We have carried out an experimental study of liquid drop impact on various superhydrophobic substrates. Our surfaces are of two kinds (1) a carpet of chemically coated nanowires and (2) a smooth warm substrate. In the latter case, the Leidenfrost effect (also called ‘boiling crisis’) ensures the existence of a thin layer of air coming from the evaporation of the drop, thus preventing the drop to touch the warm surface. Technically, in this latter situation the contact angle can then be considered as equal to 180 degrees, with no hysteresis. Due to its initial inertia, the drop experiences a flattening phase after it hits the surface, taking the shape of a pancake. Once it reaches its maximal lateral extension, the drop begins to retract and bounces back. We have extracted the lateral extension of the drop, and we propose a model that explains the trend. We find a limit initial velocity beyond which the drop (1) protrudes into the nanowire carpet (2) touches the hot plate, provoking a local violent boiling. We discuss the relevance of practical issues in terms of self-cleaning surfaces or spray-cooling.
Experimental study on the premixed lean-burn in alumina particles based porous burner
