Bulletin of the American Physical Society
66th Annual Meeting of the APS Division of Fluid Dynamics
Volume 58, Number 18
Sunday–Tuesday, November 24–26, 2013; Pittsburgh, Pennsylvania
Session M31: Biofluids: Locomotion X - Non-Newtonian Fluids |
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Chair: Thomas Powers, Brown University Room: 402 |
Tuesday, November 26, 2013 8:00AM - 8:13AM |
M31.00001: Enhanced diffusion of tracers in a bath of self-propelled particles Alexander Morozov Recent experiments have shown that micron-size tracer particles in dilute suspensions of either swimming bacteria or synthetic self-propelled particles perform diffusive motion with the diffusion coefficient significantly larger than its thermal value. Several theories have been proposed to explain the origin and magnitude of the enhanced tracer diffusivity. There is now a general agreement that it is proportional to the so-called ``active flux'' - the product of the swimmer's number density and their velocity. Here we present detailed theory and simulations of tracers diffusing in bacterial suspensions. Our work confirms the scaling with the active flux, but also unravels new important dependencies on the properties of the velocity field created by the swimmers and their kinematics. Our work is potentially relevant for understanding feeding currents and biomixing created by swimming microorganisms. [Preview Abstract] |
Tuesday, November 26, 2013 8:13AM - 8:26AM |
M31.00002: Swimming of wavy sheets in weakly viscoelastic fluids Alexander Morozov Many natural habitats of biological microswimmers include complex fluids whose mechanical response is strongly non-Newtonian. Recent attempts to understand swimming in such fluids produced a series of seemingly contradictory results. Especially, it is currently not understood whether swimming in dilute polymer solutions would be faster or slower than in Newtonian fluids like water. One of the classical models to study swimming is a 2D infinite periodic waving sheet model introduced by G. I. Taylor. For small-amplitude swimming it was shown previously that viscoelasticity of the suspending fluid reduces the propulsion speed, while simulations of a finite-size version of the same model predicted an increase of the propulsion speed followed by a decrease as the fluid becomes progressively more elastic. Here we present a mechanism for the reduction of the propulsion speed and devise a new swimmer that can go faster in viscoelastic fluids than in their Newtownian counterparts. We perform analytical and exact numerical calculations of large-amplitude swimming of both models to confirm our mechanism. [Preview Abstract] |
Tuesday, November 26, 2013 8:26AM - 8:39AM |
M31.00003: Theory for propulsion and transport in an anisotropic fluid Thomas Powers, Madison Krieger, Saverio Spagnolie Swimming microorganisms are typically found in complex fluids, which are full of polymers. When these polymers align, the fluid becomes anisotropic. We seek to understand how anisotropy affects swimming when the stroke is prescribed. We model the anisotropic fluid with a nematic liquid crystal. The swimmer is a two-dimensional sheet deforming via propagating transverse or longitudinal waves. We find that the nature of anchoring conditions for the nematic degrees of freedom plays a critical role in determining the swimming speed. Furthermore, we study the fluid transport induced by the swimmers motion by calculating the flux of fluid in the laboratory frame. Finally, we elucidate the various limits of the nematic theory, such as the six-fold symmetric hexatic case and Ericksen's transversely isotropic fluid. [Preview Abstract] |
Tuesday, November 26, 2013 8:39AM - 8:52AM |
M31.00004: The forward undulatory locomotion of Ceanorhabditis elegans in viscoelastic fluids Amy Shen, Xialing Ulrich Caenorhabditis elegans is a soil dwelling roundworm that has served as model organisms for studying a multitude of biological and engineering phenomena. We study the undulatory locomotion of nematode in viscoelastic fluids with zero-shear viscosity varying from 0.03--75~Pa$\cdot$s and relaxation times ranging from 0--350~s. We observe that the averaged normalized wavelength of swimming worm is essentially the same as that in Newtonian fluids. The undulatory frequency f shows the same reduction rate with respect to zero-shear viscosity