Study Of The Capabilities Of Electrowetting On Dielectric Digital Microfluidics (EWOD DMF) Towards The High Efficient Thin-film Evaporative Cooling Platform

Abstract

With the current technological advancement, the size of the electronic components is being reduced to smaller and compact sizes. In the meantime, the packaging density and power consumption is ever increasing. To tackle this challenge, an efficient cooling technology is required.Thin-film evaporation is a very efficient cooling technology. A practical application using thin-film evaporation is a spray cooling which is known as the cooling method that can handle very high heat flux. However, a random spray of coolant cannot guarantee a thin coolant film, therefore either local dry-out or flooding may occur, which hampers cooling efficiency significantly. Moreover, bulky spraying system is not suitable for cooling of small electronic devices. As solving the drawbacks of spraying cooling, it has been suggested making thin liquid films by delivering nanoscale liquid drops to the superhydrophilic nanoporous coating (SHNC). As soon as a nanoscale liquid drop arrives to the SHNC on a hotspot, it spontaneously spreads, forms very thin liquid film, and quickly evaporates. Electrowetting on dielectric (EWOD) digital microfluidics (DMF) is properly suited for this purpose, since it handles liquids in the form of droplets by controlling only electric fields without any bulky mechanical pumps or valves.This dissertation reports an experimental study of three essential requirements of the EWOD DMF towards the thin-film evaporative cooling platform: (1) the high accuracy and consistency in volume of coolant nanodrops dispensed from the reservoir, (2) the fast motion of coolant nanodrops to the hotspot to avoid dry-out, and (3) the simultaneous achievement of both small volume and high frequency of nanodrop that arrives to the hotspot. In this investigation, glass-based EWOD DMF and silicon-based EWOD DMF were developed, fabricated and tested. Deionized (DI) water was used as coolant due to its high heat of vaporization.To increase the volume accuracy of nanodrop, various electrode geometries of the reservoir were designed to control drop pinch-off point. A simple force balance was taken into account for the design. The minimum average volume error of 0.083 % for fifty drops of repeatable drop generation was achieved. The experimental results agreed with the numerically simulated results.To increase the speed of drop motion, three major parameters that affect the speed of drop motion were investigated: The effects of electrode size, electrode geometry and surface roughness were tested. Ten times faster speed (400 mm/s) of drop motion was achieved by modifying the electrode geometry. To achieve simultaneously high frequency and small volume of nanodrops that arrive to the hotspot, a new electrode geometry was designed to split a droplet into two while it moves toward the hotspot. Using this method, the droplet arrival frequency to the heated section was increased 4 times while the droplet volume that arrives to the heated section is 4 times smaller than the volume of droplet generated from the reservoir. By combining all of the above results, fully completed and automated EWOD DMF was designed, fabricated and characterized to deliver liquid in small volume (down to 50 nL) with high accuracy (< 5 %) and high frequency of arrival to heated zone (over 200 Hz).