Microscopic Mechanisms of Shear Thinning in Small-Molecular Lubricants

Experimental measurements of lubricant shear thinning typically do not exceed shear rates of 10⁵ s^-1, while actual working conditions often fall within the range of 10⁴–10⁹ s^-1. The power-law model and the thermal activation model can both fit the experimental data well, but their extrapolations to higher shear rates differ significantly. We employ non-equilibrium molecular dynamics simulations to shear three representative lubricant molecules—squalane, poly-alpha-olefin trimer of decene, and pristane—at strain rates between 10⁵–10¹⁰ s^-1. Power-law models attribute shear thinning to the progressive alignment of molecules with the shear direction. To assess this, we analyzed the molecular structure using orientation tensors. Atom pairs that show the largest deviation between shear and equilibrium conditions were selected for visualization using dimension reduction methods. At low pressures, these atom pairs exhibited a clear pattern with respect to strain rates, however, this pattern disappeared at high pressures, despite the fluid viscosity continuing to decrease steadily with increasing strain rate. Thermal activation models, which assume that flow occurs via stress-biased molecular rearrangements, provided a better microscopic explanation of shear thinning at high pressures, where the non-affine displacement of molecules relative to their neighbors was found to rise with increasing strain rate.