On the contact of smooth surface

Abstract

In recent years a number of experiments have shown that if a clean optically polished plate is lowered on to a similarly polished plate in air, the two surfaces do not come into contact. The top plate remains “floating” at a distance equal to the thickness of several thousand air molecules above the bottom plate. Thus Hardy and Nottage found that a polished steel cylinder placed on a steel plate remained at a distance of 4 µ above the plate in clean dry air. By applying a pressure to the top cylinder the surfaces could be made to approach more closely, but on removing the pressure the cylinder would again rise to a height of 4 µ . This height remained constant and was the same for glass on glass as for steel on steel. When the air was replaced by various organic substances, e.g. , octane, capryllic acid, octyl alcohol, the distance between the surfaces remained remarkably constant at 4 µ . When the surfaces were contaminated with a trace of palmitic acid the distance was decreased to 2·3 µ . Similarly Watson and Menon using an electrical capacity method found that the distance between polished silver or polished steel plates in air or in oil was approximately 4 µ . Experiments made by Lee are consistent with this. A full discussion of this phenomena is given by Hardy, who considers it to be “the most difficult problem of the boundary state” ( loc.cit ., p. 2). The possibility that the top plate is supported on solid particles was considered in this paper, but the experimental evidence then available argued against it, and it was thought probable that the plate was supported by a film of air or of liquid. To explain this it would be necessary to postulate oriented chains of molecules extending from the surface for several thousand molecules. This is contrary to most physical evidence, which shows that the effect of a surface on a gas does not extend for more than a few molecular layers. The first molecular layer of gas may be oriented and strongly held by the surface, but the degree of orientation decreases very rapidly and becomes inappreciable at a distance of a few molecular diameters from the surface. The point was discussed with Sir William Hardy, and at his suggestion further experimental work was undertaken on this anomalous effect.