Van der Waals forces between fused silica and fused silica covered with a chromium layer

Direct measurements of Van der Waals forces between a flat plate and a planoconvex lens are reported for separations ranging from 40 to 280nm. Both the lens and the flat plate were fused silica. Two different lenses are used for the measurements. The first one with a radius of curvature of 419.8 cm was covered with a 54 m thick chromium layer. In agreement with theoretical expectations the Hamaker coefficient was not constant in this system. The second lens with a radius of 83.75cm was covered with a chromium layer of 220nm thickness. The retarded Hamaker constant was found to be 2.1 x 10(-19) erg cm for this system.