Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session P15: Biologically Inspired Physics: Swimming, Propulsion, Bio-fluids |
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Sponsoring Units: DFD Chair: Silas Alben, Georgia Institute of Technology Room: 316 |
Wednesday, March 18, 2009 8:00AM - 8:12AM |
P15.00001: Swimming in a vortex street Silas Alben Recent studies showed that a trout swimming in a cylinder wake can save energy by ``slaloming'' through a vortex street. We present a simple model using a flexible body with vortex sheets, and find swimming shapes which maximize output power and efficiency. We find analytic solutions and compare the optimal swimming phase between the body and vortices with previous experiments and numerics. [Preview Abstract] |
Wednesday, March 18, 2009 8:12AM - 8:24AM |
P15.00002: Symmetry and Hydrodynamic Interactions of Linked-Sphere Swimmers Gareth Alexander, Julia Yeomans The motile behavior of micron-sized organisms offers an insight into a physical environment very different to our own. Micron length scales correspond to low Reynolds number conditions where viscous forces dominate over the effects of inertia [1]. A topic of growing interest is the role played by hydrodynamic interactions, both with confining walls and between organisms as a means to generate collective motility. We shall describe the form and properties of swimmer-swimmer interactions for simple models consisting of a small number of linked-spheres [2,3]. These interactions do not follow the naively expected dipolar form and moreover exhibit a strong sensitivity to the relative phase of the swimmers. Several of these features have a natural interpretation in terms of the kinematic reversibility of Stokes flows and we shall describe in particular an exact result for the scattering of two swimmers related by time reversal. [1] G. I. Taylor, Proc. R. Soc. A 209, 447 (1951); 211, 225 (1952). [2] A. Najafi and R. Golestanian, Phys. Rev. E 69, 062901 (2004). [3] C. M. Pooley, G. P. Alexander, and J. M. Yeomans, Phys. Rev. Lett. 99, 228103 (2007). [Preview Abstract] |
Wednesday, March 18, 2009 8:24AM - 8:36AM |
P15.00003: Collective locomotion of non-swimmers Eric Lauga, Denis Bartolo To achieve propulsion at low Reynolds number, a swimmer (e.g. a biological cell such as a bacterium, or a spermatozoon) must deform its shape in time in a way that is not invariant under time-reversal symmetry (non-reciprocal); this is Purcell's scallop theorem. We show here explicitly that there is no many-scallop theorem. Two active bodies undergoing reciprocal deformations - and therefore incapable of swimming when considered separately - can exploit hydrodynamic interaction to swim. If the bodies are polar, we also show that they experience effective long-range interactions. We derive our results analytically for a minimal dimers model, and generalize them to more complex geometries on the basis of symmetry and scaling arguments. Furthermore, we explain how such cooperative locomotion can be realized experimentally by shaking a collection of soft particles with a homogeneous external field, thereby making non-swimmers swim. [Preview Abstract] |
Wednesday, March 18, 2009 8:36AM - 8:48AM |
P15.00004: Self-Assembled Magnetic Surface Swimmers: Theoretical Model Igor Aranson , Maxim Belkin, Alexey Snezhko The mechanisms of self-propulsion of living microorganisms are a fascinating phenomenon attracting enormous attention in the physics community. A new type of self-assembled micro-swimmers, {\it magnetic snakes}, is an excellent tool to model locomotion in a simple table-top experiment. The snakes self-assemble from a dispersion of magnetic microparticles suspended on the liquid-air interface and subjected to an alternating magnetic field. Formation and dynamics of these swimmers are captured in the framework of theoretical model coupling paradigm equation for the amplitude of surface waves, conservation law for the density of particles, and the Navier-Stokes equation for hydrodynamic flows. The results of continuum modeling are supported by hybrid molecular dynamics simulations of magnetic particles floating on the surface of fluid. [Preview Abstract] |
Wednesday, March 18, 2009 8:48AM - 9:00AM |
P15.00005: Accumulation of microswimmers near surface due to steric confinement and rotational Brownian motion Guanglai Li, Jay Tang Microscopic swimmers display some intriguing features dictated by Brownian motion, low Reynolds number fluid mechanics, and boundary confinement. We re-examine the reported accumulation of swimming bacteria or bull spermatozoa near the boundaries of a fluid chamber, and propose a kinematic model to explain how collision with surface, confinement and rotational Brownian motion give rise to the accumulation of micro-swimmers near a surface. In this model, an elongated microswimmer invariably travels parallel to the surface after hitting it from any incident angle. It then takes off and swims away from the surface after some time due to rotational Brownian motion. Based on this analysis, we obtain through computer simulation steady state density distributions that reproduce the ones measured for the small bacteria E coli and Caulobacter crescentus, as well as for the much larger bull spermatozoa swimming near surfaces. These results suggest strongly that Brownian dynamics and surface confinement are the dominant factors for the accumulation of microswimmers near a surface. [Preview Abstract] |
Wednesday, March 18, 2009 9:00AM - 9:12AM |
