Hydrodynamic Dispersion in Microfluidic Geometries: Causes and Remedies (Plenary)

Over the past decade microfabricated chip devices have emerged as powerful tools for carrying out analytical scale separations. While these systems significantly simplify analysis procedures, their separation efficiency is largely limited due to dispersion arising from various sources. Many of these arise from the hydrodynamics of the flow through the channels themselves, including electrokinetic dispersion of solute slugs in curved geometries, dispersion due to pressure driven shear flows, the surprisingly large effect of side-walls in large aspect ratio channels, and the effect of wall absorption in open channel chromatography. All of these phenomena are essentially linked, in that they may be understood as examples of Taylor-Aris dispersion. In this paper we demonstrate how a detailed understanding of the causes of such dispersion can lead to possible remedies. In general, Taylor dispersion may be minimized by choosing microchannel designs which minimize the shear of solute slugs, or which accelerate the limiting transverse diffusion process. While optimal designs are specific to each separation process and geometry considered (and are often limited by ease of fabrication constraints), we offer several suggestions which are shown to reduce effective dispersivities by an order of magnitude or more in many instances. Such reductions directly translate into shorter required channel lengths and/or processing times.