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Flow and species transport in micro-scales experience laminar, even Stokes flow conditions. In absence of turbulence, species mixing becomes diffusion dominated, and requires very long mixing length scales (lm). This creates significant challenges in the design of micro-total-analysis-systems (µ-TAS), where mixing of macromolecules and biological species with very low mass diffusivities (D) are often desired. For such cases, the Schmidt number (Sc=ν/D) becomes extremely large, and species mixing at fixed kinematic conditions (i.e., at fixed Reynolds number, Re) scales as a function of the Peclet number (Pe≡Sc×Re). For laminar convective/diffusive transport, mixing length typically varies as lm∝Pe0.5 [1]. It is possible to reduce the mixing-length drastically by inducing chaotic stirring, which results in lm ∝ ln(Pe) for fully chaotic [2], and lm ∝ Peα ((with 0<α<1)) for partially chaotic flows [3, 4]. Chaotic advection is observed for cases, in which the equation of motion for fluid particles can be represented as a non-which induces 2-D electroosmotic flow (1-b) under an axial electric field. Time-periodic flow can be generated by altering the axial electric field in the form of a cosine wave with frequency ω. A pressure-driven unidirectional flow (1-c) is superposed to obtain continuous flow. The mixing block has half channel height h, and length L=4h. Theoretical studies assume two-dimensional flow. We assume a unique zeta-potential patterning (1-a) with three different zeta potential values, where the red and blue surfaces are positively and negatively charged with ± ζo, and the white surface is electrically neutral. Zeta potential alteration in experimental studies has been demonstrated in the literature (see section 7.4.7 in [6]). For example, in 1 mM KCl solution at pH=7.0, PMMA exhibits ζ=-34 mV [7], while Aluminum-oxide and Chromium-oxide surfaces have ζ=37 mV [8], and ζ ≈0 mV [9], respectively, which satisfies the necessary zeta potential values in the conceptual mixer

Topics: Old Dominion University
Year: 2016
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