Experimental and theoretical scaling laws for transverse diffusive broadening in two-phase laminar flows in microchannels

This letter quantifies both experimentally and theoretically the diffusion of low-molecular-weight species across the interface between two aqueous so lutions in pressure-driven laminar flow in microchannels at high Peclet num bers. Confocal fluorescent microscopy was used to visualize a fluorescent p roduct formed by reaction between chemical species carried separately by th e two solutions. At steady state, the width of the reaction-diffusion zone at the interface adjacent to the wall of the channel and transverse to the direction of flow scales as the one-third power of both the axial distance down the channel (from the point where the two streams join) and the averag e velocity of the flow, instead of the more familiar one-half power scaling which was measured in the middle of the channel. A quantitative descriptio n of reaction-diffusion processes near the walls of the channel, such as de scribed in this letter, is required for the rational use of laminar flows f or performing spatially resolved surface chemistry and biology inside micro channels and for understanding three-dimensional features of mass transport in shearing flows near surfaces. (C) 2000 American Institute of Physics. [ S0003-6951(00)02616-4].