Direct visualization of cavitation and damage in ultrathin liquid films

The Surface Forces Apparatus technique was used to observe, at the nanoscopic level, the rapid growth and disappearance of vapor cavities between two moving solid surfaces while simultaneously monitoring their effects on the deformations and damage produced on the surfaces. Both approach-separation and shear motions were studied. In both geometries, as the relative velocity between the two surfaces increases, they deform 'elastohydrodynamically'; in some regions the surfaces become flattened (corresponding to the build up of positive or compressive pressure in the adjacent liquid) while in other regions the surfaces become pointed (corresponding to negative or tensile pressure). Vapor cavities nucleate at these pointed regions once the relative velocity of the two surfaces exceeds a certain critical value. The sudden nucleation and growth of a cavity in a thin liquid film is seen to be a much more violent event than its subsequent collapse, and surface damage due to cavitation occurs during the inception rather than during the much smoother collapse of cavities. These studies are the first to directly visualize cavity formation and associated damage in ultrathin lubricating films, and provide a fairly detailed picture of both of these mechanisms and the relationship between them. The picture that emerges in complex and different from the currently accepted model, in which damage is caused during the collapse of cavities rather than their growth. The results should have a bearing on all phenomena where cavitation and cavitation damage occur in thin lubricating films.