Controlling droplet cooling with micro/nano-structured surfaces
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Heat production accounts for most of the world's primary energy consumption. A better thermal management in several applications would result in a more e‑fficient and safer design. In this context, studying the cooling of droplet impacting on a solid surface is a way to fullfil this objective. The recent advances in nanotechnology allow the possibility to control the morphology of a surface at the micro-nanoscale level, that affects the wettability and the heat transfer performance. In this regard, super-hydrophilic surfaces have been developed, in particular randomly distributed and patterned Si nanowires. A droplet cooling facility and wicking and wettability facility have been designed and validated in order to assess the performance of the nanostructures. At low surface superheat, the wicking plays a dominant role and controls the heat transfer performance. Indeed, nanowires with higher wicking show a larger reduction of the water droplet cooling time. At high surface superheat, nanowires with higher wicking show the highest Leidenfrost point shift (normalized to the droplet volume) reported in the literature. This is due to the broadening of the transition boiling region promoted by the occurrence of the lift-off phenomenon that keeps constant the droplet lifetime, as observed also in the case of Si patterned nanowires. A preliminary experimental investigation of water droplet impacting on micro/nano-cavities suggests that the size of the cavities could be important in affecting the occurrence of the Leidenfrost phenomenon. In particular, cavities with larger size seem to promote the occurrence of the lm boiling regime. Based on these results, a preliminary experiment of FC-72 droplet impacting on random Si nanowires suggests that the increase of the nucleation site density and the negligible role of the wicking promote the occurrence of the Leidenfrost phenomenon.