Boundary lubrication of rubber by aqueous surfactant

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Abstract
An apparatus has been developed which uses a laser to measure the thickness of boundary films in lubricated rubber contacts. In earlier measurements using the same interferometric principle the active optical component was a two-piece beamsplitter. This has been replaced by a simple prism. The behaviour of surfactant solution boundary films in rubber/glass contact has been re-examined using this apparatus, and films in rubber/rubber contact have been explored in detail for the first time. Stable films in static contact have been recorded at thicknesses up to 25 nm at an average pressure of 10 kPa. At higher average pressures, up to 50 kPa, the collapse of these films has been observed, which brings into question the role of electric double layers in supporting the normal load. Introducing a surfactant solution to the region around a dry, static, loaded rubber/glass contact results in the separation of the surfaces and the formation of an equilibrium film. The thickness of the film formed is the same as by squeezing rubber and glass surfaces together under the same load when immersed in the surfactant solution. This observation indicates the presence of surface repulsive forces. The improved sensitivity of the new apparatus was demonstrated during the course of experiments. In earlier measurements it was difficult to resolve the thickness of films thinner than 4 nm. It appears that films can now be resolved down to 1 nm, which is of the molecular dimensions of surfactants.