In this work we describe dispersion force (van der Waals/Casimir) measurements between gold surfaces at separations as close as 12 nm. The force measurements were performed in the plane-sphere configuration by Atomic Force Microscopy at distances ranging between 12 and 200 nm. This was accomplished by using evaporated smooth metal surfaces for both sphere and plane, and stiff cantilevers to minimize jump-to-contact problems. Finally, it is shown that below separations of 100 nm experiment and theory are found in agreement within 10 % by including in the theory the measured optical properties of gold and contribution from surface roughness.
We have investigated the morphology and surface roughness of several commercially available microspheres to determine their suitability for force measurements using the atomic force microscope. The roughness varies considerably, depending on sphere size and material, ranging from nearly ideally flat up to micrometer-sized features. Because surface roughness significantly influences the magnitude and accuracy of measurement of surface forces, the results presented here should be helpful for colloid physicists and in particular for those performing force measurements.
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