Session MW: Micro Propulsion I

Swimming in gels

Many swimming microorganisms must move through viscoelastic fluids and gels. In this talk I focus on swimming through gels. First, unlike incompressible fluids, a gel can have compressional modes with relative motion between polymer and solvent fractions. In a continuum model for a gel, we show that compressibility can increase the swimming speed of Taylor's swimming sheet. The zero-frequency shear modulus of a gel requires altered boundary conditions on the swimmer. Second, many biological gels are heterogeneous on the lengthscale of swimming microorganisms, necessitating non-continuum models that treat the gel network and swimmer on equal footing. We show that a random network modeled as dilute, immobile spherical obstacles increases the average swimming speed of a Golestanian three-sphere swimmer.   

Modeling magnetically driven synthetic microcapsules

Using computer simulations and theory, we examine how to design magnetically-responsive synthetic microcapsules that able to move in a steady manner in microfluidic channels. These compliant fluid-filled capsules encompass superparamagnetic nanoparticles in their solid shells and, thereby, can be manipulated by alternating magnetic forces. To model the magnetic capsules propelled in fluid-filled microchannels, we employ a hybrid computational method for fluid-structure interactions. This method integrates the lattice Boltzmann model for the fluid dynamics and the lattice spring model for the micromechanics of solids. We show that in circulating magnetic field the capsules propel along sticky microchannel walls. The direction of capsule motion depends on the relative location of the solid surface, whereas the propulsion speed can be regulated through the wall adhesiveness, amplitude and frequency of magnetic forces, and elasticity of capsule's shell. The results indicate that such mobile fluid-filled containers could find application in lab-on-chip systems for controlled delivery of minute amounts of fluidic samples.   

Motion of microbeads propelled by bacterial chemotaxis in a microfluidic platform

Micro actuators propelled by bacteria are of great interest in recent years, because bacterial chemotaxis has well-presented one of the very promising solutions to the utilization of the motion of flagellated bacteria. In this work, the motion of fluorescent microbeads driven by bacterial chemotaxis has been analyzed by micro-particle tracking velocimetry ($\mu$-PTV). Flagellated bacteria, Serratia Marcescens, are attached to the surface of the polystyrene (PS) microbeads spontaneously in an aqueous culture solution. Then, these particles are injected in the test medium where the linear concentration gradient of L-aspartate is maintained, which is generated by convective and molecular diffusions in a microfluidic platform. It is observed that the particles slowly move toward the high-concentration zone of L-aspartate. This work shows that migration of microbeads using bacterial chemotaxis can be one of the effective tools for the applications to actuators of micro-bio robots.   

Digital holographic PTV analysis of the motile performance of the red-tides alga ``\textit{Cochlodinium polykrikoides}''

The outbreaks of \textit{Cochlodinium polykrikoides }causes severe damages to fisheries in Korea and Japan. \textit{C. polykrikoides} actively makes chains in its exponential and stationary growth phase. The role of the chain formation has not been clearly known yet. In the present study, the motility characteristics of \textit{C. polykrikoides} was investigated using 3D digital holographic PTV technique. The moving trajectories and average velocities of the single and multi-chained \textit{C. polykrikoides} were compared to investigate the effect of the chain formation on the motile performance. Most of the multi-chained \textit{C. polykrikoides} exhibited zigzag or helical trajectory and the translational velocity of the 4-chained \textit{C. polykrikoides} was at least two-times larger than the single one.   

Cilia-induced fluid mixing in mucociliary clearance

The surfaces of the upper airways to the lung are coated with a surface liquid (ASL), which prevents inhaled pathogens from accumulating in the lungs. The ASL contains a watery periciliary layer (PCL), where a dense mat of cilia beat, propelling the viscous mucus layer above toward the trachea and mouth. We investigate fluid transport and mixing in the PCL based upon a computational model that couples the internal force generating mechanisms of cilia with external fluid dynamics, including an elastic mucus layer. We use Lagrangian Coherent Structure (LCS) methods to identify different spatial mixing regions, and study qualitative behavior of cilia with and without the mucus layer.   