P15.00006: Modeling the Behavior of Self-Propelled Microcapsules Amitabh Bhattacharya, O. Berk Usta, Anna C. Balazs Biological cells can perform complex tasks by signaling and moving autonomously in their environment. We study a system of self-propelled microcapsules, first proposed by Usta et al (2008), that mimics this process. It consists of a signaling and target microcapsule placed close to an adhesive substrate and immersed in fluid. The signaling microcapsule encases nanoparticles, which, when released, modifies the adhesive strength of the substrate. The adhesion gradients in the substrate, along with hydrodynamic interactions among the capsules, gives rise to a sustained motion of the microcapsules. In this work, we perform simulations (based on lattice Boltzmann method for the fluid and random walk simulation for nanoparticles) of several signal-target configurations, consisting of two or more rigid capsules. In particular, we examine a configuration consisting of a single signaling capsule pushing multiple target capsules in a single file. For a constant release rate of nanoparticles, the velocity of the train of capsules asymptotes to a constant value at large times. Using a low-order analytical model for this system, we show that there is a simple relationship between this asymptotic velocity and the parameters in the system (e.g. number of capsules, release rate of nanoparticles, viscosity of fluid, adhesive strength of substrate etc.). [Preview Abstract] |
Wednesday, March 18, 2009 9:12AM - 9:24AM |
P15.00007: Mixing fluid by self-propelled objects Maxim Belkin, Alexey Snezhko, Igor Aranson, Wai-Kwong Kwok Magnetic microparticles suspended at the water-air interface and subjected to an ac external driving self-assemble into dynamic structures (magnetic snakes). The snakes are accompanied by four large hydrodynamic vortices. At high enough frequencies and amplitudes of driving the snakes transform into self-propelled swimmers. Moving erratically, these swimmers mix the surface of fluid at a very high rate. We performed detailed experimental studies of these self-organized mixing. We studied space and time correlation and diffusion process in such systems. [Preview Abstract] |
Wednesday, March 18, 2009 9:24AM - 9:36AM |
P15.00008: Enskog-theory for stochastic models with self-propelled and passive particles Alemayehu Gebremariam, Thomas Ihle Macroscopic evolution equations for interacting many-body systems do not just ``emerge''; they follow from microscopic laws. However, it is often difficult to quantitatively establish this link, especially for systems which cannot be described by a Hamiltonian and which do not have pairwise additive interactions. Therefore, the general form of the macroscopic equations is usually obtained by symmetry arguments. Here, using a particle-based model with discrete time evolution steps for fluid flow I show how the macroscopic transport equations can be rigorously derived from microscopic collision rules. The approach starts with the full N-particle Liouville equation and leads to a multi-particle Enskog-equation which is treated by a Chapman-Enskog expansion. No linearization or single-relaxation time approximation of the collision operator are needed. The obtained thermo-hydrodynamic equations show excellent agreement with previous numerical results. The same approach is used to study a simple model of self-propelled, swarming birds. This model was proposed by T. Vicsek et al. [Phys. Rev. Lett. {\bf 75} (1995) 1226]; it has ``multi-particle collisions'' where birds within some interaction range align their flying directions. I analytically analyze the collision-operator for small and large bird density, and derive the hydrodynamic equations for the density and velocity fields. [Preview Abstract] |
Wednesday, March 18, 2009 9:36AM - 9:48AM |
P15.00009: The ``caterpillar'' simulation model for a biological filament Aimee Bailey, Christopher Lowe, Adrian Sutton We present a simulation model for an elastic filament in a viscous fluid, relevant for systems ranging from suspensions of paper pulp to micro-organism motility. It incorporates the Stokeslet treatment of the hydrodynamic force. We show that a non-arbitrary choice of the hydrodynamic radius is necessary to recover known dynamic behavior of a fiber with a finite cross-section. Our simulations explore configurations inaccessible by theory. We illustrate the utility of the model by considering the simple scenario of a charged filament in an electric field. Results suggest a circularly polarized electric field is a viable means for aligning microtubules in solution. [Preview Abstract] |
Wednesday, March 18, 2009 9:48AM - 10:00AM |
P15.00010: Flow and nutrient transport through porous scaffolds used for the culture of bone cells in perfusion bioreactors Dimitrios Papavassiliou, Roman Voronov, Vassilios Sikavitsas, Samuel VanGordon The goal is to understand via computation the behavior of the flow inside porous scaffolds that are used in bone tissue bioreactors. Fluid shear is an important stimulatory factor in preosteoblastic cells seeded in scaffolds and cultured under continuous flow perfusion. A Lattice Boltzmann method has been employed to simulate the flow field within porous scaffolds obtained with high resolution micro-CT. Lagrangian methods have also been used to determine the nutrient dispersion inside the scaffolds. The shear stresses calculated inside the scaffold architecture indicate that the shear stresses experienced by cells inside the scaffold can vary by orders of magnitude. This is important when designing scaffolds for bone tissue growth, since osteoblastic cells require to be stimulated by shear for growth. Moreover, cell detachment can occur when the fluid shear is too high, thus, placing a limit on the stresses that a particular scaffold design should allows. The talk will address the methodology, the validation and the correlation of scaffold structure characteristics with the shear stresses and with the rate of mass transfer. [Preview Abstract] |
