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How boundaries shape chemical delivery in microfluidics

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Science  09 Dec 2016:
Vol. 354, Issue 6317, pp. 1252-1256
DOI: 10.1126/science.aag0532

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Aspects of the design

In microfluidics systems, the small size of the channels ensures that the flow profiles are laminar, so solute mixing is largely governed by diffusion in the absence of active mixing. Aminian et al. revisited the classic phenomenon of Taylor diffusion to investigate the effects that the aspect ratio of the conduit has on the longtime axial distribution of solutes. They show both numerically and experimentally that the aspect ratio controls the skewness of this distribution, and thus pipe design alone is enough to control mixing profiles.

Science, this issue p. 1252

Abstract

Many microfluidic systems—including chemical reaction, sample analysis, separation, chemotaxis, and drug development and injection—require control and precision of solute transport. Although concentration levels are easily specified at injection, pressure-driven transport through channels is known to spread the initial distribution, resulting in reduced concentrations downstream. Here we document an unexpected phenomenon: The channel’s cross-sectional aspect ratio alone can control the shape of the concentration profile along the channel length. Thin channels (aspect ratio << 1) deliver solutes arriving with sharp fronts and tapering tails, whereas thick channels (aspect ratio ~ 1) produce the opposite effect. This occurs for rectangular and elliptical pipes, independent of initial distributions. Thus, it is possible to deliver solute with prescribed distributions, ranging from gradual buildup to sudden delivery, based only on the channel dimensions.

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