(382j) An Ac Electrokinetic Micropump Based on Field-Induced Secondary Maxwell and Back Pressure Gradients along a Continuous Wire Loop

Unlike DC electro-osmotic micro-pumps, high-frequency ACEO (AC electro-osmotic) pumps offer the advantage of not generating bubbles and can hence be embedded in micro-fluidic devices to affect better flow control. All previous ACEO micro-pump designs are based on time-averaged apparent slip velocities on disjoint electrode surfaces due to double layer polarization induced by the field sustained between the two electrodes. However, due to the opposite polarity on the two electrodes, a symmetric electrode pair can sustain a vortex pair with opposite circulations but no net flow. Previous designs of ACEO pumps are hence based on the principle of symmetry breaking, either in electrode width (Ajdari, A., Physical Review E 2000, 61(1): R45) or electrode geometry (Dmitri Lastochkin, R. Z., Ping Wang, Yuxing Ben, and Hsueh-Chia Chang, Journal of Applied Physics 2004, 3, 96), to have a dominant vortex and a net flow. Such vortex cancellation designs are extremely inefficient and often involve vortices with velocity components against the pumping direction. The flow is hence very short range and cannot extend beyond the electrode surface. Consequently, an array of disjoint electrodes must be fabricated along the entire channel to sustain the pumping action.

We have developed a new method for pumping fluid based on a secondary ACEO effect produced from a novel micro-wire geometry. The vortices are aligned with the flow direction and hence do not oppose the pumping action. The vortices in fact possess reflection symmetry in the transverse direction and are not designed for one to cancel the other to produce a net transverse flow. Instead, the circulation of the vortices, the corresponding back pressure and the Maxwell pressure vary longitudinally by design to drive a secondary flow in the longitudinal direction orthogonal to the primary vortex velocity field. The longitudinal variation is introduced by a hair-pin shaped wire loop whose field between the two side wires decreases towards the turn?the flow is towards the turn between the two side wires. Moreover, the continuous wire design with a connecting resistor allows us to introduce much higher voltage (>1 kV) than the disjointed electrodes. A scaling theory for the Maxwell pressure and ACEO slip-induced back pressure allows us to design an optimized loop geometry (width and angle between the two side wires) such that a large and highly non-uniform electric field can be sustained across the width of a micro-channel with a cross-sectional area of 40 µm2. This transverse field builds a high Maxwell pressure on the polarized electrodes whose longitudinal gradient can drive a flow. The transverse field also drives a tangential surface flow and a pair of longitudinal vortex on the wire surface. The back pressure that sustains the vortex flow also has a longitudinal gradient that can contribute to the flow. The width of the wire determines the relative importance of the Maxwell and back pressure gradients. The ratio of these two field-induced pressures is also highly frequency dependent because the ACEO slip velocity has a maximum when the frequency is at the inverse RC time of the wire/double-layer circuit while the Maxwell pressure is highest at higher frequencies. Without the deleterious effect of the counter-rotating vortex in other designs, where the back pressure gradient of the two vortices oppose rather than complement each other, the induced flow of the current design is much longer range than conventional AC electro-osmotic flow (approximately 5 times the length of the pumping device) and therefore fluid can be transported over a distance of 2 centimeters with only one small micropump at a velocity exceeding 1 mm/s and a flow rate of ~30 micro-liter/s.

The AC current is mainly carried by the wire and regulated by a large resistor outside the microchannel, thus allowing for a large rms voltage (in excess of 3500 V) to be applied across the wire without generating bubbles or other Faradaic reactions. Additionally, the device requires very little power (~40 mW) due to the fact that only a large voltage difference (electric field) between the inner ground wire and outer wires is required to induce fluid motion.