Bulletin of the American Physical Society
71st Annual Meeting of the APS Division of Fluid Dynamics
Volume 63, Number 13
Sunday–Tuesday, November 18–20, 2018; Atlanta, Georgia
Session G20: Biological Fluid Dynamics: Locomotion and Movement 
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Chair: Paul Krueger, Southern Methodist University Room: Georgia World Congress Center B308 
Monday, November 19, 2018 10:35AM  10:48AM 
G20.00001: Seastar inspired locomotion Sina Heydari, Matthew McHenry, Eva Kanso Sea stars have numerous specialized ‘tube feet’, that enable them to achieve highlycontrolled locomotion on various terrains from rocky to smooth surfaces. A tube foot consists of soft, waterfilled, muscular membranes that extend and contract through changing the hydraulic pressure. Inspired by the biomechanics of the tube feet, we propose a loworder model for a soft actuator consisting of active and passive force elements. The active element can produce either pulling or pushing forces, which correspond to contraction and extension of the tube foot, respectively. We then study the locomotion of a distributed mass driven by several such actuators. Specifically, we investigate the stability of walking by minimallycoupled multiple actuators. We find that even though the dynamics of a single foot is unstable when carrying a load, the interaction of multiple actuators can result in a stable forward locomotion. This system offers a new paradigm for walking using soft actuators, which exhibits great performance in terms of walking stability. 
Monday, November 19, 2018 10:48AM  11:01AM 
G20.00002: How an elephant trunk wraps and lifts Andrew Schulz, Jia Ning Wu, David L Hu Elephants are the construction cranes of the animal kingdom with the ability to move and lift unwieldy objects with their trunks. In this experimental study, we examine the kinematics of an elephant lifting a barbell. We show that the elephant trunk has several constraints to lift an object in this fashion. The trunk forms an Sshape with two sections of the trunk forming archlike shapes to resist deformation. To resist the bar sliding from the elephant's grip, the elephant must also wrap the tip of its trunk around the bar, in wrapping angles that increase with the amount of weight. We rationalize both the shape of the trunk and wrapping angle using mathematical models involving the trunk's elastic modulus and friction coefficient. These findings may inspire work in elephantinspired soft robotics. 
Monday, November 19, 2018 11:01AM  11:14AM 
G20.00003: Digitized gait of C. elegans' head: from mixing to propulsion Ahmad Zareei, Mir Abbas Jalali, Mohsen Saadat, Peter Grenfell, MohammadReza Alam A naturally inspired way of propulsion and mixing in low Reynolds number conditions is to replicate the locomotory function of microorganisms that have evolved to swim in highly viscous environments. Nematoda is an example of such microorganisms, and artificial mechanisms that mimic the locomotory functions of nematodes can be efficient viscous pumps. Here we simulate the motion of the head segment of Caenorhabditis elegans by introducing a reciprocating and rocking blade. We experimentally and numerically show that the bioinspired blade’s motion not only induces a flow structure similar to that of the worm, but also chaotically mixes the surrounding fluid by generating a circulatory flow. When confined between two parallel walls, the blade causes a steady Poiseuille flow with a pumping efficiency comparable with the swimming efficiency of the worm.

Monday, November 19, 2018 11:14AM  11:27AM 
G20.00004: A New Drag Model for Contracting Vorticella Revealed the Stalklengthdependence of Vorticella Stalk Contractility Sangjin Ryu, Eun Gul Chung Vorticella is a sessile protozoan, and tis stalk contracts on a millisecond timescale. Because this contraction is induced by the binding of calcium ions, the Vorticella stalk showcases calciumpowered cellular motility. In this study, the contractility of Vorticella cells of various stalk lengths were estimated using a new drag model which took account of the unsteadiness and finite Reynolds number of the water flow generated by contracting Vorticella and the wall effect of Vorticella’s residence substrate. It was found that various key contractility parameters depended on the stalk length, and the observed stalklengthdependencies were simulated using a damped spring model, which enabled estimating that the spring constant of the contracting stalk. These observed lengthdependencies of Vorticella’s contractility appear to reflect the biophysical mechanism of the spasmonemal contraction. 
Monday, November 19, 2018 11:27AM  11:40AM 
G20.00005: Scaling of thrust and drag of a rigid lowaspectratio pitching plate Uwe Ehrenstein The flow around a rigid rectangular pitching plate immersed in a free stream is numerically investigated, addressing the force and drag generated by the oscillatory motion. Several aspect ratios (span to length) ranging from 0.1 to 0.5 are considered, for a Reynolds number based on the plate's length and the incoming flow velocity of 2000. The validity of a scaling law for viscous drag, previously established for uniform plate's normal velocity, is assessed for the pitching motion. The pressure force along the moving plate is shown to decompose into a thrust part, interpreted in tems of elongated body theory, and a pressure force deficit, often analyzed as vortex drag induced by the vortices at the plate's lateral edges. Here, this timeaveraged pressure drag is interpreted as a Bernoullitype effect associated with the high transverse velocity, allowing for a scaling law using a potential model. The pressure force prediction using this composite scaling fits remarkably well with the numerical simulation results, the pressure thrust being reduced, compared to what would be predicted by the elongated body theory, by more than 30 % for the aspect ratios considered. 
