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 JA: Micro Fluids: General IV |
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Chair: Gustav Amberg, The Royal Institute of Technology, Sweden Room: Salt Palace Convention Center 150 A-C |
Monday, November 19, 2007 3:35PM - 3:48PM |
JA.00001: Modeling of the dielectrophoretic conveyer-belt assembling microparticles into large-scale structures Boris Khusid, David Jacqmin, Anil Kumar, Andreas Acrivos A dielectrophoretic conveyor-belt method for assembling negatively polarized microparticles into large-scale structures was recently developed (APL 90, 154104, 2007). To do this, first, an array of microelectrodes is energized to generate a spatially periodic AC electric field that causes the particles to aggregate into boluses in positions of the field intensity- minima, which are located mid-way along the height of the channel. The minima and their associated boluses are then moved by periodically grounding and energizing the electrode array so as to generate an electrical field moving along the electrode array. We simulate this experiment numerically via a two- dimensional electro-hydrodynamic model (PRE 69, 021402, 2004). The numerical results are in qualitative agreement with experiments in that they show similar particle aggregation rates, bolus sizes and bolus transport speeds. [Preview Abstract] |
Monday, November 19, 2007 3:48PM - 4:01PM |
JA.00002: Electrophoretic Motion of a Cylindrical Particle through a 90$^{\circ}$ Corner Kendra Sharp, Scott Davison Transient simulations of the trajectory of a cylindrical particle driven by electrophoresis through a 90$^{\circ}$ corner have been performed. The presence of the 90$^{\circ}$ corner acts to reduce the initial distribution of angles to the vertical of 90$^{\circ}$ to less than 30$^{\circ}$, demonstrating the possibility of using a corner as a passive control element as part of a larger microfluidic system. A variety of system parameters, including zeta potential, channel width, particle aspect ratio, initial vertical position of particle and initial angle with respect to the horizontal, were tested to determine their impact on the particle motion. However, the reduction in angle is limited to the area near the corner posing a limitation on this means of control. [Preview Abstract] |
Monday, November 19, 2007 4:01PM - 4:14PM |
JA.00003: Hydrodynamic effects on dielectrophoretic separation of carbon nanotubes Junichiro Shiomi, Yuan Lin, Shigeo Maruyama, Gustav Amberg Single-walled carbon nanotubes (SWNTs) are key materials in nanotechnology as potential candidates for diverse applications due to their extraordinary mechanical, thermal, optical and electrical properties. One of the current critical challenges is the separation of metallic (m-SWNTs) and semiconducting SWNTs (s-SWNTs). Among various post-synthesis separation methods devised and applied, dielectrophoretic (DEP) separation of SWNTs in dispersed form has been demonstrated to be possible with high selectivity and simplicity. The method can also be utilized to maneuver SWNTs to selected locations. Although the concept of DEP-separation in principle is simple, the system involves effects that may cause bulk flow motions such as electroosmosis, thermal convection and electrothermal flow, which is driven by a body force caused by electric field acting on gradients in permittivity and/or conductivity due to a non-uniform temperature field. In the current study, we investigate the impact of the thermo-hydrodynamics on the DEP-separation of SWNTs by formulating a dynamical model of the integrated system. The results show that, under typical experimental conditions, electrothermal flow can dominate the motion of s-SWNTs and significantly weakens the DEP separation. [Preview Abstract] |
Monday, November 19, 2007 4:14PM - 4:27PM |
JA.00004: Hybrid modeling of electromagnetic forces in microfluidic devices Justyna Czerwinska, Mohamed Gad-el-Hak Many flows in microdevices operate under condition in which electromagnetic fields are essential driving force. The majority of research in that field is experimental due to the extreme complexity of the theoretical approach. To enhance understanding as well as to help direct design in such flows, there is a need to develop fast and efficient simulation techniques. For many practical operational conditions of MEMS and NEMS, the continuum approximation breaks down. Pure atomistic modeling is too computationally intensive to be possible in the near future but for the smallest of devices. Hence, there are two approaches that can be proposed:\ mesoscale modeling such as DSMC, LBM, DPD and RD; and hybrid molecular dynamics--continuum approach, which we focus on herein. Non-equilibrium molecular dynamics simulations are applied to all fluid molecules in the immediate vicinity of the charged particles' molecules. The Navier--Stokes equations are applied in the rest of the flow field, linked to the Maxwell equations. Adding to the complexity is the fact that electromagnetic forces are long range while molecular interactions are short range, presenting an additional multiscale challenge. Our results are validated using simple flows with immersed charged particles. [Preview Abstract] |
Monday, November 19, 2007 4:27PM - 4:40PM |
JA.00005: Numerical Simulation of Electroosmotic Flow through Triangular Microchannel Vaitheeswaran Gnanaraj, V. Mohan Numerical simulation electroosmotic flow through triangular microchannels has been developed in this paper. The governing equations consist of a 2D Poisson-Boltzman equation and a 2D Navier Stoke's with Electric Double Layer (EDL) field and velocity field in the cross-section of triangular microchannel are solved analytically. The effects of channel height, electrolyte concentration, surface potential, EDL thickness and externally applied elctric field on the velocity profile of traiangular microchannels are numerically studied. The comparison of numerical simulation results shows excellent agreement with the corresponding analytical solution. The numerical simulation shows significant influences of channel cross-section geometry and volumetric flow rate. [Preview Abstract] |
Monday, November 19, 2007 4:40PM - 4:53PM |
