Bulletin of the American Physical Society
60th Annual Meeting of the Divison of Fluid Dynamics
Volume 52, Number 12
Sunday–Tuesday, November 18–20, 2007; Salt Lake City, Utah
Session FA: Micro Fluids: General II |
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Chair: Kendra Sharp,Pennsylvania State University Room: Salt Palace Convention Center 150 A-C |
Monday, November 19, 2007 8:00AM - 8:13AM |
FA.00001: A Microfluidic Approach for Studying Shear-induced ATP Release Kinetics from Red Blood Cells Jiandi Wan, William D. Ristenpart, Catherine Best, Renita Horton, Guido Guidotti, Edward H. Abraham, Howard A. Stone To understand the influence of shear stress on the kinetics of adenosine triphosphate (ATP) release from red blood cells (RBCs), we mimic arterial constrictions using a series of channels in microfluidic devices. The shear stress and duration of stress are systematically varied in different devices by changing the width and length of the constriction channels respectively. We show that the amount of released ATP increases roughly exponentially with the magnitude of the shear stress, but that there is a critical duration of stress (2 ms) required for RBCs to release significant amounts of ATP. The results suggest that RBCs are sensitive not only to the diameter of arterial constrictions but to their length, an effect with important physiological and medical implications. This work also motivated a new approach for using microfluidic methods for the measurement of enzyme kinetics, which we mention briefly. [Preview Abstract] |
Monday, November 19, 2007 8:13AM - 8:26AM |
FA.00002: The diversity of streaming patterns around bubbles at a wall David Hansen, Jih-Chiang Tsai, Sascha Hilgenfeldt Oscillating bubbles attached to plane walls, typically driven by a resonant ultrasound field, have been shown to set up powerful steady streaming flows in microfluidic geometries. Understanding these flows quantitatively is essential for designing a variety of devices making use of transport, mixing, or force actuation capabilities of bubble-driven streaming flows. As has been observed by experimentalists, a rich variety of types of flow around bubbles attached to walls is possible. The nature of such a flow depends on many factors, including the bubble position, interactions between bubble oscillation modes, the boundary conditions on the bubble, and the interaction between bubble and wall boundary layers. We discuss these different flow patterns and correlate them with experimental observations. Properly understood, the variety of possible flows could allow for great flexibility in microfluidic applications. [Preview Abstract] |
Monday, November 19, 2007 8:26AM - 8:39AM |
FA.00003: Asymmetrical microbubble streaming in a confined geometry J.-C. Tsai, David Hansen, Sascha Hilgenfeldt Ultrasound-driven oscillating microbubbles situated on a substrate induce steady streaming flows that show great potential in cellular-scale force actuation for bioengineering or in microfluidic applications [1-3]. We have demonstrated for a streaming flow of azimuthal symmetry around a single bubble that the presence of a second wall opposite to the substrate does not compromise the flow speed but instead enhances the circulation efficiency. Additionally, we show here that a continuous spectrum of flow patterns ranging from localized vortex circulations to transport-dominated directional flows can be created by breaking the symmetry either passively (by changing the subtrate topology) or actively (by imposing a large-scale flow field). The spatial confinement provides a dominant lengthscale that simplifies the flow patterns and enhances the transport efficiency. Asymmetrical microbubble streaming, with its capability to fine-tune the relative strength between the circulation and forward transport, offers a continuously adjustable tool for microfluidic applications that demand the simultaneous optimization of mixing rate and transport efficiency. Ref: [1] P. Marmottant and S. Hilgenfeldt, Nature 423, 153 (2003). [2] P. Marmottant and S. Hilgenfeldt, Proc. Natl. Acad. Science USA, 101, 9523 (2004). [3] P. Marmottant, J.-P. Raven, H. Gardeniers, J. G. Bomer, and S. Hilgenfeldt, J. Fluid Mech., vol.568, 109 (2006). [Preview Abstract] |
Monday, November 19, 2007 8:39AM - 8:52AM |
FA.00004: Formation and breakup of capillary viscous threads in square microchannels Thomas Cubaud, Thomas G. Mason We experimentally study the formation and the breakup transition of a viscous thread in a less viscous, immiscible liquid by hydrodynamic focusing into a square microchannel. Over large range of variations in both viscosities and interfacial tension, five characteristic regimes, including threading, jetting, dripping, tubing, and displacement are located on a phase-diagram based on the capillary number of each fluid. In the jetting and the dripping regimes, droplets size is measured and related to fluid properties, flow parameters, and geometry. The critical thread length before jetting as well as the critical length of a viscous tail before breakup are also examined. This study provides a means for producing supra- and sub-channel size viscous droplets in an elementary microfluidic geometry. [Preview Abstract] |
Monday, November 19, 2007 8:52AM - 9:05AM |
