The design of lubricious oil-in-water emulsions

Oil-in-water emulsions, as used in metal working and fire-resistant hydraulic fluids, often show poor lubricating performance and one reason for this is their inability to form effective elastohydrodynamic films. This paper considers how the composition of an oil-in-water emulsion contributes to its lubricating properties. A key factor in oil-in-water emulsion performance lies in the ability of the dispersed oil droplets to wet polar metal surfaces and thus promote full elastohydrodynamic lubrication, without starvation, up to high speeds. In this paper it is shown how this ability can be quantified in terms of measurable surface chemical parameters. Based on this work, three rules for designing lubricious oil-in-water emulsions are proposed. Where possible (a) the emulsifier concentration used should be just below its critical micelle concentration value, (b) the base oil should be polar and (c) the base oil should be viscous. The first two of these factors will promote oil-phase surface wetting while the third will ensure thick-film formation in the full elastohydrodynamic regime.