An atomic-force microscope was used to measure the surface forces between a sharp tungsten tip and several different types of molecularly thin organic films on a silicon substrate. The forces are dramatically influenced by the molecular films covering the surfaces. The films investigated fall into three categories: liquid for unbound films of perfluoropolyether, intermediate for bonded films of perfluoropolyether, and soft solid for multilayers of cadmium arachidate. Molecular-level origins of these forces are discussed.

• Received 8 June 1990

DOI:https://doi.org/10.1103/PhysRevLett.65.2270