Hydrodynamic channeling as a controlled flow reversal mechanism for bidirectional AC electroosmotic pumping using glassy carbon microelectrode arrays

Controlled bidirectional flow by AC electroosmotic means is achieved using asymmetric coplanar and high-aspect-ratio glassy carbon electrodes and without the involvement of moving elements. The forward and backward fluidic propulsion is the result of hydrodynamic channeling of the fluid in a microfluidic device. The asymmetric coplanar electrodes were fabricated by photolithographic patterning of SU-8 photoresist, followed by pyrolysis at 900 °C. Morphological characterization of the carbon structures was carried out by SEM and confocal microscopy. Then, Raman and EDX spectroscopies confirmed that the resulting carbon material is appropriate for electrokinetic applications. A finite element analysis was carried out to study the flow development by AC electroosmosis. Electrode arrays of three different asymmetry ratios (60 µm:20 µm, 80 µm:20 µm, and 100 µm:20 µm) were fabricated and tested. Fluid velocity was measured for an applied bias in the 2–20 V _PP amplitude range, and in the 1 kHz to 200 MHz frequency range. Overall maximum measured forward and reverse fluid velocities were 28.59 µm s⁻¹ and 338 µm s⁻¹, respectively. On an additional set of devices with the same asymmetry ratios, a second photolithography step was utilized to produce high-aspect-ratio microposts on top of the coplanar electrodes to study the effect of high electrode contact surface to generate bidirectional flow. Using the same amplitude and frequency ranges as for planar structures in experimental testing, the overall maximum measured velocities were 9.23 µm s⁻¹ and 90.66 µm s⁻¹ for the forward and reverse regimes, respectively. In contrast to the planar electrodes, microposts-containing electrodes had more balanced velocity magnitudes between reverse and forward flows as the asymmetry ratio increases. In this case, the use of this electrode topology can be useful when symmetry of the forward and backward flow is more important than the magnitude of the volumetric flow rate. (paper)