(585c) Porous Hydrophilic Coatings for Fast Evaporation

The process of evaporation is important in a wide range of industries, including electronics and spray cooling. The evaporation rate of droplets is hindered by the small surface area to volume ratio when they form a spherical cap on impermeable surfaces at small Bond numbers. To address this limitation, a thin, porous hydrophilic glass coating was employed to increase the rate of evaporation. Upon placing a droplet on the hydrophilic glass coating, the drop is drawn into the porous structure (imbibed), but the thickness of the film restricts its progress. In comparison to an impermeable glass surface, the hydrophilic glass coating enhances the evaporation rate by a factor of 8. The rate of evaporation increases by a factor of 37 when compared to a bulk porous material that shares the same chemical composition. The hydrophilic glass coating allows the droplet to spread radially on the surface, which leads to an increase in the liquid/air interfacial area. The rate of evaporation remains constant during evaporation from the porous coating. Thinner films produce faster evaporation, and the rate of evaporation is proportional to the maximum radius of the droplet within the coating. The Coffee Ring effect that we observe on these coatings suggests enhanced evaporation from the periphery of the imbibed droplet, and is consistent with evaporation proportional to radius. To analyze this phenomenon, a model was developed based on that of Hu and Larson. Our study shows that utilizing a hydrophilic glass coating is a promising method to increase the rate of evaporation in various applications.