Pressure generation at the junction of two microchannels with different depths.

In this study, we report the design of a microchip-based hydraulic pump that comprises three glass conduits arranged in a T-geometry, one of which has a 2 mm long segment shallower (0.5-3 microm in depth) than the remaining 15 microm deep microfluidic network. Upon application of an electric field across this microchannel junction, a mismatch in EOF rate is introduced due to a differential in the fluid conductivity across the deep and shallow segments. Using the reported micropump, pressure-driven velocities up to 3.2 mm/s have been generated in a 15 microm deep separation channel for an applied voltage of 1.75 kV allowing us to operate under separation conditions that yield the minimum plate height. Moreover, we have shown that this flow velocity can be maximized by optimizing the depth in the shallow region of the T-geometry. Interestingly however, a simple theory accounting for fluid conductivity differences across microchannels of different depths significantly underestimates the pressure-driven velocities observed in our experiments. The Taylor dispersion coefficient in our system on the other hand compares well with the theoretical predictions reported in the literature. Finally, the functionality of our device has been demonstrated by implementing a reverse-phase chromatographic separation that was driven by the pressure-driven flow generated on-chip.