Thermal energy transport at solid-liquid and solid-vapour interfaces

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The phase changes of fluids are used in countless industrial processes. One of their important potential applications is for electronic cooling. Spray cooling is being considered for this application. It is capable of removing more thermal energy from a solid than pool boiling, but spray cooling is recognized to be a complex process for which the energy transport mechanisms are not understood. An essential step in spray cooling is sessile-droplet evaporation. This process requires thermal energy to be transported from the solid to the liquid-vapour interface of the droplet where the liquid evaporates, reducing the temperature of the liquid and cooling the solid. Thus, a central issue is the mechanism by which thermal energy is transported from the hot solid to the liquid-vapour interface. In previous studies, we have established a new surface property of water---the surface-thermal-capacity---that can be used to determine the amount of energy transported by surface-tension-driven convection, and recently, we have confirmed that this mode of thermal-energy transport is dominate during sessile-water-droplet evaporation. It is an order of magnitude greater than thermal conduction. But this observation raises a question: how does all that energy get to the three-phase line of a sessile droplet? Based on preliminary studies, we have formulated a hypothesis: we suppose thermal energy is conducted along the solid-liquid interface of the droplet to its three-phase line, and that surface-tension-driven convection then distributes the thermal energy along the liquid-vapour interface where it is used to evaporate the liquid. We assume the thermal energy transport along the solid-liquid interface is facilitated by the liquid film adsorbed there. Based on a recently proposed isotherm, we have previously shown that the amount adsorbed may be determined by measuring the pressure dependence of the contact angle. Our proposal is to investigate our hypothesis. Thermal energy transport along a solid-liquid interface has not been previously investigated (to our knowledge). Thus, if our hypothesis proves valid, the conception of thermal-energy transport during sessile-droplet evaporation will be changed.