Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session F12: Biophysical Dynamics and Locomotion |
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Sponsoring Units: DBIO DFD Chair: Arpita Upadhyaya, University of Maryland Room: 205 |
Tuesday, March 4, 2014 8:00AM - 8:12AM |
F12.00001: Dynamic Force Patterns of an Undulatory Microswimmer Rafael Schulman, Matilda Backholm, William Ryu, Kari Dalnoki-Veress {\it C. elegans} is a millimeter-sized nematode which has served as a model organism in biology for several decades, primarily due to its simple anatomy. Using an undulatory form of locomotion, this worm is capable of propelling itself through various media. Due to the small length scales involved, swimming in this regime is qualitatively different from macroscopic locomotion because the swimmers can be considered to have no inertia. In order to understand the microswimming that this worm exhibits, it is crucial to determine the viscous forces experienced during its motion. Using a micropipette deflection technique in conjunction with high speed imaging, we have directly measured the time-varying forces generated by {\it C. elegans} during swimming. Furthermore, by analyzing the body's kinematics over time and applying a model of locomotion, we can compute the theoretical force curves. We observe excellent agreement between the measured and calculated forces. The success of this simple model has important implications in the understanding of microswimming in general. [Preview Abstract] |
Tuesday, March 4, 2014 8:12AM - 8:24AM |
F12.00002: ABSTRACT WITHDRAWN |
Tuesday, March 4, 2014 8:24AM - 8:36AM |
F12.00003: Propulsion and locomotion in hexatic liquid crystal Thomas Powers, Madison Krieger, Saverio Spagnolie The long chainlike molecules in mucus can align and lead to liquid-crystalline order. The resulting anisotropy can affect swimming behavior of spermatozoa and bacteria. We study a simple model of swimming in an anisotropic fluid, that of an infinitely long two-dimensional sheet deforming via propagating transverse or longitudinal waves and immersed in a hexatic liquid crystal. The liquid crystal is categorized by the dimensionless Ericksen number Er, which compares viscous and elastic effects. We calculate how swimming speed depends on Er for small amplitude waves, and show that our perturbative approach breaks down at large Er for transverse waves but not longitudinal waves. We also calculate the fluid transported by the swimming motion. [Preview Abstract] |
Tuesday, March 4, 2014 8:36AM - 8:48AM |
F12.00004: Efficient swimming of a plunging elastic plate in a viscous fluid Peter Yeh, Alexander Alexeev We use three dimensional computer simulations to examine the combined hydrodynamics and structural response of a plunging elastic plate submerged in a viscous fluid with Reynolds number of 250. The plate is actuated at the root with a prescribed vertical sinusoidal displacement and a zero slope (clamped) boundary condition. We explore the steady state swimming velocity and the associated input power as a function of driving frequency, added mass, and aspect ratio. We find a universal bending pattern independent of geometry and added mass that maximizes the distance traveled per unit applied work. This bending pattern is associated with minimizing center of mass oscillations normal to the direction of travel. Subsequently, the flow around the sides of the swimmer, which does not aid in propulsion, is minimized, thereby reducing viscous losses. [Preview Abstract] |
Tuesday, March 4, 2014 8:48AM - 9:00AM |
F12.00005: Kinematic Matrix Analysis of Biological Swimmers and Artificial Nanomotors Amir Nourhani, Paul Lammert, Ali Borhan, Vincent Crespi In recent years, much attention has been attracted by autonomous movers (both natural, often biological, and synthetic) which exhibit a basic deterministic motion significantly perturbed by stochastic elements. Fokker-Planck equations are a traditional tool for investigating such phenomena, but can be cumbersome to apply, especially in complex situations of the sort now attracting attention. This is partly due to their giving complete probability distributions, which is a level of detail seldom needed, and potentially obscuring of the basic physics. We present a simple yet powerful new approach which can flexibly and easily handle a large variety of elementary deterministic and stochastic component processes to yield drift and diffusion characteristics with a minimum of fuss and effort. We use the clarity and power of the new methodology to discern several new universal emergent time scales in this class of physical systems. We also describe how these methods could now serve as a platform for further advances and insights. [Preview Abstract] |
Tuesday, March 4, 2014 9:00AM - 9:12AM |