FA.00005: How to make sticky surfaces slippery? Contact angle hysteresis in AC electrowetting Frieder Mugele, Adrian Staicu, Fahong Li Pinning of contact lines at surface heterogeneities, the origin of contact angle hysteresis, is a major obstacle in micro- and nanofluidic systems involving moving three-phase contact lines giving rise to finite threshold forces. We studied the contact angle hysteresis of aqueous drops on various polymer surfaces in the presence of electric fields in an electrowetting configuration. We demonstrate the contact angle hysteresis decreases linearly with the electrostatic force applied to the contact line in the case of AC voltage at frequencies in the kHz range. In contrast, the hysteresis is essentially unaffected by a DC voltage giving rise to the same decrease in contact angle. The experimental observation is explained in terms of an instantaneous balance of surface tension forces, pinning forces, and time-dependent electrostatic forces at the contact line. To illustrate the power of the effect, we demonstrate that the threshold force required for actuating drops sandwiched between two parallel surface indeed almost vanishes for sufficiently high AC voltage. [Preview Abstract] |
Monday, November 19, 2007 9:05AM - 9:18AM |
FA.00006: Thermophoresis of Nano Platelets in Confined Space Steffen Hardt For several decades it has been known that a rigid body immersed in a gas of nonuniform temperature experiences a thermophoretic force aligned with the temperature gradient. If however, a body is placed in a narrow gap between two solid walls of uniform, but different temperature, a thermophoretic force perpendicular to the temperature gradient can be created. In this work an analytical expression is derived for the force experienced by a nano platelet between two walls of different temperature. It is assumed that the in-plane extension of the platelet is larger than the gap width and that the gas dynamics occurs in the free-molecular flow regime. For the case that the gas molecules are diffusively reflected from the walls, it can be shown that the wall collision rate is constant over the whole computational domain. Based on this insight, expressions for the force and the torque on a nano platelet in a narrow gap are derived. It is shown that the torque vanishes, whereas there is a net force perpendicular to the temperature gradient. When considering thermal fluctuations on top of the average force it becomes apparent that in such a way a thermophoretic motion of a platelet can be induced. [Preview Abstract] |
Monday, November 19, 2007 9:18AM - 9:31AM |
FA.00007: On the `piston effect' in a small-scale gap Avshalom Manela, Nicolas Hadjiconstantinou The `piston effect', the time response of a fluid to a change in the thermal properties of its boundaries, is studied for a gas confined in a small-scale (of the order of the mean free path) gap and subject to an instantaneous jump in the temperature of its boundaries. The problem is formulated for a collisionless gas in the case where the relative temperature change at each wall is small and independent of the other. An analytic solution for the probability density function is obtained and the respective hydrodynamic fields are calculated. It is found that the characteristic time scale for arriving at the new equilibrium state is of the order of several acoustic time scales. The results are compared with direct Monte Carlo simulations of the Boltzmann equation and a good agreement is found for nondimensional times (scaled by the acoustic time) not exceeding the system Knudsen number. Thus, the present analysis describes the early-time behaviour of systems of arbitrary size and may provide the initial behaviour in the counterpart continuum-limit problem. [Preview Abstract] |
Monday, November 19, 2007 9:31AM - 9:44AM |
FA.00008: Hydrodynamic computational models for nonequilibrium fluid dynamics and issues of projecting their solutions into phase space Rho Shin Myong, Byung Chan Eu There exist growing interests in developing proper mathematical models of the physical process in the mesoscopic regime. Notable examples can be found in micro- and nano-scale fluid flows and rarefied hypersonic gas flows. In this work, a method to derive high-order hydrodynamic equations for nonequilibrium fluid dynamics is presented. In addition, a slip model based on Langmuir's theory of adsorption of gases on solids is developed in order to describe the gas-surface molecular interaction in micro- and nano-scale system. Finally, issues of reconstructing a solution in phase space from information available in thermodynamic space will be discussed through the investigation of a one-dimensional shock wave flow of monatomic gases. [Preview Abstract] |
Monday, November 19, 2007 9:44AM - 9:57AM |
FA.00009: Control of capillary flow by shape perturbation Martin Heller, Mathias B{\AE}kbo Andersen, Rune Barnkob, Henrik Bruus Capillary forces offer the possibility to transport and guide liquids in microfluidic systems without active pumps. The position of the meniscus in a capillary flow can be precisely predicted in straight channels with constant cross-section. However, most chip designs require complex channel geometries, which can be achieved using accurate microfabrication techniques. We present analytical results based on first-order shape-perturbation theory for the position of an advancing liquid/gas interface in a flat microchannel with nonparallel confining boundaries. We propose to use such systems with carefully designed height variations for guided capillary filling and for avoiding trapping of air bubbles during priming of lab-on-a-chip devices. [Preview Abstract] |
Monday, November 19, 2007 9:57AM - 10:10AM |
FA.00010: The Effect of Secondary Flows on Taylor-Aris Dispersion Hui Zhao, Haim Bau We study theoretically the effects of secondary (transverse) flows on the Taylor-Aris dispersion of pressure-driven, open column flow in a conduit with a rectangular cross-section and account for the interaction of solutes with the retentive coating on the conduit's surface. A few plausible means of inducing secondary flows (that are independent of the primary, pressure-driven, axial flow) are described. The Taylor-Aris dispersion coefficient is computed as a function of the secondary flow's pattern and intensity. We show that dispersion can be significantly reduced in the presence of secondary flows. [Preview Abstract] |
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