While fluid and analyte samples are often driven through microfluidic channels using electroosmotic flow, pressure-driven transport is preferred in several applications due to its ability to move a broader range of materials and insensitivity to channel interfacial properties. Key among them is the implementation of liquid chromatography where this mode of actuation significantly improves the reproducibility and the controllability of the separations by decoupling mobile phase transport from analyte interaction with the stationary phase. Generation of pressure-driven flow on microchip devices with a high degree of control however, offers a significant challenge. Although conventional syringe pumps may be connected to microchips to realize steady hydrodynamic flows, the dynamic control over the transport rates in these systems is poor due to the large dead volumes involved.

Here, we report the design of a microchip based hydraulic pump that can generate pressure-gradients within a microchannel network via electrokinetic means allowing very precise dynamic control over the flow rates. The pump consists of 3 channel segments in a tee geometry one of which has a depth of about 100 nanometers. By applying appropriate electric fields across this design, a mismatch in electroosmotic transport rate is introduced at the micro-nanochannel junction. This occurs due to suppression of electroosmotic flow within the nanometer sized duct as the Debye layers around its inner walls overlap. The pressure-driven flow thus generated is then preferentially guided to the third channel in the tee geometry by making it deeper which reduces its hydraulic flow resistance. The third channel also connects the hydraulic pumping unit to a separation section in this device that was used for performing reverse phase liquid chromatography. Further, a solvent programming capability was realized on the device by introducing an additional channel within the network to deliver buffer at high organic solvent strength to the separation column.