Bulletin of the American Physical Society
2006 59th Annual Meeting of the APS Division of Fluid Dynamics
Sunday–Tuesday, November 19–21, 2006; Tampa Bay, Florida
Session OD: Microfluidics XII: Mixing-3 |
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Chair: Igor Mezic University of California, Santa Barbara Room: Tampa Marriott Waterside Hotel and Marina Grand Salon CD |
Tuesday, November 21, 2006 12:15PM - 12:28PM |
OD.00001: Quantification of Electrokinetic Instability Induced Micromixing using Ion Indicating Dyes Philip Wheat, Jonathan Posner Electric fields coupled with ionic conductivity gradients generates charge density in the bulk fluid which can result in electrokinetic instabilities (EKI). Electrokinetic (EK) flow with conductivity gradients become unstable when the electroviscous stretching and folding of conductivity interfaces grows faster than the dissipative effect of molecular diffusion. In this work we quantify EK micromixing in a cross-shaped microchannel using epifluorescence microscopy and ion indicating dyes. The quantum efficiency of ion indicating dyes increase when bound to specific ionic species. In this way, the degree of mixing of the ion-indicating dye and the target ion is proportional to the fluorescence intensity. Here we present a method for quantitative determination of the degree of fluid mixing. We show that mixing of these EK unstable flows increases with electric field (for a fixed ionic conductivity ratio) and that nearly complete mixing can be obtained in less than 10 microchannel widths. [Preview Abstract] |
Tuesday, November 21, 2006 12:28PM - 12:41PM |
OD.00002: Enhanced mixing in laminar flows with ultrahydrophobic surfaces Jia Ou, Geoffrey Moss, Jonathan Rothstein Ultrahydrophobic surfaces have recently been shown to produce significant drag reduction in laminar flows. In this presentation, we will talk about current research aimed at engineering these ultrahydrophobic surfaces to produce mixing enhancement in laminar flows. Our research utilizes the slip velocity along the shear-free air-water interface formed between the surface structures to produce secondary flows which stretch and fold fluid elements to produce enhanced mixing. The surfaces are fabricated with PDMS casted from silicon mold with hydrophobic patterns of microridges in different spacing and at various angles to the flow direction. The effectiveness of the surfaces is tested in micro mixing flow cell using a confocal microscope to track fluorescent die. At the inlet, to streams of fluid are brought together at a y-junction, one stream is tagged with fluorescent die. The normalized fluorescence intensity is used in experiments to calculate the degree of mixing and compared directly to the mixing predictions from numerical simulations. The kinematical mechanism of this laminar mixing enhancement method is studied though both experimental and simulations. The mixing length is shown to be dramatically reduced when compared to smooth channels and can be optimized through the design of the microridges. [Preview Abstract] |
Tuesday, November 21, 2006 12:41PM - 12:54PM |
OD.00003: Multi-frequency Electrokinetic Micromixing Frederic Bottausci, Igor Mezic We report an electrokinetic process to rapidly mix micro and nanoliter volume solutions for microfluidics applications. The method consists in initiating a flow instability that will rapidly stir the microflow streams. The effect occurs by applying multi-frequency alternative current signals in a periodic array of planar microelectrodes. The device was manufactured using bulk titanium microfabrication. It consists of an array of 24 electrodes sitting on the bottom of 200 microns wide, 30 microns deep and 6 millimeters long titanium channel. The electrodes are 20 to 40 microns wide with a pitch of 40 to 80 microns depending on the configuration studied. The device is very versatile and can be used for micro-nano particles concentration, cells sorting and micromixing depending on the input signal applied. In the present study we present some measurements of the mixing behavior. We discuss the advantage of the multi-frequency electrokinetic micromixing and show some quantitative results. [Preview Abstract] |
Tuesday, November 21, 2006 12:54PM - 1:07PM |
OD.00004: Dynamical study of a simple microfluidic device for mixing purposes Arnaud Goullet, Nadine Aubry Many microfluidic applications require the mixing of reagents, but efficient mixing in these laminar systems is often challenging. In this presentation, we consider further the method of pulsed flow mixing which takes advantage of time dependency rather than spatial complexity. Previous works show that good mixing can be achieved by considering three channels forming a T-shape geomety, with sinusoidal fluxes at the two inlets. Flow visualizations from experiments, and numerical simulations, have indicated that the majority of the mixing takes place in the confluence region. We carefully study the dynamic of tracer particles at the confluence region by using computational fluid dynamics and dynamical systems theory. We explore the parameter space in terms of the Reynolds number, Strouhal number and phase difference between the two inlet flows and show that regular and chaotic dynamics can occur at the confluence region. Poincar\'e sections and concentration plot will be presented to illustrate the mixing. The chaotic regime exhibits stretching and folding of material lines at all (large and small) scales, and is thus promising as an effective mixing tool. [Preview Abstract] |