F12.00006: Fluid flow enhances the effectiveness of toxin export by aquatic microorganisms: a first-passage perspective Nicholas Licata, Aaron Clark Aquatic microorganisms face a variety of challenges in the course of development. One central challenge is efficiently regulating the export of toxic molecules inside the developing embryo. The strategies employed should be robust with respect to the variable ocean environment and limit the chances that exported toxins are reabsorbed. In this talk we consider the first-passage problem for the uptake of exported toxins by a spherical embryo. A perturbative solution of the advection-diffusion equation reveals that a concentration boundary layer forms in the vicinity of the embryo, and that fluid flow enhances the effectiveness of toxin export. We highlight connections between the model results and recent experiments on the development of sea urchin embryos. [Preview Abstract] |
Tuesday, March 4, 2014 9:12AM - 9:24AM |
F12.00007: Evaluation of the mass transfer effect of the stalk contraction cycle of \textit{Vorticella} Jiazhong Zhou, David Admiraal, Sangjin Ryu \textit{Vorticella} is a protozoan with a contractile stalk that can contract pulling the cell body toward the substrate in less than 10 ms and return to the extended state in a few seconds. Although this stalk contraction is one of the fastest cellular motions, it is unknown why \textit{Vorticella} contracts. Because the flow field induced by \textit{Vorticella} shows different characteristics between contraction and relaxation, it has been suggested that \textit{Vorticella} augments mass transfer near the substrate based on its stalk contraction-relaxation. We investigate this hypothesis using computational fluid dynamics (CFD) simulations and particle image velocimetry (PIV) experiments. In both approaches, \textit{Vorticella} is modelled as a solid sphere that translates perpendicular to a solid surface in liquid based on the measured stalk length changes of \textit{Vorticella}. Based on the computationally and experimentally simulated flow, we evaluate the mass transfer capability of \textit{Vorticella}, for a possible application of the stalk contraction of \textit{Vorticella} as a biomimetic model system for microfluidic mixers. [Preview Abstract] |
Tuesday, March 4, 2014 9:24AM - 9:36AM |
F12.00008: Endothelial Interfaces -- Master Gatekeepers of the Cardiovascular System Sylvia Ann Junghans, Luka Pocivavsek, Noureddine Zebda, Konstantin Birukov, Mary Jo Waltman, Jaroslaw Majewski Endothelial cells, master gatekeepers of the cardiovascular system, line its inner boundary from the heart to distant capillaries constantly exposed to blood flow. Inter-endothelial signaling and the monolayer's adhesion to the underlying collagen rich basal lamina are key in physiology and disease. Using neutron scattering, we report the first-ever interfacial structure of endothelial monolayers under dynamic flow conditions mimicking the cardiovascular system. Endothelial adhesion strength (defined as the separation distance l between the basal cell membrane and solid boundary) is explained using developed interfacial potentials and intra-membrane segregation of specific adhesion proteins. Our method provides a powerful tool for the biophysical study of cellular layer adhesion strength in living tissues. [Preview Abstract] |
Tuesday, March 4, 2014 9:36AM - 9:48AM |
F12.00009: The Fast and Non-capillary Fluid Filling Mechanism in the Hummingbird's Tongue Alejandro Rico-Guevara, Tai-Hsi Fan, Margaret Rubega Hummingbirds gather nectar by inserting their beaks inside flowers and cycling their tongues at a frequency of up to 20 Hz. It is unclear how they achieve efficiency at this high licking rate. Ever since proposed in 1833, it has been believed that hummingbird tongues are a pair of tiny straws filled with nectar by capillary rise. Our discoveries are very different from this general consensus. The tongue does not draw up floral nectar via capillary action under experimental conditions that resemble natural ones. Theoretical models based on capillary rise were mistaken and unsuitable for estimating the fluid intake rate and to support foraging theories. We filmed (up to 1265 frames/s) the fluid uptake in 20 species of hummingbirds that belong to 7 out of the 9 main hummingbird clades. We found that the fluid filling within the portions of the tongue that remain outside the nectar is about five times faster than capillary filling. We present strong evidence to rule out the capillarity model. We introduce a new fluid-structure interaction and hydrodynamic model and compare the results with field experimental data to explain how hummingbirds actually extract fluid from flowers at the lick level. [Preview Abstract] |
Tuesday, March 4, 2014 9:48AM - 10:00AM |
