Dynamics of wetting on smooth and rough surfaces.

The rate of spreading of non-volatile liquids on smooth and on rough surfaces was investigated. The radius of the wetted spot was found to agree with recently proposed scaling laws (t 1/10 for capillarity driven andt 1/8 for gravity driven spreading) when the surface was smooth. However, the crossover between these regimes was not observed at a constant value of the radius. Rough surfaces exhibited at least four spreading regimes which could be rationalized in terms of a macroscopic contact angle and macroscopic deviations thereof. An interesting feature is the appearance of a wetted rim around the central drop. This rim follows a diffuce spreading law (t 1/2). Partially wetting liquids followed the same dynamics provided the spreading conditions was fulfilled