Rheological properties of small-molecule liquids in elastohydrodynamic lubrication

There is an ongoing debate concerning the rheological model that accurately captures the flow of small-molecule liquids in elastohydrodynamic lubrication (EHL). In EHL, liquids experience extreme pressures (>0.5 GPa) and strain rates (>100,000 s^-1). Rheological properties of squalane, a representative EHL fluid, are investigated using nonequilibrium molecular dynamics simulations for pressures up to 1.2 GPa, strain rates between 10^5 – 10^10 s^-1, and temperatures up to 100 C. Simulation results are consistent with experimental data for a broad range of equilibrium and nonequilibrium conditions. At high temperatures and low pressures, where Newtonian viscosity of squalane is low, shear-thinning associated with molecular alignment is observed, which can be described by power-law fluid models. As Newtonian viscosity rises above ~1 Pa-s, shear-thinning is increasingly dominated by thermally-activated flow processes. In these conditions, the stress-strain-rate behavior from simulations and available experimental data is consistent with the Eyring equation for over 10 decades in strain rate. The macroscopic rheological properties are correlated with underlying molecular orientation and dynamics.