Nanobubbles give evidence of incomplete wetting at a hydrophobic interface

The appearance of a hydrophobic surface, namely a crystalline (111) Si wafer coated with a thick soft polystyrene film, and the morphological changes along this interface depending on the polarity of an adjoining liquid phase were studied with magnetic tapping mode atomic force microscopy. Interfacially associated nanobubbles of decreasing size and number are observed as the hydrophobicity of the subphase increases. The disturbance of the water structure in the contact region induces the formation of nanobubbles. The topology of the interface is visualized, starting with the dry polymer under normal atmosphere conditions and observing the changes as air is replaced by a series of liquids. With water, the surface coverage of the substrate with bubbles is almost a close-packed configuration. The bubble shape is well approximated by spherical caps of a rather low aspect ratio. The Gaussian size distributions of bubble shape parameters are discussed. The contact angle of the nanobubbles is substantially smaller than the corresponding number measured for a macroscopic droplet. This apparent discrepancy might be resolved if the nanobubbles were assumed to exist along the interface as a connecting sublayer between a depleted water film at the hydrophobic polymer surface and an adsorbed macrodroplet.