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 KC: Microfluidics VIII: Mixing-2 |
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Chair: Hugh Fan, University of Florida Room: Tampa Marriott Waterside Hotel and Marina Grand Salon AB |
Monday, November 20, 2006 5:15PM - 5:28PM |
KC.00001: Optimization of chaotic mixing in staggered herringbone micromixer Mrityunjay Kumar Singh, Patrick Anderson, Han Meijer Mixing is vital in many microfluidic devices. In this paper, a mapping method has been applied to analyze and optimize periodic mixing protocols in different types of staggered herringbone micromixers. The mixer can be subdivided in a number of characteristic modules and for each a distribution matrix is computed by mapping concentration at the inlet to the outlet. By combining these different mapping matrices different designs of the mixer are easily evaluated, and a design has been predicted to achieve optimum mixing in the micromixer. The advantage of the mapping method is that it needs only one time computation of these mapping matrices which can be combined in various ways, which is computationally very inexpensive. To quantify mixing, the intenity of segregation has been used as a mixing measure. Further as a comparison, we calculated the maximum Lyapunov exponent for three types of mixers with three different groove depths. Our results show that the mapping method and the maximum Lyapunov exponent confirm each other regarding the effect of groove depth on mixing. [Preview Abstract] |
Monday, November 20, 2006 5:28PM - 5:41PM |
KC.00002: Investigation of Fluid Mixing in Channels with Microfabricated Ridges Zheng Xia, Louis Cattafesta, Renwei Mei, Mark Sheplak, Z. Hugh Fan Microflows in complex channels often present unusual flow behavior compared to their macroscale counterparts. Channels with microfabricated ridges are created in a plastic device using photolithography and molding. To study fluid mixing, a novel visualization technique is developed. This approach couples a conventional optical microscope with a computational deconvolution algorithm to produce the images of three dimensional flows, which are obtained by (1) optically sectioning the flow in the microchannel by collecting a series of fluorescence images at different focal planes along the optical axis, and (2) removing the out-of-focus fluorescence signal by the deconvolution method to reconstruct a corrected three-dimensional concentration image. With the counter interaction of applied pressure and electric potential fields, recirculation in the flow in these ridged channels is observed. The circulation is demonstrated by mixing a fluorescent stream and a water stream in the microchannels, as well as using microparticles. Circulation results in enhanced fluid mixing; the fluorescence intensity variation at the cross section is calculated as an index of mixing. The preliminary results show the rapid mixing in the ridged channels, verifying the existence of circulation. We also compared the experimental results with that from computational fluid dynamics simulation. [Preview Abstract] |
Monday, November 20, 2006 5:41PM - 5:54PM |
KC.00003: A Surface-Tension-Driven Nanoliter Mixer With Optimized Grooved Structures for Microchannels Chien-Cheng Chang, Chuh-Yu Kuo, Chein-Fu Chen, Chun-Fei Kung, Chin-Chou Chu, Chi-Feng Chiu The surface tension-capillary pumping is an effective driving force in a microchannel. A power-free method is explored to perform mixing in a microchannel without any external active mechanisms. The mixer is designed to have no sidewalls with the liquid being confined to flow between a bottom hydrophilic stripe and a top-covered hydrophobic substrate. It is found from both theoretical analysis and experiments that for a given channel width, the flow rate solely due to capillary pumping can be maximized at an optimal channel height. Asymmetric staggered grooved cavities are optimally arranged on the bottom substrate of the channel to enhance mixing. Furthermore, blood flows in microchannel driven by the same mechanism have been also investigated in the present study. [Preview Abstract] |
Monday, November 20, 2006 5:54PM - 6:07PM |
KC.00004: Mass and charge transport in arbitrarily shaped microchannels Henrik Bruus, Niels Asger Mortensen, Fridolin Okkels, Laurits Hoejgaard Olesen We consider laminar flow of incompressible electrolytes in long, straight channels driven by pressure and electro-osmosis. We use a Hilbert space eigenfunction expansion to address the problem of arbitrarily shaped cross sections and obtain general results in linear-response theory for the mass and charge transport coefficients which satisfy Onsager relations~[1,2]. In the limit of non-overlapping Debye layers the transport coefficients are simply expressed in terms of parameters of the electrolyte as well as half the hydraulic diameter ${\cal R}=2 {\cal A}/{\cal P}$ with $\cal A$ and $\cal P$ being the cross- sectional area and perimeter, respectively. In particular, we consider the limits of thin non-overlapping as well as strongly overlapping Debye layers, respectively, and calculate the corrections to the hydraulic resistance due to electro- hydrodynamic interactions.\newline \newline [1] N. A. Mortensen, F. Okkels, and H. Bruus, Phys. Rev. E \textbf{71}, 057301 (2005) \newline [2] N. A. Mortensen, L. H. Olesen, and H. Bruus, New J. Phys. \textbf{8}, 37 (2006) [Preview Abstract] |
