Progress in understanding wetting transitions on rough surfaces.

The abrupt change in the apparent contact angle occurring on a rough surface is called wetting transition. This change may be spontaneous or promoted by external stimuli such as pressure or vibration. Understanding the physical mechanism of wetting transitions is crucial for the design of highly stable superhydrophobic and omniphobic materials. Wetting regimes occurring on rough surfaces are introduced. Experimental methods of study of wetting transitions are reviewed. Physical mechanisms of wetting transitions on rough surfaces are discussed. Time and energy scaling of wetting transitions are addressed. The problem of the stability of Cassie wetting on inherently hydrophobic and hydrophilic surfaces is discussed. The origin and value of a barrier separating the Cassie and Wenzel wetting states are treated in detail. Hierarchical roughness increases the value of the energy barrier. The stability of Cassie wetting observed on re-entrant topographies is explained. The irreversibility of wetting transitions is explained, based on the asymmetry of the energy barrier, which is low from the side of the metastable (higher-energy) state and high from the side of the stable state. The critical pressure necessary for a wetting transition is introduced. The problem of "dimension" of wetting transition is discussed. Reducing the micro-structural scales enlarges the threshold pressure of a wetting transition. The roles of gravity and air compressibility in wetting transitions are treated. The dynamics of wetting transitions is reviewed. The results of molecular simulations of wetting transitions are presented. The trends of future investigations are envisaged.