No abstract

To respond to the dire need of smaller but more effective ways to handle non-uniform temperature distribution (a.k.a. hotspots) within an electronic device, this paper presents the concept of droplet-based cooling and its proof-of-concept demonstration. An electrowetting-on-dielectric (EWOD) digital microfluidic device with parallel plates configuration was used to control coolant droplet motion. Periodic rise and falls in hotspot temperature were measured when multiple water droplets moved over the hotspot surface while a constant heat flux was applied at the hotspot. Synchronized high-speed video data of droplet motion shows that phase-change heat transfer (i.e. evaporation) at the water droplet meniscus coincided with additional temperature drop at the hotspot. Furthermore, the heat transfer coefficient of water droplet evaporation was measured on a hydrophilic hotspot surface. The hydrophilic surface regulated the hotspot temperature within a lower envelope for a longer time.

To meet the increasing demand of efficient cooling performance in small scale, this paper presents a digital microfluidics (DMF) microscale liquid cooling system which works on the principle of electrowetting on dielectric (EWOD). In EWOD DMF, fluids are handled drop-wise by external electric field. When the dispensed liquid droplet arrives at the hotspot by EWOD DMF operation, it picks up heat and removes heat when it leaves. This process can be repeated for a series of droplets by using a completely automated LabVIEW controlled system connected to the PCB package. With the help of indium tin oxide (ITO) thin film resistance temperature detectors (RTD) and pre-calibrated temperature coefficient of resistance (TCR) data, the temperatures of the hotspot before and after the residence of liquid droplet (i.e., cooling) can be recorded for different frequencies (dwelling time period of droplet on the hotspot) of the drop motion and varying heater power. Future work will involve RTD resistance data collection to plot the heat flux and the temperature difference (before and after cooling) for different frequencies of drop motion. Although the primary focus is to study single phase cooling, the DI water drop will experience considerable evaporation resulting in higher cooling performance. The single phase cooling studies will help in establishing a robust platform for future two-phase cooling analysis in which evaporation effects will be considered.