Turn-induced isotachophoretic focusing in microfluidic channels.

Multifunctional microfluidic platforms that use ITP for sample stacking require the use of serpentine channels. When ITP is carried out in serpentine passages, the sample band shows dramatically different behavior than that observed in conventional on-chip CE. The band skew, defined as the distance between the leading and trailing edge of the band, is reduced and the sample is focused on the outside wall of the turn. This effect is explained using a simplified analytical model for the infinite Peclet number limit and the predicted trends are compared to full numerical simulations and experimental measurements. The outward focusing is driven by a wall-normal electric field that results from the skewed leading buffer-trailing buffer (LB-TB) interface. The results indicate that the degree of outward focusing is controlled by the ratio of the LB and TB conductivities, the ratio of the channel width to turn radius, and the Peclet number. Increasing the band skew, which shows a complex dependence on the nondimensional parameters, maximizes the outward focusing effect.