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 JI: Micro Fluids: Mixing |
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Chair: Timothy Ameel, University of Utah Room: Salt Palace Convention Center 250 C |
Monday, November 19, 2007 3:35PM - 3:48PM |
JI.00001: Mixing Potential of an Oscillating Circular Cylinder in a Micro-Channel Bayram Celik, Unal Akdag, Ali Beskok Using h/p type finite element algorithms based on the Arbitrary-Lagrangian-Eulerian formulation, we investigated mixing potential of forced transverse oscillations of a circular cylinder in a microchannel at Reynolds number 100 and oscillation amplitude to cylinder diameter ratio of 0.8. Simulations are performed for two fluids entering the channel that are stirred by the oscillating cylinder in the Strouhal frequency range of 0.4-1.6. These frequencies are selected to be both in the lock-in and non-lock-in regimes using the natural vortex shedding frequency of the stationary cylinder placed in the channel. The relationship between the Strouhal frequency and resulting flow characteristics such as vortex dynamics and the force exerted on the cylinder is investigated. Mixing simulations are performed at Peclet numbers of 100 and 1000. Computational results show that mixing characteristic is highly related to the resulting vortex pattern and the wake behind the cylinder. The lock-in cases have shown better mixing potential than the non-lock-in cases, which is a result of their relatively shorter formation lengths and vortex patterns. [Preview Abstract] |
Monday, November 19, 2007 3:48PM - 4:01PM |
JI.00002: Numerical Modeling of Two-phase Electrohydrodynamic Instability Mixing Venkat Narayanan, Jianbo Li, Jeffrey Zahn, Hao Lin Organic-aqueous liquid (phenol) extraction is one of many standard techniques to efficiently purify DNA directly from cells. Mixing of the two fluid phases increases the surface area over which biological component partitioning may occur. Electrohydrodynamic (EHD) instability provides a means for effective two-phase micromixing, and has been experimentally demonstrated. In this work, analysis and simulation are combined to study two-phase EHD instability; main focus is placed on simulating the nonlinear instability regime and droplet formation. Specifically, the work investigates the effects of the applied electric field, geometry, surfactants, and convective flow rate on instability and mixing characteristics. The eventual objective is to maximize surface area and dispersion of the organic phase for optimized DNA extraction. The Taylor-Melcher leaky dielectric model is implemented using a finite volume, immersed boundary method, and preliminary results are presented and discussed. [Preview Abstract] |
Monday, November 19, 2007 4:01PM - 4:14PM |
JI.00003: Chaotic Mixing in Three-Dimensional Microchannel Flow Thuy Hong Van Le, Young Kwoen Suh, Sangmo Kang The quality of chaotic mixing in three-dimensional microchannel flow has been numerically studied using particle tracking techniques such as Poincare section and Lyapunov exponents. The chaotic mixing was generated by applying alternating current to electrodes embedded on the bottom wall at a first half period and on the top wall at a second half period. The equations governing the velocity and concentration distributions were solved using Fractional-step method (FSM) based on Finite Volume approach. Results showed that the mixing quality depended significantly on the modulation period. The modulation period for the best mixing performance was determined based on the mixing index for various different initial conditions of concentration distribution. The optimal values of modulation period obtained by the particle tracking techniques were compared with those from the solution of concentration distribution equation using FSM and CFX software and the comparison showed their good match. [Preview Abstract] |
Monday, November 19, 2007 4:14PM - 4:27PM |
JI.00004: Experimental and numerical investigation of the three-dimensional flow in a T-shaped micromixer. Ralph Lindken, Jeanette Hussong, Joep van Esch, Jerry Westerweel Three-dimensional stereoscopic-micro-PIV measurements reveal the symmetry breaking of the laminar stationary flow in a symmetric micro-scale T-shaped mixer. Liquid flows from both side arms into the T-mixer, meets in the junction region and enters the mixing channel under a 90-degree flow direction change. At a geometry-dependent Reynolds number of about 140 the flow pattern changes from parallel co-flow to a complex three-dimensional flow pattern. The mixing process changes from diffusion only to advection with mixing times of less than 1 ms. CFD simulations show that for undercritical Reynolds numbers the parallel co-flow is dominated by two Dean vortex pairs. At the critical Reynolds number the flow pattern changes from four Dean vortices to two stationary co-rotating vortices in the mixing channel. The interaction of the corner vortices in the junction region and the Dean vortex pairs results in an engulfment process of the entering liquid streams that causes the change of the flow pattern. The overcritical flow pattern has two stable symmetric solutions. The existence of this bifurcation is proven numerically. The novel laminar bifurcation is also verified experimentally. [Preview Abstract] |
