We report an electrowetting-controlled cooling system with site-specific treatments on the heat source (evaporator or hot spot) surfaces. Electrowetting-on-dielectric (EWOD) has great potential in thermal management because EWOD-driven droplet transport has unique characteristics of prompt response, low power consumption and programmable paths without the need for any mechanical moving parts. Prompt and fast droplet transport is necessary for adaptive and active cooling of high heat flux targets. Using a multi-channel DC/AC control system, we carried out sequenced activation of AC voltages on coplanar electrodes and transmitted a droplet to the spot target along a programmable path. With high positioning accuracy at the chip level, we have successfully transmitted a water droplet of 15 μL at speeds as high as ∼10 cm/s. We further improved electrowetting cooling performance by coating a fine copper screen on the cooling targets. The capillarity associated with the copper screen facilitates the delivered droplets automatically spreading and clinging to the target surfaces. As a result, heat transfer is in the more efficient form of filmwise evaporation at the evaporator sites. To maintain a thin film with proper thickness on the hot spots, we implemented EWOD-assisted droplet splitting and merging to precisely control the droplet volume to avoid fluid flooding (accumulation) on the hot spot surfaces. Our investigation indicates that thin-film evaporation is a high-efficiency heat transfer mechanism on a hydrophilized hot spot surface. Based on EWOD technique with surface treatments, the superheat on a hot spot of 4mm × 4mm was maintained well below 30°C even when the heat flux reached as high as 80W/cm2. The closed loop of this novel thermal management system can potentially function as a wickless vapor chamber or heat pipe with enhanced heat dissipation capabilities.
- Nanotechnology Institute
Spot Cooling Using Electrowetting-Controlled Thin Film Heat Transfer
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Cheng, J, & Chen, C. "Spot Cooling Using Electrowetting-Controlled Thin Film Heat Transfer." Proceedings of the ASME 2012 Third International Conference on Micro/Nanoscale Heat and Mass Transfer. ASME 2012 Third International Conference on Micro/Nanoscale Heat and Mass Transfer. Atlanta, Georgia, USA. March 3–6, 2012. pp. 275-281. ASME. https://doi.org/10.1115/MNHMT2012-75032
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