Bulletin of the American Physical Society
67th Annual Meeting of the APS Division of Fluid Dynamics
Volume 59, Number 20
Sunday–Tuesday, November 23–25, 2014; San Francisco, California
Session H11: Microscale Flows: Locomotion |
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Chair: David Saintillian, University of California at San Diego Room: 3007 |
Monday, November 24, 2014 10:30AM - 10:43AM |
H11.00001: Investigation of fluid flow and pumping due to a bacterial flagellum in its various polymorphic forms rotating above a no-slip boundary James Martindale, Henry Fu Recently, the fabrication of magnetically actuated rotating bacterial flagella attached to a planar substrate has been achieved. An array of such flagella may have applications as a microscale pump. In order to understand pumping, velocity, and other flow properties of anchored bacterial flagella rotating above a no-slip plane in the Stokes flow regime, a model of a single flagellar filament under constant torque near a no-slip boundary is considered for various polymorphic forms. The method of regularized Stokeslets, whose code is verified by examining a classical problem, is employed to create a benchmark case which can in turn be used to justify a slender body theory approach that drastically decreases computational cost. We investigate the flow for all 11 polymorphic forms of bacterial flagella, as well as the effect of tilt on several flow metrics for the various polymorphic forms. [Preview Abstract] |
Monday, November 24, 2014 10:43AM - 10:56AM |
H11.00002: Enhanced Monopropellant Fuel Decomposition by High Aspect Ratio, Catalytic CNT Structures for Propulsion of Small Scale Underwater Vehicles Kevin Marr, Jonathan Claussen, Brian Iverson Both maneuverability and efficiency for reagent-based propulsion systems of small-scale exploratory devices, such as autonomous underwater vehicles (AUVs), is largely dependent on their maximum fuel decomposition rate. Reagent-based systems, however, require large catalyst surface area to fuel volume ratios in order to achieve the fuel decomposition rates necessary for locomotion. This work demonstrates the utility of platinum-coated, carbon nanotube (CNT) scaffolds as high surface area catalysts for decomposition of hydrogen peroxide (H$_{2}$O$_{2})$ in a flowing environment. Usage of these functionalized microchannels ensures that both the maximum distance between fuel and catalyst is only half the microchannel diameter, and that the fuel concentration gradient increases due to boundary-layer thinning. These conditions facilitate intimate contact between fuel and catalyst and, therefore, faster decomposition rates. Electrochemical testing revealed that electroactive surface area to volume ratios of approximately 61.4 cm$^{-1}$ can be achieved for samples fabricated using a static Pt deposition scheme. Thrust measurements were taken using a small-scale submersible which indicated a maximum thrust of 0.114 N using 50 weight percent H$_{2}$O$_{2}$ exposed to eight inline 2.867 cm$^{2}$ Pt-CNT scaffolds. [Preview Abstract] |
Monday, November 24, 2014 10:56AM - 11:09AM |
H11.00003: Self-sustained motion of microcapsules on a substrate controlled via the repressilator regulatory network Henry Shum, Victor Yashin, Anna Balazs We design microcapsules that undergo self-induced motion in a fluid along a substrate and are able to collectively self-organize when controlled by a biomimetic signaling network. Three microcapsules act as localized sources of distinct chemicals that diffuse through the fluid. The production rate of each chemical is modulated by a regulatory network known as the repressilator: each species represses the production of the next in a cycle. We show that this system can exhibit sustained oscillations. We then allow the diffusing species to adsorb onto the substrate, altering the surface interaction energy. Gradients in surface energy lead to motion of the microcapsules. We find that regulation via the repressilator gives rise to qualitatively different outcomes. Chemical oscillations can facilitate aggregation of the microcapsules and the aggregate can undergo sustained translational or oscillatory motion. Numerical simulation of the fluid flow, microcapsule dynamics and concentration fields is achieved by a combination of the lattice Boltzmann, immersed boundary and finite difference methods. We assess the role of hydrodynamic interactions by comparison with a simplified model that assumes a constant drag coefficient relating the force on a microcapsule to its velocity. [Preview Abstract] |