JA.00006: Transient Currents, Capacitance and an Electrical Analogy of a Finite Length Microchannel Ali Mansouri, Subir Bhattacharjee, Larry Kostiuk Numerical simulations with the fluid mechanics based on the unsteady Navier-Stokes equations, and the Poisson-Nernst-Planck formulation of electrostatics and ion transport, were used to explore the transient transport of charge through a finite length cylindrical microchannel that is driven by a pressure difference. The evolution of the trans-capillary potential from a no-flow equilibrium to the steady-state-steady-flow streaming potential was analyzed by following the convection, migration and net currents. Observations of the unsteady characteristics of the streaming current, electrical resistance, and capacitance lead to an electrical analogy. This electrical analogy was made from a current source which was placed in parallel with a capacitor and a resistor. A parametric study involving a range of geometries, fluid mechanics, electrostatics, and mass transfer states allowed predictive sub-models for the current source, capacitor and resistor to be developed based on a dimensional analysis. [Preview Abstract] |
Monday, November 19, 2007 4:53PM - 5:06PM |
JA.00007: Direct Numerical Simulation (DNS) of Suspensions in Spatially Varying Electric Fields M. Janjua, Sai Nudurupati , Pushpendra Singh, Nadine Aubry We have developed a new direct numerical (DNS) scheme to simulate the motion of dielectric particles suspended in a dielectric liquid in uniform and nonuniform electric fields. The motion of particles is tracked using a distributed Lagrange multiplier method (DLM) and the electrostatic forces acting on the particles are calculated by integrating the Maxwell stress tensor (MST) over their surfaces. The MST is deduced from the electric potential which, in turn, is obtained by solving the electrostatic problem. We show that the error in the trajectories given by the point dipole method, which assumes that the presence of particles does not alter the imposed electric field, increases as the distance between the particles decreases. In addition, the error is relatively large when the particle radius is comparable to the domain size, and also increases as the difference between the dielectric constants of the particles and the fluid increases. The final steady positions of the particles, including the orientations of the line joining their centers relative to the electric field direction, obtained by using the MST method are different from those resulting from the point dipole method. [Preview Abstract] |
Monday, November 19, 2007 5:06PM - 5:19PM |
JA.00008: Streaming Potential generated by two-phase flow in a capillary Etienne Lac, John Sherwood Streaming potentials generated by two-phase flow in a porous medium are much less well understood than those generated by single phase flow. We study the Stokes flow of a droplet in a straight capillary as a function of drop size, capillary number and ratio of the viscosity of the drop to that of the surrounding fluid. The electrical double layer on the wall of the capillary is assumed to be thin and the $\zeta$-potential is small; the drop is assumed to be perfectly insulating. Boundary integral methods are used to solve both the Stokes equation and the Laplace equation for the electric field within the capillary. The change in streaming potential due to the presence of the drop is computed, as is the change in pressure drop and in electrical resistivity. The results show that the ratio between the increased pressure drop and increased streaming potential can take either sign, depending upon the drop viscosity and capillary number. The effect of a constriction within the capillary will also be discussed. [Preview Abstract] |
Monday, November 19, 2007 5:19PM - 5:32PM |
JA.00009: The stochastic dynamics of a triangular atomic force microscope near a solid boundary Matt Clark, Mark Paul The stochastic dynamics of an atomic force microscope (AFM) cantilever with a complex planform and in a finite-sized fluid filled container is of direct relevance to industry and the development of new technologies. We study the Brownian driven dynamics of an industrially available AFM cantilever with triangular planform immersed in water at room temperature. The stochastic dynamics are determined using a thermodynamic approach based upon the fluctuation-dissipation theorem that requires only deterministic calculations. Using this approach with finite element simulations of the complete fluid-solid interaction problem we quantify the equilibrium fluctuations in cantilever tip displacement and in tip angle. We use our results to explore the increased fluid dissipation that arises as the cantilever is brought near a solid boundary, and the corresponding reduction in quality factor and resonant frequency of the cantilever. Approximations based on long-thin cantilevers predict higher dissipation than found in our approach, suggesting that care be used when applying the theory to cantilevers with complex planform. [Preview Abstract] |
Monday, November 19, 2007 5:32PM - 5:45PM |
JA.00010: Design of Optimal Microfluidic Components Using a Genetic Algorithm Search David Mott, Keith Obenschain, Peter Howell, Joel Golden Mott et al. [1] describe the automatic design of optimal microfluidic components based on performance criteria. The approach constructs a complex component by adding geometric features, such a grooves of various shapes, to a microchannel. The net transport produced by each of these features in isolation was pre-computed and stored as an ``advection map'' for that feature, and the complex flow through a composite geometry that combines these basic features was calculated rapidly by applying the corresponding maps in sequence. An exhaustive search of feature combinations produced optimized mixer designs of moderate size and complexity. In the current work, a genetic algorithm replaces the exhaustive search of Ref. [1], enabling the optimization of much more complex components with far more degrees of freedom. New metrics for characterizing surface delivery and sample dispersion (i.e., the spreading of a sample plug within the pressure-driven flow) are developed, and the software is applied to design new components that optimize surface delivery and that minimize sample dispersion. \newline [1]. Mott, D.R., Howell, P.B, Golden, J.P., Kaplan, C.R., Ligler, F.S., and Oran, E.S., Lab on a Chip, Vol. 6, No. 4, 2006, pp. 540-549. [Preview Abstract] |
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