F12.00010: Investigation of ciliary propulsion of \textit{Tetrahymena Pyriformis} in viscous solution Ilyong Jung, Eva Lyubich, James Valles Recent experiments by our group showed that the ciliated protist \textit{Paramecium Caudatum }swims with a constant propulsive force in solutions with viscosities 1 \textless $\eta $/ $\eta_{\mathrm{w}}$\textless 7 where $\eta_{\mathrm{w}}$ is the viscosity of water. Measurements of the geometry of its helical swimming trajectory combined with high speed video of the ciliary motion provided insight into this behavior. Using a phenomenological model we found that the body cilia beating frequency decreases while the beating angle remains roughly constant to produce the constant propulsive force dependence on viscosity. In this talk, we present studies of another ciliated protozoa, \textit{Tetrahymena Pyriformis} to determine whether the behavior of \textit{Paramecium} is general. Preliminary results indicate that \textit{Tetrahymena Pyriformis} also swims with a nearly constant propulsive force with increasing viscosity. Investigations similar to those performed on \textit{Paramecium} are underway and the latest results will be presented. This work was supported by NSF PHY0750360 and at the NHMFL by NSF DMR-0084173 [Preview Abstract] |
Tuesday, March 4, 2014 10:00AM - 10:12AM |
F12.00011: The behavioral space of zebrafish locomotion and its neural network model Kiran Girdhar, Maria Benitez-Jones, Ha Pham Thi, Mark Nelson, Martin Gruebele, Yann Chemla How does one describe quantitatively the complex motion of vertebrates? To answer this question, we investigated a model system for vertebrate locomotion: zebrafish swimming. We performed a quantitative analysis of all stereotyped behavioral swimming patterns of zebrafish larvae: spontaneous swimming, escape response to stimulus, and prey tracking. Previous attempts to analyze zebrafish swimming motion quantitatively have imposed many arbitrary parameters. Here, we instead used a~parameter-independent method that produces an orthogonal set of ``eigen-shapes'' of fish backbones to describe swimming motion in a low-dimensional space. We show that a linear combination of only three such ``eigen-shapes'' is sufficient to describe 97{\%} of zebrafish shapes. Moreover, stereotyped swimming behaviors fall on two low-dimensional attractors embedded in this three dimensional behavioral space. We also show using a two-dimensional correlation analysis that ``scoots'' and ``R-turns,'' which were previously described as discrete behavioral states, in fact represent extrema in a continuum in this low-dimensional behavioral space. To understand the neural basis of~the~behavior, we have also developed a neural network model of spontaneous swimming of fish larvae. We present a set of neural parameters such as synaptic conductance, stimulus amplitude that produces swimming behavior and reconstructed the low-dimensional behavioral space obtained from experimental results. [Preview Abstract] |
Tuesday, March 4, 2014 10:12AM - 10:24AM |
F12.00012: Active microrheology of fluids inside developing zebrafish Mike Taormina, Raghuveer Parthasarathy Biological fluids are a source of diverse and interesting behavior for the soft matter physicist. Since their mechanical properties must be tuned to fulfill functional roles important to the development and health of living things, they often display complex behavior on length and time scales spanning many orders of magnitude. For microbes colonizing an animal host, for example, the mechanical properties of the host environment are of great importance, affecting mobility and hence the ability to establish a stable population. Indeed, some species possess the ability to affect the fluidity of their environment, both directly by chemically modifying it, and indirectly by influencing the host cells' secretion of mucus. Driving magnetically doped micron-scale probes which have been orally micro-gavaged into the intestinal bulb of a larval zebrafish allows the rheology of the mucosal layer within the fish to be measured over three decades of frequency, complementing ecological data on microbial colonization with physical information about the gut environment. Here, we describe the technique, provide the first measurement of mucosal viscosity in a developing animal, and explore the technique's applicability to other small-volume or spatially inhomogeneous fluid samples. [Preview Abstract] |
Tuesday, March 4, 2014 10:24AM - 10:36AM |
F12.00013: Alignment of active particles with hydrodynamic interactions and formation of a self-assembled pump Katrin Wolff, Marc Hennes, Holger Stark Hydrodynamically interacting active particles in an external harmonic potential are known to form a self-assembled pump at large enough Peclet numbers [1]. Here, we give a quantitative criterion for the formation of the pump for active Brownian particles depending on the rotational diffusion of particles, their swim speed and the strength of the harmonic trap. The emerging flow field caused by the swimmers corresponds to a regularized stokeslet and stabilises the pump. We find that the particle distribution settles into a non-equilibrium steady state with non-vanisihing flux. The particle orientations can be mapped onto an equilibrium system as they align along a common ``pump axis'' in analogy to dipoles in an electric field. We perform Brownian dynamics simulations with hydrodynamic interactions and compare the many-particle simulations with an analytically tractable mean field system. \\[4pt] [1] R. W. Nash et al., \emph{Phys. Rev. Lett.}~\textbf{104}, 258101 (2010) [Preview Abstract] |
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