Measurement of van der Waals Dispersion Forces in the Range 1.4 to 130 nm

Abstract

THIS communication describes an experimental study of the van der Waals dispersion forces between curved mica surfaces. The mica specimens were glued to cylindrical glass mountings and supported with their axes at right angles; the contact resembles that between a sphere and a flat. For separations in the range 1.4 to 20 nm the forces were measured by the “jump” method described by Tabor and Winterton¹. In this method one of the surfaces (the upper surface) is mounted at the end of a double cantilever spring, and the lower surface, facing the upper, is supported on a stiff piezoelectric transducer. If the lower surface is moved towards the upper, at some point — depending on the stiffness of the spring—the two will jump into contact. The measurement of this jump distance as a function of the spring stiffness is the basis of the jump method. In the present apparatus the spring stiffness was varied by adjusting the clamping position along its length, and the distance between the surfaces was measured to about 0.2 nm using multiple beam interferometry¹. In the earlier work the smallest separation was 5 nm, but we have extended the separation down to 1.4 nm and improved the accuracy of measurement. Experiments were carried out in air at atmospheric pressure.