The direct measurement of normal and retarded van der Waals forces

A direct measurement has been made of the forces in air between two cylindrical sheets of mica arranged with their axes mutually at right angles. The contact resembles that between a sphere and a flat. The mica sheets are glued to glass formers, their concave face being first slightly silvered. The contact region and the distance of approach are measured by multiple beam interferometry using fringes of equal chromatic order (f. e. c. o.). This gives an accuracy for the distances between the surfaces of ± 0.3 nm. Since the surfaces are molecularly smooth it is possible to bring them very close to one another. One surface is held on a rigid support, the other on a light cantilever beam. The surfaces are slowly brought together and at a critical separation ‘flick’ together. The ‘flick’ distance depends on the stiffness of the cantilever and this in turn provides a direct measure of the surface forces. By using cantilevers of different stiffnesses the method has proved effective for separations ranging from 5 to 30 nm. The results show that for separations less than about 10 nm the forces operating are ‘normal’ van der Waals forces whilst for distances greater than 20 nm they are ‘retarded’ van der Waals forces.