Bulletin of the American Physical Society
63rd Annual Meeting of the APS Division of Fluid Dynamics
Volume 55, Number 16
Sunday–Tuesday, November 21–23, 2010; Long Beach, California
Session EM: Microfluids: General III: Microchannels |
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Chair: Evelyn Wang, Massachusetts Institute of Technology Room: Long Beach Convention Center 202B |
Sunday, November 21, 2010 4:10PM - 4:23PM |
EM.00001: Taylor-Aris dispersion in time-dependent laminar channel flows S{\O}ren Vedel, Henrik Bruus The effective axial diffusion of solute concentrations advected in channel flows is known as Taylor-Aris dispersion [1,2]. Due to the no-slip condition, particles near the walls are displaced less than those close to the channel center axis, leading to concentration gradient perpendicular to the axis and an enhanced axial diffusivity. In many applications the velocity field is unsteady, but concentration dispersion in such time-dependent flows is largely unexplored, except for transient dispersion of an initial concentration profile in a steady flow [3], and dispersion in a velocity field with one harmonically oscillating component superimposed on a steady component [4]. We present a mathematical theory for Taylor-Aris dispersion in a straight channel with an arbitrary time- dependent flow, based on Fourier expansion of the velocity field, valid for all times and all values of the P\'eclet number. The theory is applied to different time-dependent flows in channels of different cross sections, and we discuss the new phenomena arising by adding an increasing number of higher harmonics. \\ \noindent{}[1] Taylor, \textit{Proc. Roy. Soc. Lond. A} \textbf {219}, 186 (1953)\\ \noindent{}[2] Aris, \textit{Proc. Roy. Soc. Lond. A} \textbf {235}, 67 (1956)\\ \noindent{}[3] Barton, \textit{J. Fluid Mech.} \textbf{126}, 205 (1983)\\ \noindent{}[4] Mukherjee and Mazumder, \textit{Acta Mech.} \textbf{74}, 107 (1988) [Preview Abstract] |
Sunday, November 21, 2010 4:23PM - 4:36PM |
EM.00002: Designing patterned microchannels to separate colloid-polymer suspensions Hassan Masoud, Alexander Alexeev Using dissipative particle dynamics, we examine the flow of colloidal suspensions in microfluidic channels with patterned walls. The distribution of colloids in a channel is set by the competition between diffusion and hydrodynamic effects. We show that the distribution can be altered by introducing tilted nanoscopic posts protruding from internal walls of a microchannel. Specifically, we demonstrate that depending on the post orientation, the patterned walls can either hydrodynamically attract nanoscale objects suspended in the flowing fluid or prevent their depositions by repelling them away from solid walls. Furthermore, surfaces decorated with tilted posts can discriminate nanoscopic entities with regard to their shape and, thus, can be utilized for separating colloid-polymer mixtures. [Preview Abstract] |
Sunday, November 21, 2010 4:36PM - 4:49PM |
EM.00003: The Effect of Aspect Ratio on Taylor Dispersion in Oscillatory Poiseuille Flow in Rectangular Channels Jinkee Lee, Elejdis Kulla, Anubhav Tripathi, Anuj Chauhan The presence of size walls is known to lead to significant increase in dispersion in uniaxial Poiseuille flows even for very large aspect ratios. This presentation focuses on exploring the effect of the side walls on dispersion in oscillatory Poiseuille flows in rectangular channels. The method of multiple time scales with regular expansions is utilized to obtain analytical expressions for the effective dispersivity $D_{3D}^\ast$ and analytical results are compared with CFD simulations. The effective dispersivity is of the form $D_{3D}^\ast =Pe^2f\left( {\Omega ,Sc,\chi } \right)$ where its dependency on the dimensionless oscillating frequency $\Omega$, the Schmidt number and the aspect ratio $\chi$ of the channels is non-explicit. The effect of various parameters on dispersion coefficient is explored numerically and also through asymptotic expressions that are valid in some limiting cases. For small $\Omega $ the dispersion coefficient for the oscillatory flow approaches the time averaged dispersion of the unidirectional Poiseuille flow and for large $\Omega$, $D_{3D}^\ast$ scales as ${Pe^2} \mathord{\left/ {\vphantom {{Pe^2} \Omega }} \right. \kern-\nulldelimiterspace} \Omega ^2$. We believe that the results of this study will enhance our understanding of transport in microscale systems that are subjected to oscillating flows. [Preview Abstract] |
Sunday, November 21, 2010 4:49PM - 5:02PM |
EM.00004: Dynamics of pulsatile flows through elastic microtubes Omer San, Anne Staples We investigate pressure driven transient flows of incompressible Newtonian fluids through circular microtubes with thin elastic walls under the long-wavelength and small deformation assumptions, which are valid for many industrial and biological process. An analytical solution of the coupled fluid and solid equations is found using the Navier slip boundary condition and is shown to include some existing Womersley solutions as limiting cases. The effect of the slip length at the fluid-solid interface of the flexible microtube is analyzed for oscillatory pressure gradients using a range of slip-ratio and frequency parameters. We find that for a steady pressure gradient, slip at the boundary simply adds a translational velocity and does not lead to material deformations, while for pulsatile flows with oscillating pressure gradients, the influence of the slip length becomes nonlinear and affects the flow rate, velocity profile, and shear stress. We compare the solutions for elastic and rigid walls with and without slip boundary conditions for broad ranges of the relevant parameters. We show that the elasticity of the microtube couples nonlinearly with the slip velocity and can greatly enhance the flow rate, sigfinifcantly changing its maximum value and effective range as a function of Wormersley number, compared to the no-slip case. Additionaly, we find that increasing the slip length produces less shear stress, which is consistent with the nearly frictionless interfaces observed in many microscale experiments. [Preview Abstract] |
Sunday, November 21, 2010 5:02PM - 5:15PM |
