Wetting failure condition on rough surfaces

Wetting states and processes attract plenty of interest of scientific and industrial societies. Air entrainment, i.e., wetting failure, on smooth plate in wetting process has been investigated carefully before. Liquid bath entries of “rough” silicon wafers are studied experimentally in the present work, and the air entrainment condition is analyzed specially with the lubrication theory. The roughness effects on the moving contact lines are therefore explored. The contact line pinning is found to be the main reason for the dynamically enhanced hydrophobicity of rough surface, which implies an effective microscopic contact angle of θ_e=θ_Y+90° where θ_Y is the Young's contact angle of the material. Our results suggest that the solid surfaces can be considered as hydrophobic ones for a wide range of dynamic process, since they are normally rough. The work can also be considered as a starting point for investigating the high-speed advancing of moving contact line on rough surfaces.