Monday, November 24, 2014 11:09AM - 11:22AM |
H11.00004: Collective dynamics and mixing in a suspension of micro-rotors Enkeleida Lushi, Kyongmin Yeo, Petia Vlahovska We investigate theoretically and computationally the dynamics of many interacting micro-rotors suspended in fluid. As a particle rotates due to intrinsic or external torques, it disturbs the surrounding fluid and the motion of neighbouring particles. It can be shown that the motion of less than four point rotors is periodic and above that number their trajectories can become chaotic, a dynamics reminiscent to that of 2D point-vortices. If the full hydrodynamical interactions and lubrication effects between the particles are accounted for in a finite domain, a richer dynamics emerges. We exploit this coupled dynamics between micro-rotors and the structure of the generated fluid flows to mix a passive dye field or passive sphere particles also immersed in the fluid. The efficiency of the mixing for a variety of parameters will be discussed as well as experimental realizations. [Preview Abstract] |
Monday, November 24, 2014 11:22AM - 11:35AM |
H11.00005: Rotation axes, bistability, and controllability of rigid achiral magnetically rotated microswimmers Farshad Meshkati, Henry Fu We investigate magnetically actuated microswimmers through analytical and numerical schemes which are applicable to arbitrary rigid geometries. We examine the dynamics of a simple nonhelical, achiral, rigid swimmer composed of three connected colloidal beads. We consider magnetic fields that can rotate either perpendicular to its rotation axis, or at a nonperpendicular angle to its rotation axis. We find the steady rotating orbits of the swimmer and evaluate them for stability. We show that certain experimental conditions, determined by magnetic field strength, rotation frequency, and angle of field relative to rotation axis, can result in more than one stable orbit. We compare this to experimental observations of bistability of helical swimmers. We scrutinize the dependence of the rotation axis of the swimmer on experimental parameters and compare it to the experimental observations of wobbling in the literature. Finally, we show that the controllability of these types of swimmers can be improved by manipulating the angle between the direction of the magnetic field and its axis of rotation. [Preview Abstract] |
Monday, November 24, 2014 11:35AM - 11:48AM |
H11.00006: Design of helical magnetically rotated microswimmers for controllability Henry Fu Microswimmers or microrobots have recently received much attention due to their possible applications in microscale sensing and actuation, including many biomedical applications such as drug delivery, in vivo diagnostics, and tissue manipulation. We have developed a modeling framework to describe the dynamics of rigid microswimmers that can be propelled through bulk fluid (rather than only near surfaces) when rotated by an external magnetic field. Here, this modeling framework is used to identify stable steady rotating orbits of the helical microswimmers under development by many research groups. I investigate how the swimming properties depend on the magnetization direction and geometry (pitch and radius) of the helix. In general, these swimmers have nonlinear dependence of velocity on frequency due to changes in the rotation axis of the swimmer as frequency is changed. However, a linear dependence would enhance velocity control and precise positioning of these swimmers. I identify magnetization directions which keep the rotation axis constant as a function of frequency, hence lead to linear velocity-frequency dependence. I also identify helical geometries which lead to maximal swimming velocities and rotation axes closest to the helical axis of the swimmer. [Preview Abstract] |
Monday, November 24, 2014 11:48AM - 12:01PM |
H11.00007: Geometrical Performance of Electrocatalytic Nanomotors Amir Nourhani, Paul E. Lammert, Vincent H. Crespi, Ali Borhan We provide a general analytical expression for the speed of electrocatalytic nanomotors in terms of surface cation flux, interfacial potential and physical properties of motor environment in the linear regime and thin electric diffuse layer. We model the motor geometry by a prolate spheroid which covers a range of geometries from sphere to rod-shape and slender bodies. We obtain a functional that turns the surface cation flux distribution into a motive utility factor. For a spherical motor the kernel of the functional reduces to the first Legendre polynomial of the first kind and with increase in the aspect ratio of the motor, the kernel tends to give more significance to the cation flux near the ends of the motor and the motor velocity becomes less sensitive to the flux distribution around the equator of the spheroid. [Preview Abstract] |