EM.00005: An examination of collisional Lattice Boltzmann Method for microchannel flows Boe Green, Prakash Vedula A new computational approach for prediction of microchannel flows, which accounts for the full collision operator of the Boltzmann equation via a lattice framework, is presented. Unlike the widely used Lattice Boltzmann Method (LBM), our approach, called the collisional Lattice Boltzmann Method (cLBM), does not make any a priori assumptions on the equilibrium state and hence is capable of handling general nonequilibrium flows (i.e. over a wide range of Knudsen numbers). In cLBM, an operator splitting approach is used for solution of the Boltzmann equation, where representative populations of notional particles are streamed along the underlying lattice from all lattice nodes and the effects of collisional relaxation at each node are accounted for via a solution of a system of differential equations, derived from the full collision operator. This approach not only preserves several symmetries of the full collision operator, but is also structured to account for the evolution of selected generalized moments of the distribution (including conservation of mass, momentum, energy). Simulations of microchannel Couette and Poiseulle flows (including pressure driven and body force driven cases) over a broad range of Knudsen numbers, using a D3Q27 lattice structure, show that the results obtained from cLBM are in good agreement with those obtained from conventional LBM (relying on equilibium based BGK model). [Preview Abstract] |
Sunday, November 21, 2010 5:15PM - 5:28PM |
EM.00006: Droplet and Slug Detachment and Entrainment in Microchannel Gas Flows Brian Robinson, Brian Carroll, Carlos Hidrovo Liquid droplet and slug dynamics in a confined microchannel high speed gas flow is an important phenomenon with applications in two-phase micromixers, spray cooling for point source heat rejection, and water management in proton exchange membrane fuel cells. Thus, the ability to understand, predict, and control droplet growth, detachment, entrainment, and possible breakup is crucial. When subjected to a gas flow in a standard T-junction arrangement, experimental studies have shown that droplet and slug detached characteristics are determined by the gas Reynolds number, site geometry, and liquid/solid interfacial tension. Increasing the gas Reynolds number reduces the volume of the detached droplets and slugs while injection geometry and interfacial tension influence droplet and slug tail growth and formation of liquid films. Additionally, droplets can grow and detach with and without contact with adjacent channel walls due to site contaminants, geometry imperfections, and surface treatments, thereby adding complexity to the detachment process. [Preview Abstract] |
Sunday, November 21, 2010 5:28PM - 5:41PM |
EM.00007: Gas-flow animation by unsteady boundary heating in a microchannel Nicolas Hadjiconstantinou, Gregg Radtke, Avshalom Manela We study the response of a one-dimensional gas layer due to unsteady boundary heating. Analytical results are presented for the slip-flow/Navier-Stokes and collisionless limits. The latter is applicable to gas layers that are thinner than the molecular mean free path or to layers of arbitrary size with heating time-scales that are shorter than the mean collision time. Our analytical results are complemented by low-variance simulations of the Boltzmann equation, which are useful for establishing the limits of validity of the closed-form predictions, as well as bridging the gap between them. In particular, we consider the gas response to step-jump heating and show that the slip-flow solution captures the correct gas behavior for times as short as few collision times. The Navier- Stokes slip-flow solution is also used to elucidate a singular limit reported in the literature for oscillatory heating of a dynamically incompressible fluid. [Preview Abstract] |
Sunday, November 21, 2010 5:41PM - 5:54PM |
EM.00008: ABSTRACT WITHDRAWN |
Sunday, November 21, 2010 5:54PM - 6:07PM |
EM.00009: Effect of channel turn on the trajectory of an electrophoretic particle Dustin House, Haoxiang Luo Streamlines of non-particle-laden flow are often used as a convenient method to predict the trajectory of particles driven through a microchannel by electrophoresis. However, the validity of this approach it is not clear when the channel geometry is complex and when the particle size is large compared to the characteristic length scale of the channel. To address this issue, we have developed an accurate numerical approach based on the boundary-element method to solve the coupled electric field, flow and particle motion. From this, we simulate a spherical particle moving in a bent cylindrical channel. In the simulation, both the particle and channel walls are non-conducting, and the electrical double layers adjacent to the solid surfaces are assumed to be thin with respect to the particle radius and to the particle-wall gap. The result shows that the particle trajectory deviates from the flow streamlines (in the absence of the particle) when the turning radius is small and the particle is close to the inner side of the turn. The effect of the particle-to-cylinder size ratio will be also be presented. [Preview Abstract] |
Sunday, November 21, 2010 6:07PM - 6:20PM |
EM.00010: Effect of surface condition on the flow in segmented gas-liquid microreactors Shahram Pouya, Manoochehr Koochesfahani The mixing process within segmented gas-liquid microreactors is of significance importance in design and optimization of devices for high throughput material synthesis. In a typical slug flow regime the liquid slugs are connected through a thin liquid film that plays an important role in hydrodynamics of the microreactor flow. Among the parameters that can influence the thin film layer, and the overall flow, is the surface condition of microchannel walls. We present preliminary results of this influence in the segmented gas-liquid flow of Ethanol/Nitrogen within PDMS microreactors. The results are presented specifically for microreactors with different level of roughness on the channel walls. The range of stable slug flow regime and behavior of liquid film are studied as a function of surface roughness. [Preview Abstract] |
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