The measurement of van der Waals dispersion forces in the range 1.5 to 130 nm

Abstract

.This paper describes an experimental study of the van der Waals dispersion forces between curved mica surfaces. For separations in the range 1.4 to 20 nm the forces were determined by the jump method described in earlier work by Tabor & Winterton (1969). For larger separations the forces were determined by a new dynamic method. One surface was supported on a rigid piezo-electric crystal and could be set vibrating at very small amplitudes over a convenient range of frequencies. The other was supported, facing it, on a stiff spring: its natural frequency depended both on the spring stiffness and on the van der Waals force exerted upon its by the first surface. By determining the resonant frequency as a function of separation the law of force was deduced in the range 10 to 130 nm. Both methods thus covered the range from 1.4 to 130 nm. In this way it is shown that there is a gradual transition between non-retarded and retarded forces as the separation is increased from 12 to 50nm. Experiments have also been carried out on the influence of adsorbed fatty acid monolayers on the van der Waals interaction between mica surfaces. The results show that for separations greater than about 5 nm the forces are as for bulk mica: for separations less than 3 nm the forces are slightly less and appear to be dominated by the van der Waals properties of the monolayers themselves.