Measurement of Sub-Nanometer Lubricant Films using Ultra-Thin Film Interferometry

The ultra-thin film interferometric method of measuring the thickness of very thin films in lubricated contacts has been refined so as to be able to measure films down to 0.3 nm with a standard deviation of 0.15 nm. The main remaining source of measurement variation for films below 3 nm thick is the surface roughness of the contacting solids. This modified technique has been applied to study the film-forming properties of three fluids, hexadecane, a dilute solution of surfactant in hexadecane, and cyclohexane. Purified hexadecane shows a very slightly enhanced oil-film thickness below 1 nm. The long-chain surfactant forms a boundary film 2 nm thick. Cyclohexane behaves as though it forms a surface layer about 1 nm thick with viscosity three times the bulk fluid viscosity.