Drop Evaporation on Rough Hot-Spots: Effect of Wetting Modes

Abstract

Hot-spots are a common occurrence in power electronics which become increasingly hotter as chips become denser. Novel cooling technologies are emerging to cope with this increasing heat load, which imbed a condenser to supply cooling drops to the evaporator resting on the hot-spots. Nonetheless, the evaporation process of the drops has been overlooked. Here, we conducted a series of experiments to understand how the evaporation and motion of drops are influenced by the wetting mode of rough hot-spots. We fabricated three different surfaces exhibiting full (Wenzel) or partial (Cassie–Baxter) wetting and the hot-spot is imposed by laser irradiation. We report a direct link between drop motion and wetting mode with the partial wetting drops being highly mobile, attributable to lower pinning energy based on an energy analysis. This study provides a framework for future modifications in hot-spot cooling to account for drop motion which should greatly influence the overall heat removal performance.

Notes on contributors

Huacheng Zhang is a M.E. student in the School of Mechanical Engineering, Kyushu University, Japan, under the supervision of Prof. Yasuyuki Takata. He received his B.E. degree in mechanical engineering in 2016 from Beijing University of Chemical Technology, China. His current research interest is the Marangoni flow during drop evaporation.

Yutaku Kita is an assistant professor in Mechanical Engineering at Kyushu University, Japan. He is a young research fellow of the Japan Society for the Promotion of Science (JSPS). He received his Ph.D. degree in mechanical engineering in 2019 from the Kyushu University. His research interests include evaporation, wetting, and thermocapillary instabilities of sessile drops.

Dejian Zhang is a Ph.D. candidate in the Graduate School of Engineering, Kyushu Institute of Technology, Japan, under the supervision of Prof. Gyoko Nagayama. He received his M.E. degree in mechanical engineering in 2016 from Qingdao University of Technology, China. His research focus is on the wetting behavior of the nano/microstructured surfaces.

Gyoko Nagayama is a professor in the Department of Mechanical Engineering, Kyushu Institute of Technology, Japan. She received her Ph.D. degree from Kyushu Institute of Technology in 2001. She was a postdoctoral fellow in the Hong Kong University of Science and Technology (2001–2002), an assistant professor in Tokuyama College of Technology (2003–2005), associate professor in Kyushu Institute of Technology (2005–2017), a visiting associate professor in the University of British Columbia (2011). Having been engaged in the research of engineering thermophysics, presently, she is focused on nano/microscale interfacial transport phenomena at the liquid–vapor interface, solid–liquid interface, solid–liquid–vapor triple phase interface and its application in micro-fuel cell and micro-heat pipe.

Yasuyuki Takata is a professor of thermofluid physics in the Department of Mechanical Engineering and a Lead Principal Investigator of the Thermal Science and Engineering Research Division at the International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), Kyushu University, Japan. His research area covers phase change heat transfer, micro JT cooler, micro heat transfer devices, and thermophysical properties of hydrogen, as well as a database of thermophysical properties of fluids. He received The JSTP Award for Outstanding Achievement in 1995, The JSTP Best Paper Award in 2010 from the Japan Society of Thermophysical Properties, Heat Transfer Society Award for Scientific Contribution in 2002 from the Heat Transfer Society of Japan, and JSME Thermal Engineering Achievement Award in 2010 from the Thermal Engineering Division of the Japan Society of Mechanical Engineers. He is a fellow and executive board director of the Japan Society of Mechanical Engineers (JSME), and served as an executive board director of international affairs of the Heat Transfer Society of Japan (HTSJ) and the President elect of the Japan Society of Thermophysical Properties (JSTP). He is a vice-president of Asian Union of Thermal Science and Engineering (AUTSE) since November 2015.

Khellil Sefiane is a Professor of Thermophysical Engineering at the University of Edinburgh. He has been associate editor for the Int. Journal of Multiphase Flows, Elsevier and the ASME Journal of Heat Transfer. He has a long interest in heat and mass transfer and multiphase flow problems with a focus on both novel experimental techniques and numerical modeling. His studies of evaporating droplets have included identification of self-excited hydrothermal waves, influence of substrate conductivity, adsorption dependence, infrared themography to study contact line dynamics, flow transition within evaporating binary mixtures and suppression of ring stain formation during polymer solution droplet evaporation. In 2009, he received the Institute of Physics Printing & Graphics Science Group Prize for his “Fundamental studies on droplet evaporation”. He holds an ExxonMobil fellowship awarded by the Royal Academy of Engineering (2000), is a Fellow of the Royal Society of Chemistry, a Japan Society for the Promotion of Science Fellow (Kyushu University, Japan, 2011), an elected UK representative on EUROTHERM Com., an elected member of the Scientific Council of the International Centre for Heat and Mass Transfer. He has published more than 140 refereed journal papers and more than 10 contributions to books.

Alexandros Askounis is a lecturer in Mechanical Engineering at the University of East Anglia. He received his Ph.D. in Engineering from the University of Edinburgh in 2014 and then was a postdoctoral fellow at the Thermal Science and Engineering Research Division at the International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), Kyushu University, Japan. His research focuses on wetting, phase-change, and heat transfer physics at multiple scales.