Design and analysis of folded channels for chip-based separations

Analytical methods are employed to investigate band broadening in a microchannel turn and adjoining straight channel segments for species transport by electrophoresis or electroosmotic flow. On the basis of closed-form solutions, we find that turn-induced broadening is negligible relative to total broadening when the radius of the turn exceeds some minimum. This minimum radius is about six-tenths of the product of the channel width and the Peclet number. We also find that the minimum radius is significantly reduced when a straight channel segment adjoins the turn in a folded configuration. Such straight segments noticeably reduce the minimum radius even for segment lengths comparable to the turn radius. The application of these results to folded and spiral channels is discussed, and sample calculations for practical conditions are presented. New pleated and coiled geometries for the compact layout of separation channels are also presented.