Monday, November 19, 2018 11:40AM  11:53AM 
G20.00006: A needleshaped Brownian microswimmer in a channel JeanLuc Thiffeault, Saverio Eric Spagnolie, Jacob Gloe We consider a microswimmer modeled as a onedimensional line segment  a needle  with a fixed swimming velocity. The direction of swimming changes according to a Brownian process, and the swimmer is confined to an infinite channel. This is a standard model for a simple microswimmer, or a confined wormlike chain polymer. Using natural assumptions about reflection of the swimmer at boundaries, we compute the invariant distribution across the channel, and the statistics of spreading in the longitudinal direction. When the needle length is longer than the channel width, we compute the mean drift velocity of the swimmer. Otherwise, we examine the time it takes for the swimmer to reverse direction, and the effective diffusion constant of its large scale motion. 
Monday, November 19, 2018 11:53AM  12:06PM 
G20.00007: Transitions in the synchronization modes of elastic filaments through basalcoupling Hanliang Guo, Kirsty Y Wan, Janna C Nawroth, Eva Kanso Cilia and flagella often beat in synchrony. They can also switch between different synchronization modes. However, the exact mechanisms responsible for such switching remain elusive. While previous evidence suggests that cilia coordination can be a result of hydrodynamic coupling, recent experimental findings show that defects in the intracellular basal coupling significantly affect the synchronization modes. Here, we account for basal coupling between two elastic filaments, driven by a geometric switch moment, by introducing a linear elastic spring that connects the two filaments. We quantitatively examine the effects of this coupling on the synchrony of the filaments. We find that their coordination is strongly affected by the stiffness of the basal spring and the geometric switch parameters. Specifically, we observe a bistable region in the parameter space where the filaments could synchronize in either breaststroke or freestyle fashion depending on their initial conditions. We use the model to explain the transitions between various synchronization modes observed experimentally for the algae cells Chlamydomonas. 
Monday, November 19, 2018 12:06PM  12:19PM 
G20.00008: Successive instabilities in elastic filaments driven by molecular motors Yi Man, Eva Kanso Active microfilaments are ubiquitous in cellular and subcellular processes that involve motility and cargo transport. Examples include the beating motion of cilia and flagella, the formation of the mitotic spindle during cell division, and cytoskeletondriven motility. In these systems, molecular motors bind to microtubules, exerting axial forces along these filaments. Experimental assays have demonstrated that motordriven microtubules exhibit rich dynamical behaviors from straight to curled configurations. Here, we theoretically investigate the dynamic instabilities of elastic filaments, with freeends, due to a single follower force. Using the resistive force theory at low Reynolds number, and a combination of numerical techniques with linear stability analysis, we show the existence of five distinct regimes of filament behavior, including a buckled state with locked curvature similar to experimental observations. 
Monday, November 19, 2018 12:19PM  12:32PM 
G20.00009: Spontaneous oscillations and hydrodynamics of active microfilament Brato Chakrabarti, David Saintillan Cilia and flagella are thin hairlike cellular projections that play a variety of crucial roles from propulsion at low Reynolds number to longrange hydrodynamic transport. The movement of the cilium is produced by the bending of its core, known as the axoneme that consists of 9 pairs of microtubules. In presence of ATP, molecular motors carrying cargo undergo cycles of attachment and detachment generating sliding forces that are in turn converted to waving motion of the filaments. We present a microscopic bottomup model that accounts for the detailed stochastic kinetics of molecular motors and as well as feedback from the geometry of the axoneme represented as an Euler elastica. Through direct numerical simulations that account for hydrodynamics, we find that beyond a critical activity of motors the elastica starts beating spontaneously resulting in propagating bending waves similar to those observed in sperm flagella. In the parameter space of the model, we are also able to observe whipping patterns of cilia and the breast stroke of Chlamydomonas. Using PCA, we construct reducedorder representations of the dynamics and flow fields in terms of fundamental singularities of Stokes flow. We also explore hydrodynamic interactions and collective behavior of interacting filaments. 
Monday, November 19, 2018 12:32PM  12:45PM 
G20.00010: Wake Pattern Identification Using Graph Matching Mohammadreza Zharfa, Michael Hahsler, Eli Olinick, Paul S. Krueger Flow patterns generated in the wakes of immersed objects such as bluff bodies and swimming animals are related to the flow generation mechanisms (e.g., vortex shedding) and can provide a window to understanding hydrodynamic performance such as drag or propulsive efficiency. In this work the essential flow features are represented by critical points of the velocity field. The pattern is encoded in a graph constructed from the critical points, with node properties determined by the critical point character and graph edges (connections between critical points) carrying a scaleindependent “weight” property associated with proximity of the connected points. Flows are compared by determining a best match between the weighted graphs of the two flows, accounting for constraints based on the character of matched critical points. Relative similarity is assessed from differences in weights of the matched edges. The approach is able to correctly match flow fields of the same type from potential flow representations of several bluff body and aquatic locomotion flows, even when distortions of up to 20% of the vortex spacing are applied to the flows. 
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