Micro-Heat Exchangers and AC Micropumps

An AC Faradaic micropump has been developed that uses high-frequency (>100 KHz) and strong (>10 V) AC fields to induce AC Faradaic reaction on fabricated electrodes. This reaction polarizes the electrode surface and produces a net AC electro-osmotic flow. The Faradaic reaction is reversible and the net production of ion and gas products is zero. As seen in the insert of the figure to right, gas generation does not occur for voltages as high as 20 V. In contrast, DC electric fields would typically generate gas bubbles at 1 V. The 50-micron T-shaped electrodes shown on the left can propel the electrolyte solution upto a speed of 1 mm/s, as seen on the right. Open symbols correspond to Pt electrodes, a very good electrode catalyst, with growing Faradaic polarization and velocity at low frequencies due to the loner reaction time. Closed symbols correspond to relative inert gold electrodes whose Faradaic reaction and polarization equilibrate at low frequencies. It is an electrokinetic pump without moving parts and , unlike DC pumps, do not generate bubbles or produce ions. A prototype micropump is can pump strong electrolytes at flow rates beyond one cc per hr.                                   

A separate micro-bubble jet AC micro-pump has been developed. It uses Faradaic reaction at a tip electrode to produce a train of fast moving micro-bubble train that drives a liquid flow with a linear speed of 10 cm/s. The gas bubbles dissolve into the solution far from the tip and do not block the flow channel. Such high-speed micropump can be used for CPU cooling and liquid fuel cells.

The cooling of micro-devices is especially challenging due to the small coolant thermal capacitance and flow rates in the micro-channels. Turbulent heat transfer is unattainable nor is large heat-transfer areas. The only recourse is evaporative heat transfer which uses the large heat of vaporization to compensate for the small flow. However, two-phase evaporative heat transfer generates bubbles which are often difficult to drive in micro-channels. We are investigating a new evaporative driven flow that can transport bubbles in a closed two-phase system between two thermal reservoirs. The pressure gradient within the closed loop is kept to a minimum by using non-circular conduits. Bubble release is preferentially directed in one particular direction of the loop by using a frit that controls the evaporation location. The bubbles then drive a continuous flow of evaporating and condensing bubbles between the two thermal baths without significant capillary or other pressure gradients. Due to the existence of the two phases within the loop, the temperature gradient is also vanishingly small between the two reservoirs, representing the best possible thermal communication between them. By thermally insulating these loops in the longitudinal direction, we are then able to achieve maximum transeverse thermal communication between the hot and cold streams but minimum longitudinal thermal communication along each stream.

Patent: Fast-igniting Catalytic Converters with Bypass (with David Leighton), United States Letters Patent No. 6,428,754, 2002.

For Electrokinetic Displacement of Bubbles, see CRC MEMS Handbook Review: [132] ref below.[132]

Relevant Publications: [132],136].[143],[146],[147],[148],[154].[156]

Others: refer to " Reaction and Thermal Engineering" section.