Superlubricity of polyethylene glycol solutions: Running-in effects, thickness changes, and rheology

The role of additives in liquid superlubricity is regarded as a crucial element of the running-in process due to their role in reducing friction. Nevertheless, there has been minor investigation into rheological changes that occur during the process. This paper presents an examination of the evolution of film thickness over time and its subsequent behavior. The primary experiments were performed on an optical ball-on-disk tribometer, with the ability to control the percentage of slip. The film thickness was evaluated by optical interferometry and its rheological behavior was subsequently researched by rotational rheometer and viscometer. It was discovered that the primary contribution to the reduction in friction during running-in is better contact separation caused by the evaporation of water. However, the global behavior of the solution was found to have been changed by formation of a convoluted compound and probably by adsorption to contact surfaces. It causes a behavior that is more complex than that predicted by common elastohydrodynamic equations, but may result in a reduction of friction due to an increased separating layer.