constitutively. As a result, the cells emit fluorescent light all the time. The cells are placed in a 5mm diameter and 1.5 mm depth well before filming. The 3D trajectories of multiple swimming {\it E.coli} cells are obtained for the first time using a novel defocused particle tracking technique. Various types of locomotion of bacteria are observed, running, tumbling, and wobbling. Using the track data, we evaluated the diffusion coefficient of the swimming cells. It demonstrated a ballistic behavior at the early time, and gradually develop into a random walk in later time. The Diffusion coefficient is about $10^{3}$ orders of magnitude larger than a system with nonmotile microorganisms of similar size. [Preview Abstract] |
Wednesday, March 23, 2005 11:51AM - 12:03PM |
P23.00004: A Computational Model of Deformable Cell Rolling in Shear Flow Charles Eggleton, Sameer Jadhav, Kostantinos Konstantopoulos Selectin-mediated rolling of polymorphonuclear leukocytes (PMNs) on activated endothelium is critical to their recruitment to sites of inflammation. The cell rolling velocity is influenced by bond interactions on the molecular scale that oppose hydrodynamic forces at the mesoscale. Recent studies have shown that PMN rolling velocity on selectin-coated surfaces in shear flow is significantly slower compared to that of microspheres bearing a similar density of selectin ligands. To investigate whether cell deformability is responsible for these differences, we developed a 3-D computational model which simulates rolling of a deformable cell on a selectin-coated surface under shear flow with a stochastic description of receptor-ligand bond interaction. We observed that rolling velocity increases with increasing membrane stiffness and this effect is larger at high shear rates. The average bond lifetime, number of receptor-ligand bonds and the cell-substrate contact area decreased with increasing membrane stiffness. This study shows that cellular properties along with the kinetics of selectin-ligand interactions affect leukocyte rolling on selectin-coated surfaces. [Preview Abstract] |
Wednesday, March 23, 2005 12:03PM - 12:15PM |
P23.00005: Pressure-driven polymer dynamics in nanofluidic channels Derek Stein, Wiepke Koopmans, Cees Dekker The pressure-driven transport of different lengths of DNA molecules in flat, rectangular nanofluidic channels was studied using fluorescence microscopy. The molecular speeds were observed to fall between the maximum fluid velocity in the parabolic flow profile and the average fluid velocity. The dependence of polymer speed on channel height was characterized by two distinct transport regimes: In channels larger than $\sim $1 $\mu $m, 21 $\mu $m-long molecules traveled faster than 3.8 mm-long molecules. In channels smaller than $\sim $1 $\mu $m, the observed speeds coincided. This behavior reflects the dynamical properties of polymer coils in solution, whose statistical size is characterized by the radius of gyration, R$_{g}$. In large channels, DNA coils can explore all regions of the channel cross section up to a distance $\sim $R$_{g}$ from the walls. The center of mass of large molecules is therefore confined to regions of higher fluid velocity than small molecules, and travel faster in a pressure-driven flow. In thin channels, molecular conformations are confined in height, leading to cross-sectional profiles that are independent of length, and that travel at the same speed. [Preview Abstract] |
Wednesday, March 23, 2005 12:15PM - 12:51PM |
P23.00006: Polymer dynamics and fluid flow in actin-based cell motility Invited Speaker: In living cells, nonequilibrium protein polymerization reactions are frequently used to convert chemical energy into mechanical energy and thereby generate useful force for cellular movements. We have examined the polymer and fluid dynamics in two biological cases where the assembly of branched actin filament networks generates force: the intracellular movement of the bacterial pathogen \textit{Listeria monocytogenes}, and the extension of the leading edge of skin epithelial cells during wound-healing. In both cases, net actin filament assembly occurs at the front of the network structure and net disassembly occurs at the rear. Actin protein subunits and other network components must be recycled through the fluid phase to the front of the polymerizing network in order for forward movement to continue at steady state. For actin-based movement of \textit{Listeria monocytogenes}, we have found that actin recycling is not rate-limiting; instead, the speed of movement is governed by the cooperative dissociation of groups of noncovalent protein-protein bonds attaching the filamentous network to the bacterial surface. In contrast, rapid actin-based extension at the leading edge of moving epithelial cells is associated with unusual perturbations in intracellular fluid flow. [Preview Abstract] |
