Direct measurement of van der Waals forces between solids in air

Non-retarded van der Waals forces between a pair of macroscopic bodies have been measured as a function of separation in the range 2-30 nm for three systems: (a) mica/mica, (b) calcium stearate/calcium stearate, and (c) silver/mica. The values of the Hamaker constant, which is a measure of force strength, are slightly larger than those predicted by a simple version of Lifshitz theory, but the ratios of the Hamaker constants are in agreement with theory. Experimental reasons connected with the details of surface profile measurements, and theoretical reasons for the differences are discussed.