Paper VIII(iv) The elastohydrodynamic behaviour of simple liquids at low temperatures

The relationship between molecular structure and elastohydrodynamic rheological properties of lubricants is only poorly understood. One reason for this is that normal liquid lubricants, such as mineral oils and esters, enerally have complex multi-atom molecular structures and are also often mixtures. It is thus difficult to correlate EHD performance with structural details. This paper describes a feasibility study into the technique of using very low temperatures to convert simple, low viscosity liquids into lubricants and thus systematically investigate the influence of structure on elastohydrodynamic (EHD) properties. An optical EHD ball on plate rig has been modified to study the lubricating properties of simple fluids down to −120°C. At these temperatures many simple fluids are found to attain the useful viscosity range for lubrication without freezing. Two classes of fluid have been studied, the phthalate esters and the alkylbenzenes. Both gave measurable EHD film thicknesses though the alkylbenzenes were effective over a very limited and low temperature range.