Monday, November 19, 2007 4:27PM - 4:40PM |
JI.00005: Experimental Investigation of Mixing in a T-channel Susan Thomas, Tim Ameel Flow in a 20mm, square T-channel geometry was experimentally investigated and characterized for Reynolds numbers from fifty to six-hundred. The dynamic conditions and square channel geometry are relevant to microscale mixing. A T-shaped geometry is especially attractive because it is easy to fabricate and integrate into complex systems. In this study, PIV and LIF were used to track the evolution of three flow regimes with varying Reynolds number. Of particular interest to mixing was the development of an asymmetric flow instability at moderate Reynolds number. Additionally, LIF measurements in the outlet channel were used to characterize downstream mixing. The effect of asymmetric inlet velocities and temperature gradients was also investigated. Further, implications for microscale mixing were extracted. [Preview Abstract] |
Monday, November 19, 2007 4:40PM - 4:53PM |
JI.00006: Liquid mixing in T-shaped and zigzag-shaped microreactors M. Hoffmann, M. Schlueter, N. Raebiger Microreactors are basic components of microfluidic systems for chemical applications and the large area-to-volume ratio allows for a higher yield and selectivity than conventionally designed processes. To take advantage of the full potential of this technology, a fundamental understanding of the transport processes on the relevant time and length scales is necessary. One approach is the use of non-invasive measurement techniques, i.e. micron resolution particle image velocimetry (micro-PIV) and micron resolution laser induced fluorescence (micro-LIF) in order to measure the velocity and concentration fields (mixing of a passive tracer as well as reactive mixing). By using a confocal laser scanning microscope it is possible to remove out-of-focus emitted light and thus improve the lateral and axial spatial resolution. Hence a three-dimensional concentration field inside the mixing channel (T- and zigzag-shaped micromixers) can be rendered and a determination of the tracer distribution at different cross sections is basis for a quantitative analysis. The experimental analysis of the velocity fields (2 dimensions + 2 components) are basis for the calculation of the out-of-plane velocity component by using the continuity equation. With the knowledge of all three velocity components the 3D streamlines can be visualized and a calculation of the local rate of energy dissipation is possible. [Preview Abstract] |
Monday, November 19, 2007 4:53PM - 5:06PM |
JI.00007: The Optimization Design of An AC-Electroosmotic Micro mixer Yangyang Wang, Yongkweon Suh, Sangmo Kang We propose the optimization design of an AC-electroosmotic micro-mixer, which is composed of a channel and a series of pairs of electrodes attached on the bottom wall in zigzag patterns. The AC electric field is applied to the electrodes so that a fluid flow takes place around the electrodes across the channel, thus contributing to the mixing of the fluid within the channel. We have performed numerical simulations by using a commercial code (CFX 10) to optimize the shape and pattern of the electrodes via the concept of mixing index. It is found that the best combination of two kinds of electrodes, which leads to good mixing performance, is not simply harmonic one. When the length ratio of the two kinds of electrodes closes to 2:1, we can get the best mixing effect. Furthermore, we will visualize the flow pattern and measure the velocity field with a PTV technique to validate the numerical simulations. In addition, the mixing pattern will be visualized via the experiment. [Preview Abstract] |
Monday, November 19, 2007 5:06PM - 5:19PM |