Wednesday, March 23, 2005 12:51PM - 1:03PM |
P23.00007: Formation of 2D ligand-receptor bonds under shear Nelly Henry, David Pointu, David Leboeuf Formation and dissociation of specific molecular links between opposing surfaces are omnipresent events of the living world. They very often take place in highly dynamic conditions like in blood stream where wall shear rates are believed to vary from 150 to 1600 s$^{-1}$. Thus, a better understanding of shear effects on 2D ligand-receptor bonds formation is a key step towards improving our conception of biological molecular recognition. Using streptavidin-biotin as a model receptor-ligand pair, we have probed the establishment of specific bonds between the surface of a B-lymphocytes and grafted micrometric particles under controlled shear stress. The results showed that shear stress had a determining effect on cell-particle interactions, introducing a ligand surface density condition for the binding and very likely cell mechanical compliance leading to increased binding at high shear. This view will be discussed in the light of other results that we have obtained using a purely colloidal experimental model, made of particles grafted either with streptavidin or biotin and brought into contact under controlled shear stress. [Preview Abstract] |
Wednesday, March 23, 2005 1:03PM - 1:15PM |
P23.00008: Swimming, Stirring, and Hydrodynamic Scaling in the {\it Volvocales} Cristian Solari, Sujoy Ganguly, John Kessler, Raymond Goldstein The {\it Volvocales} constitute a family of colonial algae ranging in size from tens to hundreds of microns. The surface of these nearly spherical algae is packed with cells, each having two flagella. Their incessant, periodic flailing moves the water in which the organisms live. The {\it Volvocales} and especially their largest member {\it Volvox} constitute a model system to study the coordinated action of multiple flagella in self-propulsion and the fluid mixing they produce. Using flow visualization and micromanipulation we have measured the swimming speed, fluid velocities, and propulsive forces for {\it Volvocales} over several orders of magnitude in organism size. The associated Peclet number varies from $\sim 0.01$ for the smallest species to $\sim 100$ for the largest, spanning the range from diffusion-dominated to advection-dominated transport of vital molecules dissolved in the suspending medium. Over this same range we quantify scaling relations for swimming speed and propulsive efficiency. These results are interpreted in terms of metabolic and physical tradeoffs. [Preview Abstract] |
Wednesday, March 23, 2005 1:15PM - 1:27PM |
P23.00009: The effect of solution conditions on the conformation of clathrin triskelions Matthew Ferguson, Kondury Prasad, Dan Sackett, Peter Schuck, Hac\`ene Boukari, Eileen Lafer, Ralph Nossal The major component in the protein coat of certain endocytic vesicles is clathrin, a three-legged heteropolymer (known as a ``triskelion'') that assembles into polyhedral cages composed primarily of pentagonal and hexagonal facets. In vitro, this assembly depends on the pH, with cages forming more readily at low pH and less readily at high pH. We have developed novel techniques to make physical measurements of the clathrin triskelion under conditions where assembly occurs. By sedimentation velocity and laser light scattering, we measure changes in Stokes radius, r$_{H}$ and radius of gyration, r$_{g}$ of the clathrin triskelion as the pH is lowered. Calculations, with the program HYDRO, on a rigid molecular bead model of clathrin show that measured changes may be accounted for by a pH dependent puckering of the arms at the vertex. This is consistent with the idea that a change in clathrin conformation may play a role in clathrin cage assembly. [Preview Abstract] |
Wednesday, March 23, 2005 1:27PM - 1:39PM |