Monday, November 20, 2006 6:07PM - 6:20PM |
KC.00005: Effect of surface roughness on a shear dependent slip flow of simple fluids Nikolai Priezjev The behavior of the slip length in a flow of simple fluids subject to a constant force is investigated using molecular dynamics simulations. A nonlinear shear rate dependence of the slip length is observed for the weak surface attraction and incommensurable structures of the liquid/solid interface. A variation of the wall-fluid interaction potential produces a gradual transition in the functional dependence of the slip length on the shear rate. Thermal roughness of the wall atoms is found to modify the slip behavior and, in the case of large spring stiffness, the slip length is increased by a constant value in a range of accessible shear rates. Periodically and randomly corrugated surfaces strongly suppress shear rate dependence of the slip length even for a weak wall-fluid attraction. A relation to recent slip flow experiments will be discussed. [Preview Abstract] |
Monday, November 20, 2006 6:20PM - 6:33PM |
KC.00006: Application of electrokinetic instability to control fluid mixing inside micro-channels Zheyan Jin, Hui Hu An experimental investigation is conducted to utilize electrokinetic instability to control/enhance fluid mixing inside Y-shaped micro-channels. Microscopic particle image velocimetry and microscopic laser induced fluorescence techniques are used to conduct qualitative flow visualization and quantitative velocity and concentration field measurements to examine the enhanced mixing process inside the micro-channels. Difference designs of the Y-shaped micro-channels, which including adding microstructures such as steps or cavities inside the mixing channels, will be investigated and compared quantitatively. The objectives of the study is to understand the fundamental nature of electrokinetic instability, to study the effects of the relevant parameters such as the magnitude and frequency of the applied electric fields on the effectiveness of the mixing control/enhancement, and to explore/optimize design paradigms for the development of robust electrokinetic micromixers for various microfluidics or ``lab-on-a-chip'' applications. [Preview Abstract] |
Monday, November 20, 2006 6:33PM - 6:46PM |
KC.00007: New experimental results on microbubble streaming J.-C. Tsai, David Hansen, Sascha Hilgenfeldt Fast, ultrasound-driven oscillation of air bubbles situated at or close to a stationary wall induces a steady Stokes flow (microstreaming) of the surrounding fluid. The flow achieves high speeds and can exert strong hydrodynamic forces capable of rupturing cells [1]. In the semi-infinite space bounded by a wall it can be successfully described by analytical means [2]. Here, we report new results including the possibility of flow reversal by amplitude regulation, the excitation of rotating flows, and the effects of an additional confining wall on the microstreaming. Time-resolved imaging of passive tracers from different view angles reveals the 3D structure of the flow field. The results show that ultrasound-driven streaming is effective and versatile in a number of practically relevant geometries and suggests a unique combination of effective transport and effective mixing capabilities in microfluic applications. \newline [1] P. Marmottant and S. Hilgenfeldt, Nature 423, p153 (2003); \newline [2] D. Hansen, P. Marmottant, J.-C. Tsai, and S. Hilgenfeldt (submitted to Journal of Fluid Mechanics, 2006) [Preview Abstract] |
Monday, November 20, 2006 6:46PM - 6:59PM |
KC.00008: Dielectric coating dynamics in electrified coaxial jets Alvaro G. Marin, Ignacio G. Loscertales , Antonio Barrero Electrified coaxial jets generated from compound electrified menisci (compound electrosprays) have demonstrated their ability to produce particles with complex core-shell structure (I.G. Loscertales et al., Science, 295, 1695 (2002); JACS, 126, 5376 (2004)). Since at least one of the liquids forming the compound meniscus must be relatively conductive, two configurations are possible: the conducting liquid being in or out. In this work we shall consider the first configuration, whose applications range from simple protection of volatile liquids by non-volatile dielectric liquids to encapsulation by using dielectric polymer melts. Although the scaling law for the electric current transported by the electrified coaxial jets has been recently investigated (Lopez-Herrera et al.,JAS ,34, 535 (2003)), the dynamics of the coating layer is still not completely understood. The aim of this communication is to gain insight on the dependence of the coating thickness on the injected liquid flow rates, the viscosities of both liquids and surface tensions (liquid-liquid and liquid-air). [Preview Abstract] |
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