Isotachophoresis (ITP) is a powerful electrophoretic technique for sample concentration and separation; when used on microfluidic chips it may be necessary to use serpentine passages or expansion sections to meet design requirements. We have observed novel two-dimensional focusing effects in these geometries. Using a simple analytical model for the behavior in a turn, we illustrate that the focusing effect occurs because the spatial distortion of the leading buffer-trailing buffer interface generates a wall-normal electric field. Using this analytical model we identify the controlling non-dimensional parameters. We present the results of a computational parametric study of this focusing behavior in turns as well as a comparison to experimental visualizations. Finally, we present preliminary computational results for ITP in expansions.
