Dispersion reduction in pressure driven flow through microetched channels

Fluid is often moved about microetched channels in lab-on-a-chip applications using electrokinetic flows (electrophoresis or electroosmosis) rather than pressure-driven flows because the latter result in large Taylor dispersion. However, small pressure gradients may arise unintentionally in such systems due to a mismatch in electroosmotic flow rates or hydrostatic pressure differentials along the microetched channel. Under laminar flow conditions, Doshi et al, (Chem. Eng, Sci, 1978, 33, 795-804) have shown that for a channel with rectangular cross-section of width W and depth d, longitudinal diffusivities can attain values as large as similar to8 K-0 for small values of the aspect ratio d/W, where K-0 is the value of the longitudinal diffusivity obtained by ignoring all variations across the channel, Microchannels in lab-on-a-chip geometries are often not rectangular in cross-section. Isotropic etching techniques, for example, lead to channels with quarter-circular ends. In this paper we examine the effect of this geometry on the magnitude of longitudinal dispersivity for pressure-driven flows and also investigate modifications to this design which may minimize such dispersion. Optimal channel profiles are shown to lead to dispersivities approaching K-0, the theoretical minimum, for small values of d/W.