Modeling and Measurement of Contact Angle Hysteresis on Textured High-Contact-Angle Surfaces

A set of surfaces with different Cassie fractions were fabricated. For a given Cassie fraction surfaces had different pillar diameter and spacing combinations. Advancing and receding contact angles of both water and ethylene glycol were measured on the fabricated surfaces. We examined the effects of both surface feature size and Cassie fraction on advancing and receding contact angles and found no relationship between size and contact angle when Cassie fraction was held constant. Also examined was the effect of the Cassie fraction on contact angle hysteresis, which led to the development of a new theoretical framework for understanding contact angle hysteresis on rough surfaces that helps unite previous theory and observations with current observations. The theoretical framework includes empirically determined contact-line pinning forces. From measurements, we found a constant contact-line pinning force for the receding contact angles. For the advancing contact angles, there was also a contact-line pinning force, but one that changed with changing Cassie fraction.