(72g) Pressure-Generation at the Intersection of Two Microchannels with Different Depths Via Electrokinetic Means

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 glass channel segments in a tee geometry; one of which is shallower (about 1-2mm deep) than the other two (about 5-15mm deep). By applying appropriate electric fields across this design, a mismatch in electroosmotic transport rate is introduced at the junction of the shallower and the deeper microchannels which occurs due to a differential in the effective fluid conductivity in these two segments. For example, it has been observed that the effective fluid conductivity in glass microchannels increases with a reduction in the channel depth due to an increasing contribution from surface conductance [1]. The pressure-driven flow generated in the system as a result of this mismatch is then guided to a separation section in this device using the third segment in the tee geometry. With our current design, pressure-driven flow velocities up to 5mm/s have been generated in the separation column for an applied voltage of 2kV. In addition, experiments have been also performed to understand the effect of the buffer ion concentration and the relative depth of the shallower channel segment in this design on the pressure-generation capability of our device. Finally, the functionality of our device has been demonstrated by performing an open channel liquid chromatography of fluorescent dyes under pressure-driven flow conditions in a 5mm separation channel.

References:

[1] Stein, D., Kruithof M, and Dekker C, ?Surface charged governed ion transport in nanofluidic channels?, Phys. Rev. Lett. 2004, 93, Article No: 035901