INTERFACIAL FRICTION OF WETTED MONOLAYERS

A ''molecular tribometer'' has been constructed [1] that measures dire ctly, with great accuracy, the forces and the displacements that act b etween metallic surfaces bearing organic layers in a liquid medium as they slide past each other. Its originality is due to its low complian ces (2 x 10(-7) m N-1 for the normal direction, 2 x 10(-6) m N-1 for t he two other directions), which permit mechanical control of the thin layer. In this work, it is shown that an anisotropic thin monolayer of stearic acid adsorbed an each cobalt surface is more compliant than t he contact of the cobalt surfaces and hence reduces the apparent shear moduli of the contact zone and controls the shear process. The ''inte rfacial'' friction depends on the sliding speed and is very sensitive to the very small variations of the film interface thickness. Two comp etitive and opposing processes occur during the sliding: crushing and lifting of the monolayer. It is suggested that the crushing appears at very low speed, when the transit time is close to that of the molecul es monolayer. Our findings have direct applications for the properties of boundary lubricants and for the rheological behaviour of both surf actant and polymer molecules.