Surface roughness effect on the solid equilibrium contact angle

To collect high quality contact angles (CAs), which are close to equilibrium CAs, is a prerequisite for adequate calculation of the surface energy from a selected theory. In this study, we investigated how changes in the surface roughness affect the deviation of apparent CAs from equilibrium CAs. Static, advancing, and receding CAs of water were measured on six different types of materials, each with three arithmetic mean roughnessess R_a(0.1, 1.0, and 2.0 μm), as measured by a profilometer. Other height and spatial roughness parameters were also calculated. The equilibrium CA of each material was calculated from the corresponding advancing and receding CA data. The roughness data showed different topographies among material surfaces with statistically similar R_avalues, particularly in spatial terms. Both the static and advancing CAs measured on the "smooth" (R_a= 0.1 μm) surfaces showed good fit with the equilibrium CAs, whereas "moderately rough" (R_a= 1.0 μm) and "rough" (R_a= 2.0 μm) surfaces had CAs with larger deviations from the equilibrium values. The findings suggest that surface energy values are only valid when calculated from CAs measured on carefully prepared smooth surfaces.