Bulletin of the American Physical Society
66th Annual Meeting of the APS Division of Fluid Dynamics
Volume 58, Number 18
Sunday–Tuesday, November 24–26, 2013; Pittsburgh, Pennsylvania
Session A6: Microfluids: Mixing |
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Chair: Igor Mezic, University of California, Santa Barbara Room: 328 |
Sunday, November 24, 2013 8:00AM - 8:13AM |
A6.00001: Synergetic Fluid Mixing from Viscous Fingering and Alternating Injection Birendra Jha, Luis Cueto-Felgueroso, Ruben Juanes We study mixing of two fluids of different viscosity in a microfluidic channel or porous medium. We show that the synergetic action of alternating injection and viscous fingering leads to a dramatic increase in mixing efficiency at high Peclet numbers. Based on observations from high-resolution simulations, we develop a theoretical model of mixing efficiency that combines a hyperbolic mixing model of the channelized region ahead, and a mixing-dissipation model of the pseudo-steady region behind. Our macroscopic model quantitatively reproduces the evolution of the average degree of mixing along the flow direction, and can be used as a design tool to optimize mixing from viscous fingering in a microfluidic channel. [Preview Abstract] |
Sunday, November 24, 2013 8:13AM - 8:26AM |
A6.00002: The Impact of Miscible Viscous Fingering on Mixing Jane Chui, Pietro de Anna, Ruben Juanes Viscous fingering is a hydrodynamic instability that occurs when a less viscous fluid displaces a more viscous one. Instead of progressing as a uniform front, the less viscous fluid forms fingers that vary in size and shape to create complex patterns. The interface created from these patterns affects mixing between the two fluids, and therefore is of critical importance in applications such as enhanced oil recovery and microfluidics. This work focuses on how the evolution of the fingering interface affects mixing between two miscible fluids, specifically in a radial configuration. We measure the local concentration field temporally and spatially with the use of a fluorescent tracer in the injected fluid, and with this high resolution information are able to calculate various measures of mixing, such as mixing efficiency, scalar dissipation rate, and the areal mixing zone for different fluid injection rates and various viscosity ratios. We propose a scaling theory based on experimental observations for the growth of the mixing zone and the overall rate of mixing. [Preview Abstract] |
Sunday, November 24, 2013 8:26AM - 8:39AM |
A6.00003: Diffusive and inertial instabilities during miscible fluid thread formation in microgeometries Thomas Cubaud, Sara Notaro We study the formation and stability of miscible fluid threads having large difference in viscosity using hydrodynamic focusing sections. Miscible core annular flows are useful for transporting viscous materials and can be destabilized for enhancing mass transfer. Here, we delineate phase-diagrams of the formation of miscible threads from low to large viscosity contrasts with various diffusion coefficients. Depending on fluid properties and flow rates of injection, microflows are classified into diffusive, stable, and inertial regimes. For low P\'{e}clet numbers, we examine threads dynamics when diffusive effects strongly influence flow structures. Another regime is investigated for moderate Reynolds numbers where small threads are rapidly destabilized in the inertial flow field of the sheath fluid at the junction. [Preview Abstract] |
Sunday, November 24, 2013 8:39AM - 8:52AM |
A6.00004: Diffusion Effects on the Chaotic Fluid Mixing for AC Electrothermal Flows by Blinking Vortices Sophie Loire, Igor Mezic We present a computational study of AC electrothermal chaotic mixing using blinking of asymmetric electrothermal vortices. Electrothermal flows are modeled by finite element method using COMSOL software based on an enhanced electrothermal model. We use the mix-norm on numerical trajectory simulations to evaluate mixing at different scales including the layering of fluid interfaces by the flow, a keypoint for efficient mixing. The blinking vortices method greatly improve mixing efficiency. The effect of blinking frequency and particle size is studied. A large influence of diffusion on the mixing efficiency is observed as well as on the optimal blinking frequency. [Preview Abstract] |
Sunday, November 24, 2013 8:52AM - 9:05AM |
