Contact line length dominance in evaporation of confined nonspherical droplets

Abstract

Free droplets are spherical within capillary lengths and become nonspherical when trapped in a confined space. Confined nonspherical droplets are as common as spherical droplets. Yet, their evaporation dynamics are not fully understood because of their geometrical complexity. We use monochromatic synchrotron x-ray microtomography to investigate the evaporation dynamics of confined nonspherical water droplets trapped by micropillars based on three-dimensional geometrical information with time. We find two types of confined nonspherical droplets: Wenzel droplets with single-sided air-water interfaces, and Cassie-Baxter droplets with double-sided interfaces. Both droplets show similar sphericity at large volumes but approach different ones at small volumes: Wenzel droplets follow a thin film limit, whereas Cassie-Baxter droplets follow a spherical sessile limit. Despite the geometrical complexity of confined nonspherical droplets, the vapor diffusion mechanism suggests that the evaporative flux is maximal at the contact line, which governs the evaporation dynamics, as proven by observations. The proportionality of the evaporation rate to the contact line length demonstrates the contact-line-length-dominant evaporation dynamics of confined nonspherical droplets. The findings of this study can unify the evaporation mechanism for spherical sessile and confined nonspherical droplets, even with geometric complexity.