Field-Effect Control of Electroosmotic Pumping Using Porous Silicon–Silicon Nitride Membranes

Field-effect control of electroosmotic (EO) flow, which has been demonstrated on single microchannels, provides a novel method to modulate the zeta potential and, therefore, the EO flow through a porous membrane. To realize field-effect control of EO pumping with a membrane, a SiN_x-coated porous silicon membrane was designed and fabricated. The heavily doped silicon core was used as a conducting electrode to apply the transverse gate potential, which modulates the zeta potential of the channel walls and, thereby, the EO flow rate with constant externally applied electric fields along the channels. We observed significant electrolytic-rectification effect, i.e., for gate voltage (V_g) < 0, a substantial current leakage through the SiN_x was observed, whereas negligible leakage current was detected for V_g > 0. Significant EO flow control, nearly 70% reduction in flow velocity, was observed for positive gate bias, while only 15 % flow-velocity enhancement was observed for negative gate bias of similar magnitude. This first demonstration of field-effect control on porous membranes opens the door for making high-flow rate EO pumps with porous membranes of low zeta potential materials.