What Can We Learn from the Vibration of Drops Deposited on Rough Surfaces? Wetting Transitions Occurring on Rough Surfaces

Wetting transitions were studied with vertically and horizontally-vibrated drops on various artificial and natural rough substrates. Alternative pathways of wetting transitions were observed. The model of wetting transition is presented. Multiple minima of the Gibbs free energy of a drop deposited on a rough surface explain the alternative pathways of wetting transitions. It is demonstrated that the wetting transition occurs when the constant force resulting from vibration, Laplace and hydrostatic pressures acts on the triple line. It is shown that the final wetting state is mainly the Cassie impregnating wetting state with water penetrating the pores in the outer vicinity of the droplet or the Wenzel state with water inside the pores under the droplet whereas the substrate ahead the drop is dry. For horizontally vibrated drops, the resonance character of wetting transition is noteworthy: the threshold amplitude necessary for transition is decreased dramatically in the vicinity of the eigenfrequencies of surface waves.