In situ confocal Raman spectroscopy of lubricants in a soft elastohydrodynamic tribological contact

Abstract

This paper describes an experimental setup that combines friction measurements with in situ confocal Raman microscopy on a lubricating fluid film. The setup allows the determination of film thickness and shear-rate profiles, the position-dependent lubricant composition within the rubbing contact, as well as optical imaging of the contact and its surroundings. The tribological contact can be investigated at continuous rubbing conditions over a range of sliding speeds from 1×10−3to2m∕s. The setup’s capabilities are demonstrated for a soft poly(dimethylsiloxane) sphere pressed and rotated against a quartz flat. Friction forces are presented in the boundary, mixed, and elastohydrodynamic lubrication (EHL) regimes for both a hydrophilic and a hydrophobic flat surface. Optical images of the contact show that, for single-phase aqueous Newtonian lubricants in the EHL regime, starvation of the contact occurs above a critical value of the product of the entrainment speed and the lubricant viscosity. Starvation results in a lower film thickness and lower friction values than predicted by numerical calculations that do not take starvation into account. The capability to determine, using Raman spectroscopy, the composition of the lubricant within the contact is demonstrated for oil-in-water emulsions, stabilized by a nonionic surfactant (Tween 60). The oil concentration was found to depend on both the sliding speed and the position within the rubbing contact. For the investigated tribological conditions, the oil content within the contact was lower than that of the bulk emulsion. This is attributed to the fact that the aqueous matrix phase preferentially wets the hydrophilic quartz slide, which prevents oil droplets from entering the contact if the film thickness is of the order of the droplet size. The ability to determine friction forces, lubricant composition and film thickness, and shear-rate profiles will provide invaluable information in the pursuit of a better understanding of lubrication of soft and hard contacts by multiphase complex fluids.