In Situ Droplet Microgoniometry Using Optical Microscopy

Solid-liquid interactions are ubiquitous phenomena in the nature and industry. Wettability of a liquid on a solid if governed by the chemical heterogeneity and physical roughness of the solid surface and can be characterized by measuring the advancing and receding contact angles of the liquid droplet residing on the solid. To characterize contact angle, goniometry or the Wilhelmy plate method have been widely used. Although powerful, these methods have difficulty characterizing microdroplets, can be cumbersome and expensive, and have trouble handling surfaces with local wetting heterogeneity and deformed non-circular contact lines. Furthermore, past methods are incapable of measuring contact angle in situ during experiments (e.g. condensation). Here, we develop a simple yet powerful contact angle measurement technique using conventional optical microscopy that utilizes focal plane shift imaging, ray optics, and wave interference. We used our techniques to study the wetting characteristics for a wide range...