Droplet asymmetry and wetting dynamics on irregularly roughened surfaces

This study illustrates pervasive challenges in studying wetting dynamics, including dynamic contact angles, on irregularly roughened surfaces. We demonstrate that asymmetric water droplet shapes occur more than 50% of the time during static and dynamic contact angle measurements on sandblasted Zn-plated stainless steel with a polymeric overcoat. The pinning that causes the asymmetric drop shape distortion on horizontal surfaces also influences the sliding behavior on inclined surfaces. These effects lead to a poor correlation between the measured dynamic contact angles and the observed sliding angles (critical tilt angles). Our work emphasizes that large variations in the values of these dynamic wetting parameters are inherent to the heterogeneity of the surface roughness, and thus they limit the usefulness of standard dynamic wetting criteria. These findings have implications for academic and industrial research focused on making coated materials that have consistent wettability properties throughout their usage life cycle.