JI.00008: Fast diffusiophoretic mixing of colloids in microchanels Cecile Cottin-Bizonne, Benjamin Abecassis, Christophe Ybert, Armand Ajdari, Lyderic Bocquet Diffusio-phoresis is the movement of a colloid particle induced by a gradient of concentration of a molecular solute. This form of transport originates at the colloid surface, within a nanometric layer where the interaction of the solute with the colloid surface differs from that of the solvent, {\it e.g.} the electric Debye layer for a charged solute. The solute concentration gradient induces a pressure drop within the interfacial layer along the surface, hence propelling the particle in a direction which depends on the interaction potential between the solute and the particle's surface. Using this effect we have realized enhanced mixing of colloids in microfluidic devices. We have shown experimentally that, as predicted by theory, in presence of a gradient of concentration of salt the colloids obey a diffusive like behaviour but with an effective diffusion coefficient which is orders of magnitude greater than the colloid's one. Depending on the solute nature and the particle's zeta potential, the amplitude of the phenomenon can be finely tuned. This provides a very simple and efficient way of mixing macromolecules in microfluidic devices. [Preview Abstract] |
Monday, November 19, 2007 5:19PM - 5:32PM |
JI.00009: Flow-enhanced mixing in nanoscale channels: Linking changes in shape to flow kinematics Howard Stone, Myoung-Woon Moon, Kyu Hwan Oh, John Hutchinson, Emmanuel Villermaux We first experimentally study mixing of miscible liquids during pressure-driven flow in nanoscale channels that are created by patterned buckling in compressed films on silicone substrates. The buckled films display the telephone cord morphology with a characteristic configuration of a zig-zag shape along the length direction, where the cross-sectional shape, which is almost semi-circular, has a periodic asymmetry. The experiments demonstrate flow enhancement of the mixing. Second, we present a model for the low Reynolds number mixing based on an asymptotic (lubrication) analysis of the flow accounting for the periodic axial changes in cross-sectional shape. The variations in shape produce secondary flows that give rise to exponential stretching of material lines. The theory that thus describes the nanoscale mixer provides a complete kinematical characterization of the flow, which mixes using transverse shears that are out-of-phase. We compare the theoretical predictions to the experimental measurements. [Preview Abstract] |
Monday, November 19, 2007 5:32PM - 5:45PM |
JI.00010: Resonant chaotic mixing in cellular flows Dmitri Vainchtein, John Widloski, Roman Grigoriev We present a quantitative theory of resonant mixing in time-dependent volume-preserving $3D$ flows using a model cellular flow as an example. Specifically, we show that chaotic advection is dramatically enhanced by a time-dependent perturbation for certain resonant frequencies. We compute the fraction of the mixed volume as a function of the frequency of the perturbation and show that essentially complete mixing in $3D$ is achieved at every resonant frequency. [Preview Abstract] |
Monday, November 19, 2007 5:45PM - 5:58PM |
JI.00011: ELISA and DNA reactions enhancement using micromixing Frederic Bottausci, Igor Mezic We report new experimental results on biological reactions enhancement using two different micromixing processes. The first process consists of perturbing the unmixed fluids by using the kinetic of jet flows emanating from a series of transverse channels. The jet flows are pressure driven (the pressure does not exceed 0.1 Bar). Complete mixing is achieved in 10ms. In this device, we present stop-flow study on complimentary single strain-DNA reactions and on ELISA reaction. We present a second process involving electrokinetic fluid flow mixing where the method consists in initiating a flow instability that will rapidly stir the microflow streams. Depending on the fluids properties, we induce AC-electrothermal or AC-electroomotic flow to manipulate the fluids. In this device, we show about an order of magnitude enhancement for biological reaction time. In the present study we present some measurements of the mixing behavior. We discuss the advantages of each process and show some quantitative results on biological reactions enhancement. [Preview Abstract] |
Monday, November 19, 2007 5:58PM - 6:11PM |
JI.00012: Stresses in Binary Particulate Systems Jin Liu, Shiyi Chen, Duanzhong Zhang A unified framework of equations for binary particulate systems are first derived based on two-equation model, these equations are correct from molecular systems to disperse two-phase flows. It is shown that the interactions between different species not only result in an exchange force but also an interspecies stress, which is crucial for recovering the averaged equations in the limit of disperse two-phase flow. The behaviors of exchange force, intraspecies stresses and interspecies stresses are explored using direct numerical simulations of binary particulate system in a periodic box undergoing relative motion, based on which the possible closure for derived equations is also discussed. [Preview Abstract] |
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