in viscoelastic fluids as that found in the Newtonian fluids, meaning that the undulatory frequency is mainly controlled by the fluid viscosity. However, the moving speed $V_m$ of the worm shows more distinct dependence on the elasticity of the fluid and exhibits a 4\% drop with each 10-fold increase of the Deborah number $\mathrm{De}$, a dimensionless number characterizing the elasticity of a fluid. To estimate the swimming efficiency coefficient and the ratio $K=C_N/C_L$ of resistive coefficients of the worm in various viscoelastic fluids, we show that whereas it would take the worm around 7 periods to move a body length in a Newtonian fluid, it would take 27 periods to move a body length in a highly viscoelastic fluid. [Preview Abstract] |
Tuesday, November 26, 2013 8:52AM - 9:05AM |
M31.00005: A mechanism for non-Newtonian swimming enhancement Yi Man, Eric Lauga Polymeric solutions and suspensions are prone to display slip due to the presence of thin low-viscosity fluid layers near boundaries. Using theoretical modeling, we investigate the role of such reduction in fluid friction on locomotion of model microorganisms. Addressing two- and three-dimensional situations, we demonstrate how even very thin regions of reduced fluid friction can dramatically enhance locomotion speeds. Our results suggest a mechanism for enhanced swimming in complex fluid environments. [Preview Abstract] |
Tuesday, November 26, 2013 9:05AM - 9:18AM |
M31.00006: Locomotion of microorganisms near a no-slip surface in a viscoelastic fluid Shahrzad Yazdi, Arezoo Ardekani, Ali Borhan Microorganisms are exposed to complex fluids in their natural habitats, especially during biological processes. In many of these processes, microorganisms swim in confined domains such as spermatozoa in mucus of mammalian reproduction tracts or bacteria in extracellular polymeric matrices during biofilm formation. Thus, it is important to understand the kinematics of propulsion in a viscolastic fluid near a no-slip surface. We used a squirmer model with a time-reversible body motion to analytically investigate the swimming kinematics in an Oldroyd-B fluid near a no-slip surface. Our results show that the time-averaged propulsion for a pusher (puller) is towards (away from) the no-slip surface. We present the swimming trajectory as a function of Deborah number, initial distance from the surface, and initial swimming direction. [Preview Abstract] |
Tuesday, November 26, 2013 9:18AM - 9:31AM |
M31.00007: Non-Newtonian rotational swimming: experiments S. Gomez, F.A. Godinez, R. Zenit, E. Lauga Recently Pak et al. (PoF, 2012) showed that a device composed of two unequal spheres (snowman) could swim in a viscoelastic fluid under a rotational actuation. By symmetry such device isn't able to move in a Newtonian fluid but because of its geometrical asymmetry is able to generate asymmetric elastic response and generate a purely viscoelastic thrust. We implemented this swimmer experimentally using a magnetic snowman driven by an external rotating magnetic field. We demonstrate that the snowman swims solely as a result of fluid elasticity. We conduct tests in Newtonian and Boger fluids, varying the sphere size ratio and rotation speed. We also conducted measurements in a confined environment, which showed an improved swimming performance. [Preview Abstract] |
Tuesday, November 26, 2013 9:31AM - 9:44AM |
M31.00008: Undulatory Swimming in Fluids with Polymer Networks David Gagnon, Xiaoning Shen, Paulo Arratia In this talk, we systematically investigate the motility behavior of the nematode Caenorhabditis elegans in polymeric solutions of varying concentration using tracking and velocimetry methods. As the polymer concentration is increased, the solution undergoes a transition from the semi-dilute to the concentrated regime, where these rod-like polymers entangle, align, and form networks. Remarkably, we find an enhancement in the nematode's swimming speed of approximately 65 percent in concentrated solutions compared to semi-dilute solutions. Using velocimetry methods, we show that the undulatory swimming motion of the nematode induces an anisotropic mechanical response in the fluid. This anisotropy, which arises from the fluid micro-structure, is responsible for the observed increase in swimming speed. [Preview Abstract] |
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