A6.00005: AC Electrokinetic 3D blinking micro-mixer Marin Sigurdson, Sophie Loire, Marko Budisic, Igor Mezic An AC electrokinetic 3D mixer is presented, which has the potential to accelerate bio-reactions in both lab-on-a-chip and microplate formats. AC voltage across electrodes on a micro-chamber floor generate vortices in the buffer via the electrothermal effect, which is particularly effective for ionic buffers used in bioassays. Controlling these vortices over time, for example, by periodic switching between overlapping vortices (blinking), creates time dependent 3D chaotic mixing. This mixing was studied via 2 methods. First, the full 3D, 3 component velocity field was measured with our original Proper Orthogonal Decomposition PIV method for each vortex configuration. These velocity fields were then used to numerically evaluate mixing predictive parameters such as ergotic quotient and fluid layering. These parameters help identify regions of good and poor mixing, aiding electrode shape design. Second, mixing of low and high diffusivity particles was optically evaluated, through the Mix Variance Coefficient. The blinking pattern and frequency was then optimized to yield the fastest mixing for each case. Finally, work is underway to demonstrate reaction rate acceleration on the order of 10 fold as the result of this mixing. [Preview Abstract] |
Sunday, November 24, 2013 9:05AM - 9:18AM |
A6.00006: Effective mixing strategies with microbubble streaming flows Cheng Wang, Bhargav Rallabandi, Lin Guo, Sascha Hilgenfeldt Homogeneous mixing of chemical/biological samples and reagents is one of the essential preparation steps for lab-on-a-chip systems. As effective Stokes flows driven by fast time scale oscillatory flows, microbubble streaming flows are a tool uniquely positioned between passive and active mixing approaches. Guided by thorough theoretical understanding of the flows and of micromixing itself, we investigate various designs of microbubble mixers, employing two key strategies: (a) introducing controlled unsteadiness in the acoustic driving pattern, e.g. by duty-cycling and driving frequency modulation, and (b) optimizing the arrangement of multiple bubbles, such as the number, position, and orientation of the microbubbles, particularly to generate 3D chaotic flow patterns. Both of these approaches significantly improve mixing over that of previous steady 2D bubble micro-mixers, and the strategies can be combined for greater effect. [Preview Abstract] |
Sunday, November 24, 2013 9:18AM - 9:31AM |
A6.00007: Microfluidic mixing using orbiting magnetic microbeads Matthew Ballard, Drew Owen, Wenbin Mao, Peter Hesketh, Alexander Alexeev Using three-dimensional simulations and experiments, we examine mixing in a microfluidic channel that incorporates a hybrid passive-active micromixer. The passive part of the mixer consists of a series of angled parallel ridges lining the top microchannel wall. The active component of the mixer is made up of microbeads rotating around small pillars on the bottom of the microchannel. In our simulations, we use a binary fluid lattice Boltzmann model to simulate the system and characterize the microfluidic mixing in the system. We consider the passive and active micromixers separately and evaluate their combined effect on the mixing of binary fluids. We compare our simulations with the experimental results obtained in a microchannel with magnetically actuated microbeads. Our findings guide the design of an efficient micromixer to be used in sampling in complex fluids. [Preview Abstract] |
Sunday, November 24, 2013 9:31AM - 9:44AM |
A6.00008: ABSTRACT WITHDRAWN |
Sunday, November 24, 2013 9:44AM - 9:57AM |
A6.00009: Mixing and transport by ciliary carpets Yang Ding, Janna Nawroth, Margaret McFall-Ngai, Eva Kanso Cilia are hair-like micro-structures observed on surfaces of many biological systems such as the human lungs. Cilia usually beat asymmetrically in a coordinate manner and serve for flow generation and sensing. Here, we use a 3D computational model to study the fluid transport and mixing due to the beating of an infinite array of cilia. In accord with recent experiments, we observed two distinct regions: a fluid transport region above the cilia and a fluid mixing region below the cilia tip. We examined the effect of the metachronal wave (due to phase differences between neighboring cilia) on the net flow and mixing rate. We found that the metachronal wave can enhance both transport and mixing rate of the fluid, often simultaneously. Our results suggest that the simultaneous enhancement in fluid transport and mixing is due to the enhancement in shear flow. As the flow above the cilia increases, shear rate in the fluid increases and such shear enhances stretching, which is an essential ingredient for mixing. Estimation of the time scale of the mixing indicates that, compared to diffusion, the mixing due to the cilia beat may be significant or even the dominate way of distributing molecules in some biological systems. [Preview Abstract] |
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