Here, we report on a micropump that generates hydraulic pressure owing to a mismatch in EOF rates of microchannels and submicrometer cylindrical glass capillaries integrated on silicon. The electrical conductance of such capillaries in the dilute limit departs from bulk linear behavior as well as from the surface-charge-governed saturation in nanoslits that is well described by the assumption of a constant surface charge density. The capillaries show rather a gradual decrease in conduction at low salt concentrations, which can be explained more aptly by a variable surface charge density that accounts for chemical equilibrium of the surface. The micropump uses a traditional cross-junction structure with ten identical capillaries integrated in parallel on a side arm and each with a 750 nm diameter and 3 mm length. For an applied voltage of 700 V, a hydraulic pressure up to 5 kPa is generated with a corresponding flow velocity nearly 3 mm/s in a straight field-free branch 20 μm wide, 10 μm deep, and 10 mm long. The micropump utility has been demonstrated in an open tubular LC of three fluorescently labeled amino acids in just less than 20 s with minimal plate height values between 3 and 7 μm. The submicrometer capillaries are self-enclosed and produced through a unique process that does not require high-resolution advanced lithography or wafer-bonding techniques to define their highly controlled precise structures.
Cao, Zhen, Levent Yobas