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Cooling performance of bio-mimic perspiration by hydrogel
Keywords: electronics cooling, electronic packaging, biomimic material
In Nature, plants and animals are autonomously adaptive to hot environment through transpiration and sweating cooling via evaporation of water, which has one of the highest latent heats among various fluids. Inspired by these natural passive cooling phenomena, several self-adaptive cooling technologies involving bio-inspired artificial skins have been reported[1] [2] [3] [4] [5]. One of the promising materials is based on superabsorbent polymers, or hydrogels, which can contain more than ~90 wt% of water in their fully swollen state[6] [7]. By sweating the surface with water contained in hydrogels, the heat dissipation rate is enhanced such that the surface temperature can be reduced by 10-30¡ãC upon the application of hydrogels [8-15]. This remarkable autonomous cooling capability makes hydrogels an attractive candidate for energy-efficient cooling. A novel passive cooling solution, Bio-mimic Perspiration Cooling (BP-Cooling), was recently proposed. In this paper, the heat and mass transfer characteristics of BP-Cooling are investigated. The temperature and humidity fields of BP-Cooling are measured by the Twyman-Green interference technique and modeled by computational fluid dynamics (CFD) simulations. The validated CFD model is further used to study the impacts of different usage conditions, e.g. ambient temperature, ambient humidity, and the starting temperature of BP-Cooling, on the BP-Cooling performance. Results show that BP-Cooling can improve passive cooling performance up to twenty times above natural convection and may be powerful enough to enable next-generation mobile phones perform like personal computers in a wide design envelope. Besides, we reported for the first time the application of a highly stretchable and tough hydrogel as a regenerable ¡®sweating skin¡¯ for cooling. Compared with conventional hydrogels used in prior cooling studies [8-15], the tough hydrogel used in this work has a significantly higher fracture energy (~9,000 Jm-2) [16], which is on the same order of magnitude with natural rubber. We have demonstrated high-performance and remarkably regenerable cooling capability of tough hydrogels. Compared with conventional hydrogel layers, the cooling power and water absorption capability of tough hydrogels are retained after at least 50 cycles. We envision that these remarkable mechanical and thermal properties of tough hydrogels, especially the significantly improved cyclability, will significantly expand the applications of hydrogels for energy-efficient thermal management of various devices and systems, such as for buildings, electronic devices, occupational clothing, and batteries.
Shuang CUI, Graduate Research Assistant
University of California, San Diego
La Jolla, California

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