(21f) Molecular Dynamics Simulations of Shear Thinning of Lubricants at High Strain Rates

Rheological properties such as shear thinning of lubricants are difficult to measure in experiments at high strain rates, e.g, > 10⁵ s^-1. The lack of experimental data has given rise to debate surrounding the appropriate rheological model to describe shear thinning of lubricants under these high rates at high pressures (e.g., > 500 MPa) such as those often encounted in elastohydrodynamic lubrication. We employ non-equilibrium molecular dynamics simulations to explore high-strain-rate shear thinning of three lubricants: squalane (SQL), poly-alpha-olefin trimer of decene (PAO4), and n-triaoctane (nC30), which share the same chemical formula (C₃₀H₆₂) but exhibit different molecular shapes with branched, star, and linear architectures respectively. Our simulations span a wide range of pressures (from ambient to ~1 GPa) and strain rates (~ 10⁶ to 10¹⁰ s^-1). For all three liquids, the onset of shear thinning is pushed to smaller strain rates as pressure increases. Access to a large set of simulation data enables us to credibly compare the rheological models for shear thinning, in particular, the power-law shear-thinning models (e.g., Carreau) and thermal-activation based flow models (e.g., Eyring). Further, we connect the predictions of these phenomenological models to microscopic mechanisms via the computation of changes in diffusion coefficient, molecular order, and molecular rearrangement timescales with strain rate under different pressures. Together, the macroscopic trends and associated microscopic mechanisms reveal the effect of molecular shape on shear rheology of lubricants, at both low pressures associated with relatively low Newtonian viscosities, and at high pressures characterized by high Newtonian viscosities.