Particle Entrainment in Lubricants Leads to Extended Mixed Lubrication in Soft Tribology

Particle-laden lubricants provide a versatile platform for tuning frictional behavior across boundary, mixed, and elastohydrodynamic (EHL) regimes in soft tribological systems. In this study, we examine the evolution of an extended mixed regime between poly(dimethylsiloxane) (PDMS) tribopairs lubricated with poly(methyl methacrylate) (PMMA) suspensions in low-viscosity squalene and high-viscosity diisodecyl phthalate (DIDP). Smaller particles at low concentrations delay entrainment and slow film formation, while larger particles consistently reduce friction by enhancing load distribution and promoting separation that limits solid-solid contact. These effects, driven by particle confinement and structural rearrangements, generate clear deviations from classical Stribeck behavior. Two characteristic points mark the onset and termination of this extended mixed regime. We observe that the velocity corresponding to the beginning of mixed regime transition, Um1, is largely independent of particle size and volume fraction in squalene suspensions due to a restrictive particle confinement within the tribopairs that primarily allows solvent movement through the gap. In contrast, DIDP suspensions have a lower onset velocity (Um1) as the higher solvent viscosity promotes particle entrainment into the lubrication gap. The velocity corresponding to the end of the mixed regime transition (Um2) decreases with increasing volume fraction, indicating the formation of a fully developed lubrication film that separates the PDMS tribopairs. Hamrock-Dowson film thickness estimates combined with experimental relaxation measurements are used to quantify the development of the lubrication film as sliding speed increases and the system approaches the EHL regime. The findings from this study establish particle entrainment and suspension viscosity as key mechanisms for expanding and stabilizing the extended mixed regime.