Rupture and dewetting of water films on solid surfaces.

An experimental study was conducted to observe rupture and dewetting of water films, 0.5-2mm thick, on solid surfaces. The effects of surface roughness, wettability, protrusions on surfaces, and air entrapment between films and surfaces were studied. Film thickness measurements were made and film rupture and surface dewetting photographed. Experiments showed that liquid films ruptured first along the highest edges of test surfaces. Placing a protrusion on the surface had no effect-the liquid film continued to rupture along the edges. A thermodynamic model was developed to show that protrusions lower the surface energy of the system and promote wetting. Increasing surface roughness therefore increases film stability by resisting rupture and dewetting. Water films could be punctured by introducing an air bubble that burst and created a hole. The hole would close if the film was thick and the solid-liquid contact angle was either small or large; the hole would grow larger if the film was thin and the contact angle was in the mid-range (∼80°). An analytical model that calculates the difference between the surface energies of the two states can be used to predict whether a hole would lead to surface dewetting or not.