How the change of contact angle occurs for an evaporating droplet: effect of impurity and attached water films

Abstract

On a hydrophobic surface, the contact angle of an evaporating droplet decreases with time and becomes much smaller than its receding contact angle; the rate of decrease is accelerated with time. When we use impurity-concentrated water produced by partial distillation, the decrease in contact angle is remarkably accelerated with time. In contrast, for purified water, the decrease in contact angle is significantly reduced and the start of stage 3 is delayed. These results indicate that the submicron-sized impurities cause the decrease in contact angle. Also, we found that a number of attached thin films of water are generated around the periphery of impure droplets. We derived the contact angle equation under the Cassie-Baxter model by regarding the water film as a heterogeneous layer. We compared the theoretical model to the experimental data, and these results suggest that the attached film should be considered as one of the direct causes for the large deviation from the Young's angle of evaporating droplets.