Elastohydrodynamic Lubrication and Glassy Flow: Linking Experiments and Simulations at High Rates and Pressures

Elastohydrodynamic lubrication (EHL) is important in many practical devices and produces extreme pressures (> 1 GPa) and shear rates (10⁵ s^-1 to 10⁷ s^-1). As a result, viscosity of EHL fluids rises by orders of magnitude leading to a glass-like rheological response. Further, the high strain rates lead to large shear stresses and shear-thinning in these fluids. This talk will present nonequilibrium molecular dynamics (MD) simulations of the model EHL fluid squalane from 10⁵ s^-1 to 10¹⁰ s^-1 and a wide range of temperatures and pressures. The results are consistent with experimental data for a broad range of equilibrium and nonequilibrium stress and viscosity measurements and allow shear response to be correlated to molecular orientation and dynamics. At high temperatures and low pressures, where the Newtonian viscosity is low, the fluid shows simple shear-thinning associated with molecular alignment which can be described by power-law fluid models. As the Newtonian viscosity rises above ~1 Pa-s, shear-thinning is increasingly dominated by activated hopping. Over more than 10 decades in strain rate, the stress rises logarithmically with rate as implied by Eyring theory. Extrapolating Eyring fits to simulations at 10⁶ s^-1 and above yields Newtonian viscosities that are consistent with available low-rate experiments, and allows predictions to higher pressures and lower temperatures. Implications of our results to glass transition and EHL phenomena are discussed. Perspectives in the use of molecular simulation to extract accurate viscosities of glassy systems are offered.