Tuesday, November 21, 2006 1:07PM - 1:20PM |
OD.00005: Laminar mixing via steady streaming Using an active ionic polymer actuators C. Clawson, A. Williams, M. Paul, P. Vlachos Low Reynolds number and microscale mixing has been garnering significant attention in recent years due to the demands of developing microfludics. In this work we develop a novel active micro-mixer in a two-dimensional closed chamber with one active wall. This ``active skin'' is a flexible surface composed of an ionic active polymer transducer that deflects on the order of microns under an applied voltage potential. The effectiveness of the ionic polymer over a range of frequencies and actuation amplitudes was explored under an initially quiescent flow. Time Resolved Digital Particle Image Velocimetry (TRDPIV) was employed to resolve the resulting flow structures. The principle of operation of this mixing device is based on capitalizing on steady streaming effects. Bodies oscillating with the appropriate frequency and amplitude produce steady streaming. Steady streaming occurs when nonlinearities within the viscous boundary layer rectify the oscillatory flow and cause a steady, time-averaged flow outside the boundary layer. Orders of magnitude analysis shows a significant improvement over diffusion alone. This research indicates that laminar mixing can be greatly enhanced by steady streaming, thus exploiting a new avenue for the development of future micro-mixing devices. [Preview Abstract] |
Tuesday, November 21, 2006 1:20PM - 1:33PM |
OD.00006: The effect of unsteadiness on mixing and transport in microscale circulating cavity flows Derek Rinderknecht, Mory Gharib Micro mixers are an important category of functional units on microfluidic devices and have demonstrated their viability in applications such as protein crystallization, and DNA hybridization. Steady flows within micro cavities have been shown to produce controllable circulations capable of manipulating cells and biological molecules. Here we present experimental studies examining the effect of pulsatile free stream flow on mixing within microscale rectangular cavities. Pulsatile flow is provided upstream of the cavity by an impedance pump, a device which utilizes the pressure wave interactions to generate a pulsatile flow output. Micro cavities were analyzed under steady and pulsatile flow conditions for mean free stream Re spanning 0.01 to 100. Laser induced fluorescence was performed to activate caged fluorescein dye to visualize fluid interactions between the free stream and cavity region. Flow patterns emerging during a period of flow pulsation were observed using phase averaged PIV. Mixing was quantified by integrating the velocity fields to calculate an average residence time of a random distribution of fluid particles seeded in the cavity. [Preview Abstract] |
Tuesday, November 21, 2006 1:33PM - 1:46PM |
OD.00007: Optimal Control of Mixing in Stokes Fluid Flows George Mathew, Igor Mezic, Symeon Grivopoulos, Umesh Vaidya, Linda Petzold Motivated by the problem of microfluidic mixing, the problem of optimal control of advective mixing in Stokes fluid flows is considered. The velocity field is assumed to be induced by a finite set of spatially distributed force fields that can be modulated arbitrarily with time and a passive material is advected by the flow. To quantify the degree of mixedness of a density field, we use a Sobolev space norm of negative index. We pose a finite-time optimal control problem where we aim to achieve the best mixing for a fixed value of the action (time integral of the kinetic energy of the fluid body) per unit mass. We derive the first order necessary conditions for optimality that can be expressed as a two point boundary value problem and we discuss some elementary properties that the optimal controls need to satisfy. A conjugate gradient descent method is used to solve the optimal control problem and we present numerical results for two problems involving arrays of vortices. A comparison of the mixing performance shows that optimal aperiodic inputs can do better than periodic inputs. [Preview Abstract] |
Tuesday, November 21, 2006 1:46PM - 1:59PM |
OD.00008: Electrokinetic Microstrirring to Enhance Immunoassays Hope Feldman, Marin Sigurdson, Carl Meinhart Electrokinetic microstirring is used to improve the sensitivity of microfluidic heterogeneous immuno-sensors by enhancing the transport in diffusion-limited reactions. The AC electrokinetic force, Electrothermal Flow, is exploited to create a circular stirring fluid motion, thereby providing more binding opportunities between suspended and wall-immobilized molecules. This process can significantly reduce test times, important for both field-portable biosensors and for lab-based assays. A 2-D numerical simulation model is used to predict the effect of electrothermal flow on a heterogeneous immunoassay resulting from an AC potential applied to two parallel electrodes. The binding is increased by a factor of 7 for an applied voltage of 10 Vrms. The effect was investigated experimentally using a high affinity biotin-streptavidin reaction. Microstirred reaction rates were compared with passive reactions. The measurements show on average an order of magnitude increase in binding between immobilized biotin and fluorescently-labeled streptavidin after 5 minutes. Therefore, this technique shows significant promise for reducing incubation time and enhancing the sensitivity of immunoassays. [Preview Abstract] |
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