Swimming of helically-undulating rings in a Stokes fluid

Dinoflagellates swim due to the action of two eucaryotic flagella - a trailing, longitundinal flagellum that propagates planar waves, and a transverse flagellum that propagates helical waves. The transverse flagellum wraps around the cell in a plane perpendicular to the trailing flagellum, and is thought to provide both forward thrust along with rotational torque. Motivated by the intriguing function of this transverse flagellum, we study the fundamental fluid dynamics of a helically-undulating ring in a Stokes fluid. We use slender-body theory to compute the steady-state transverse and rotational swimming velocities of the ring in free- space, due to an imposed helical traveling wave. In addition, we study the dynamics of an undulating, elastic ring moving in both free-space and near a plane wall using the method of regularized Stokeslets.   

A lightning stab in the dark: fluid dynamics of attack jumps of ambush feeding copepods

A large class of marine zooplankton, in particular among copepods, are ``ambush feeders'', who wait for their prey and capture them by surprise attacks. The successful attack must happen so rapidly and unexpectedly that the prey cannot escape and the fluid disturbances created by the attack must be so small that the prey is not pushed away by the flow created by the much larger approaching copepod. Detailed high speed video\textit{ in vivo} reveals that the nearly blind copepod manages to perform the attack by precision maneuvering during a rapid jump of a few milliseconds moving approximately one body length. The prey is pushed only by around 10{\%} of this distance, and thus the bulk flow must be close to potential with small boundary layers. From this, we argue that the smallest ambush feeding copepod, \textit{Oithona davisae}, is close to the size limit for the ambush feeding strategy. REF: T. Ki{\o}rboe, A. Andersen, V. J. Langlois, H. H. Jakobsen and T. Bohr, PNAS (2009)   

Propulsion of water striders: capillarity and hydrodynamics

We present a numerical investigation of the propulsion of water striders. The flow produced by the stroke of the leg is modeled as that around a hydrophobic circular cylinder astride the interface. We use a diffuse-interface model to compute the moving contact line and the interfacial deformation, and to probe the origin of the propulsive force acting on the leg. The movement of the leg produces a significant deformation of the interface: The upstream meniscus is curved by the dynamic pressure, while the downstream meniscus is flattened by the low-pressure wake associated with vortex shedding. Due to this asymmetry, the force produced by interfacial tension dominates the propulsion.   

Comparison of real and idealized cetacean flippers

We explored the consequences of the idealization process by creating exact scale models of cetacean flippers using CT scans, creating corresponding idealized versions, then determining the hydrodynamic characteristics of the models via water tunnel testing. We found that the majority of the idealized models did not exhibit fluid dynamic properties that were drastically different from those of the real models, although multiple consequences resulting from the idealization process were evident. Drag performance was significantly improved by idealization. Overall, idealization is an excellent way to capture the relevant effects of a phenomena found in nature, which spares the researcher from having to painstakingly create exact models, although we have found that there are situations where idealization may have unintended consequences such as one model that exhibited a decrease in lift performance.   

The hydrodynamics of two species of copepods: temperate and subtropical \textit{Euchaeta}

Different species of the copepod genera \textit{Euchaeta} live in polar, temperate, and subtropical ocean environments. \textit{Euchaeta elongata} is a species found in temperate waters and is roughly double the size of the subtropical species \textit{Euchaeta rimana}. The kinematic viscosity of the ocean water in the temperate latitude (8 deg C) is roughly 50{\%} greater than that of subtropical environments (23 deg C). We hypothesize that these species have adapted to the local fluid environment to create flow disturbances that facilitates optimal prey capture and predator avoidance. Particle Image Velocimetry (PIV) was used to quantify the flow surrounding each copepod species during cruising and escaping behaviors. Seven to nine replicates for each species were collected for free swimming specimens during both cruise and escape behavior. The average Reynolds number of both species was found to be on the order of 10 for cruising behavior and 1000 for escapes. During cruising, the spatial extent of the region of flow disturbance, defined by a threshold of the maximum principle rate of deformation, was not significantly different between species. In contrast, the spatial extent of the region of flow disturbance during escapes was larger for \textit{E. elongata}. Further, the viscous dissipation rate was similar for the species during cruising, whereas \textit{E. elongata} had a significantly greater viscous dissipation rate during escape behavior.