P23.00010: Lizard locomotion on weak sand Daniel Goldman, Wyatt Korff, Homero Lara, Robert Full Terrestrial animal locomotion in the natural world can involve complex foot-ground interaction; for example, running on sand probes the solid and fluid behaviors of the medium. We study locomotion of desert-dwelling lizard {\it Callisaurus draconoides} (length 16 cm, mass=20 g) during rapid running on sand. To explore the role of foot-ground interaction on locomotion, we study the impact of flat disks ($\approx$ 2 cm diameter, 10 grams) into a deep (800 particle diameters) bed of $250~\mu m$ glass spheres of fixed volume fraction $\phi \approx 0.59$, and use a vertical flow of air (a fluidized bed) to change the material properties of the medium. A constant flow $Q$ below the onset of bed fluidization weakens the solid: at fixed $\phi$ the penetration depth and time of a disk increases with increasing $Q$. We measure the average speed, foot impact depth, and foot contact time as a function of material strength. The animal maintains constant penetration time (30 msec) and high speed (1.4 m/sec) even when foot penetration depth varies as we manipulate material strength. The animals compensate for decreasing propulsion by increasing stride frequency. [Preview Abstract] |
Wednesday, March 23, 2005 1:39PM - 1:51PM |
P23.00011: Choice of High-Efficacy Strains for the Annual Influenza Vaccine Michael Deem We introduce a model of protein evolution to explain limitations in the immune system response to vaccination and disease [1]. The phenomenon of original antigenic sin, wherein vaccination creates memory sequences that can increase susceptibility to future exposures to the same disease, is explained as stemming from localization of the immune system response in antibody sequence space. This localization is a result of the roughness in sequence space of the evolved antibody affinity constant for antigen and is observed for diseases with high year-to-year mutation rates, such as influenza. We show that the order parameter within this theory correlates well with efficacies of the H3N2 influenza A component of the annual vaccine between 1971 and 2004 [2,3]. This new measure of antigenic distance predicts vaccine efficacy significantly more accurately than do current state-of-the-art phylogenetic sequence analyses or ferret antisera inhibition assays. We discuss how this new measure of antigenic distance may be used in the context of annual influenza vaccine design and monitoring of vaccine efficacy. 1) M. W. Deem and H. Y. Lee, Phys. Rev. Lett. 91 (2003) 068101. 2) E. T. Munoz and M. W. Deem,q-bio.BM/0408016. 3) V. Gupta, D. J. Earl, and M. W. Deem, ``Choice of High-Efficacy Strains for the Annual Influenza Vaccine,'' submitted. [Preview Abstract] |
|
P23.00012: Hydrodynamic Forcing of Spontaneous Helical Growth Ariel Balter, Jay Tang Inspired by the biological system of actin ``comet tails'' formed during bacterial motility, we have modelled the natural evolution of a cylindrical gel growing from a fixed object. We have found that a stable solution, is a kind of helical growth in which the helix pitch and diameter increase, and the pitch angle varies, but the axis remains constant. The growth can be defined by a rotation vector $\vec k$. The instantaneous magnitude of this vector is related to the intantaneous pitch and diameter, and is a simple inverse linear function of the contour length, $s$, along the helix: $\left| \vec k \left( s \right) \right| \approx c s$ for some constant c. The instantaneous pitch angle $\chi(s)$, is a function of the components of $\vec k$. Helical growth can spontaneously emerge from random initial conditions. However, by starting with "seed" helices of various shapes we found that a critical value of the pitch angle, $\chi_{crit} \approx \tan^{-1}\frac{4}{\pi}$, governs the growth of the helices. For example, when $\chi (s) = \chi_{crit}$ then $\frac{d\chi}{ds} = 0$. However, this critical value is an unstable fixed point, so growth does not converge to $\chi_{crit}$. [Preview Abstract] |
|
P23.00013: A Newtonian fluid meets an elastic solid: Simulating fluid flow past compliant walls Rolf Verberg, Gavin Buxton, David Jasnow, Anna Balazs We present a novel algorithm that couples the dynamics of a fluid with the mechanical behavior of the confining walls. The fluid is simulated with the lattice Boltzmann model, an efficient solver of the Navier-Stokes equations. The solid walls are modeled by the lattice spring model, which simulates the dynamics of a continuum elastic material. We implemented solid-fluid interactions that give stick boundary conditions for the fluid and allow for a dynamic interaction between the elastic walls and the confined fluid. Here, the fluid and the solid are coupled through pressure and shear forces that are excerted across the interface. We applied the model to a study of the impact of a microcapsule (a fluid-filled elastic shell) with a regular smooth wall as well as an adhesive surface for varying fluid and shell properties. The results show that we can accurately and efficiently simulate the interaction between microcapsules and a variaty of solid surfaces. This is crucial in the study of many bio-mechanical applications. [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