Wednesday, March 18, 2009 10:00AM - 10:12AM |
P15.00011: Instabilities and waves in thin films of living fluids Sumithra Sankararaman, Sriram Ramaswamy We formulate the thin-film hydrodynamics of a suspension of polar self-driven particles and show that it is prone to several instabilities through the interplay of activity, polarity and the existence of a free surface. Our approach extends, to self-propelling systems, the work of Ben Amar and Cummings [Phys Fluids {\bf 13} (2001) 1160] on thin-film nematics. Based on our estimates the instabilities should be seen in bacterial suspensions and the lamellipodium, and are potentially relevant to the morphology of biofilms. We suggest several experimental tests of our theory. [Preview Abstract] |
Wednesday, March 18, 2009 10:12AM - 10:24AM |
P15.00012: Tuning inter-virus interactions in natural aquatic environments Nathan W. Schmidt, Andrew K. Udit, Leonardo Gutierrez, Thanh H. (Helen) Nguyen, M.G. Finn, Gerard C.L. Wong Polymeric natural organic matter (NOM) originating from plants and animals is ubiquitous in natural aquatic environments. Many water-borne pathogens, including viruses, readily associate with NOM, which has a statistical distribution of charged and hydrophobic groups. Virus-NOM association influences the transport of viruses in groundwater environments, but little is known about this interaction, or how NOM can induce new inter-virus interactions. To better understand the interaction between NOM and aqueous contaminants, we use the MS2 and Qbeta viruses (diameters $\sim $ 27nm) as surrogate water-borne pathogens. Small Angle X-ray Scattering is used to characterize the inter-particle interaction between viruses over a range of NOM concentrations and different salt types and concentrations. [Preview Abstract] |
Wednesday, March 18, 2009 10:24AM - 10:36AM |
P15.00013: Computational studies on characteristic fluid behavior in the stented cerebral aneurysm Miki Hirabayashi, Makoto Ohta, Daniel A. R\"ufenacht, Bastien Chopard We present a computational analysis of the fluid behavior in the stented aneurysm. It is important to reveal the complex mechanism of the velocity reduction of the flow in the stented aneurysm in order to design the effective stent, which is a tubular mesh of wires placed for the treatment of the cerebral aneurysm. To understand the effect of a stent we already proposed a qualitative analysis of the flow pattern in the stented aneurysm. Here we present a quantitative analysis of the transition of the pressure and the shear stress caused by the changes of the flow pattern to verify the velocity reduction mechanism of the stent. We expect that our study will lead to a new suggestion for the effective treatment of the cerebral aneurysm by the stent. [Preview Abstract] |
Wednesday, March 18, 2009 10:36AM - 10:48AM |
P15.00014: Run length is the dimension that characterizes path integrals useful for designing passive bacterial pumps David Liao, Guillaume Lambert, Peter Galajda, Robert Austin Asymmetric funnels have been used as passive pumps to concentrate \textit{E. coli} in nanofabricated devices (Austin 2007). Funnel geometry changes pump efficiency, which could be important when driving cell sorters (Whitesides 2008). The large set of funnel geometries that could be considered when designing pumps motivated us to derive a path-integral-like formula to predict the flux produced by arbitrary funnel geometries. We applied this equation to a two-dimensional wedge-shaped funnel. Model and experiment agree that the steady-state ratio between concentrations on two sides of a funnel open to $60^{\circ}$ is 3 when the aperture is one fifth the bacterial run length and 1 when the aperture is 16 times the run length, an example of how the run length here has a role loosely analogous to the wavelength in quantum mechanical path integrals. [Preview Abstract] |
Wednesday, March 18, 2009 10:48AM - 11:00AM |
P15.00015: Selective transport through nano-channels: do we understand it? Anton Zilman, T. Jovanovic-Talisman, B. Chait, M. Rout, S. Di Talia, M. Magnasco Functioning of living cells requires selective molecular transport, which is provided by transport channels that are able to selectively transport certain molecular species while filtering others, even similar ones. Such channels can selectively transport their specific molecules in the presence of vast amounts of non-specific competition. In many cases, efficient and selective transport occurs without direct input of metabolic energy and without transitions from an `open' to a `closed' state during the transport event. Examples include selective permeability of porins and transport through the nuclear pore complex. Mechanisms of selectivity of such channels have inspired design of artificial selective nano-channels, which mimic the function of selective biological channels. Mechanisms of selectivity of such nano-channels are still unknown. I present a theoretical model to explain the selectivity of transport through nano-channels, which contains only the essentials of stochastic kinetics inside the channel. The theory provides a mechanism for selectivity based on the differences in the kinetics of transport through the channel between different molecules. The theory explains how the specific molecules are able to filter out the non-specific competitors - and proposes a mechanism for sharp molecular discrimination. The theoretical predictions account for previous experimental results and have been verified in ongoing experiments [Preview Abstract] |
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