In Situ Droplet Microgoniometry Using Optical Microscopy

Solid-liquid interactions are ubiquitous phenomena in nature and industry. Wettability of a liquid on a solid is 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 and 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 noncircular contact lines. Furthermore, past methods are incapable of measuring contact angle in situ during experiments (e.g., condensation). Here, we develop simple yet powerful contact angle measurement techniques 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 of water droplet diameters (10 μm < D < 600 μm) and apparent contact angles (0° ≤ θ^app ≤ 180°). The outcomes of this work establish a powerful tool to more easily and rapidly characterize microscale droplet advancing and receding contact angles.