Enhancement of mixing and adsorption in microfluidic devices by shear-induced diffusion and topography-induced secondary flow

Continuum simulations are used to assess the effects of shear-induced diffusion and secondary flow kinematics on the enhancement of mixing and adsorption during flow of suspensions in microfluidic channels. Unidirectional flow in rectangular channels is considered, as well as flow in channels with a topographically patterned wall that generates transverse flow. Patterns that lead both to chaotic and nonchaotic kinematics are considered. Effects of shear-induced diffusion due to the presence of suspended particles are incorporated via an empirical shear-rate dependent diffusivity. It is observed that for the bulk mixing case the most significant enhancement is due to convection. Channels with chaotic flow have the best mixing characteristics, followed by channels with swirling, nonchaotic flow. Only a small increase in mixing due to shear-induced diffusion is observed. For the case of adsorption from the bulk to a channel wall, on the other hand, it is observed that the most significant enhancement is due to shear-induced diffusion. Channels with secondary flows, both chaotic and nonchaotic, circulate solute-depleted fluid away from the adsorbing boundary but this is not sufficient to guarantee high fluxes toward the surface when the diffusivities are small. The most effective way to enhance adsorption is through the combination of both secondary flow and shear-induced diffusion. Secondary flow circulates fluid between bulk and boundary layer, while shear-induced diffusion enhances transport across the boundary layer. Nevertheless, under the large Peclet number conditions considered here, only a maximum of 30% of the solute is adsorbed to the surface for channels with length of 300 channel heights; for smooth channels without shear-induced diffusion this fraction is only 3%. (C) 2008 American Institute of Physics.