An integrated micropump and electrospray emitter system based on porous silica monoliths

The work presents the design of an integrated system consisting of a high-pressure electroosmotic (EO) micropump and a microporous monolithic emitter, which together generate a stable and robust electrospray. Both the micropump and electrospray emitter are fabricated using a sol-gel process. Upon application of an electric potential of sufficient amplitude (>2 kV), the pump delivers fluids with an electroosmotically induced high pressure (>1 atm). The same potential is also harnessed to electrostatically generate a stable electrospray at the porous emitter. Electrokinetic coupling between pump and spray produces spray features different from sprays pressurized by independent mechanical pumps. Four typical spray modes, each with different drop sizes and charge-to-mass ratios, are observed and have been characterized. Since the monolith is silica-based, this integrated device can be used for a variety of fluids, especially organic solvents, without the swelling and shrinking problems that are commonly encountered for polymer monoliths. The maximum pressure generated by a 100 microm id monolithic pump is 3 atm at an applied voltage of 5 kV. The flow rate can be adjusted in the range of 100 nL/min to 1 microL/min by changing the voltage. For a given applied voltage across the pump and emitter system, it is seen that there exists one unique flow rate for which flow balance is achieved between the delivery of liquid to the emitter by the pump and the liquid ejection from the emitter. Under such a condition, a stable Taylor cone is obtained. The principles that lead to these results are also discussed.