Monday, November 24, 2014 12:01PM - 12:14PM |
H11.00008: Designing a bio-inspired self-propelling hydrogel micro-swimmer Svetoslav Nikolov, Peter Yeh, Alexander Alexeev Artificial micro-swimmers have found numerous applications in microfluidics, drug delivery systems, and nanotechnology. In our current research we use dissipative particle dynamics to design and optimize a self-propelling hydrogel micro-swimmer with an X-shaped flat geometry and bi-layered hydrogel structure. The two polymeric layers that bind to each other have identical material properties but distinctive chemical responses to external stimuli. In the presence of outside stimuli one of the layers swells where the other remains passive resulting in hydrogel bending. Our simulations demonstrate that under periodic applications of an external stimulus this actuation routine is capable of creating time-irreversible motion in a low Reynolds number environment. Initially, when the external stimulus is introduced a forward stroke is initiated, as the swimmer first expands and then bends. When the outside stimulus is removed the forward stroke is terminated and a backward stroke begins, as the swimmer contracts and then straightens. Propulsion results due to the difference in momentum exchange between the forward and backward strokes. We use our simulations to probe how alterations in the material properties of the bi-layered hydrogel can affect swimming performance. [Preview Abstract] |
Monday, November 24, 2014 12:14PM - 12:27PM |
H11.00009: Phase behavior of monolayer suspensions of counter rotating rotors Kyongmin Yeo, Enkeleida Lushi, Petia Vlahovska The dynamics of monolayer suspensions of counter-rotating spherical rotors is investigated by using the force coupling method. The motions of the suspended rotors are confined to the horizontal plane perpendicular to the axis of rotation. The suspensions are equally divided by two species of spherical particles, which are rotating under equal-magnitude opposite-sign torques. Unlike the previous results in non-hydrodynamic limit, it is shown that the conversion rate of the rotational kinetic energy to the translational kinetic energy increases slowly with the increase in volume fraction ($\phi$) and eventually exhibits a sharp drop around a critical volume fraction ($\phi \simeq 0.54$). A closer investigation of suspension microstructure reveals that the rotors of the same torque start to form a cluster for $\phi \ge 0.30$. Around the critical volume fraction, hexagonal structures emerge in the suspensions and the particle mobility is significantly hindered by the caging effects. [Preview Abstract] |
Monday, November 24, 2014 12:27PM - 12:40PM |
H11.00010: Acoustophoretic particle motion in a square glass capillary Rune Barnkob, Alvaro Marin, Massimiliano Rossi, Christian J. K\"{a}hler Acoustofluidics applications often use complex resonator geometries and complex acoustic actuation, which complicates the prediction of the acoustic resonances and the induced forces from the acoustic radiation and the acoustic streaming. Recently, it was shown that simultaneous actuation of two perpendicular half-wave resonances in a square channel can lead to acoustic streaming that will spiral small particles towards the pressure nodal center (Antfolk, Anal. Chem. 84, 2012). This we investigate in details experimentally by examining a square glass capillary with a 400-$\mu$m microchannel acoustically actuated around its 2-MHz half-wave transverse resonance. The acoustic actuation leads to the formation of a half-wave resonance in both the vertical and horizontal direction of the microchannel. Due to viscous and dissipative losses both resonances have finite widths, but are shifted in frequency due to asymmetric actuation and fabrication tolerances making the channel not perfectly square. We determine the resonance widths and shift by measuring the 3D3C trajectories of large particles whose motion is fully dominated by acoustic radiation forces, while the induced acoustic streaming is determined by measuring smaller particles weakly influenced by the acoustic radiation force. [Preview Abstract] |
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