Molecular dynamics methods predict transport properties of model lubricants

The ability to calculate and precisely predict pressure-viscosity data of lubricants is important for the characterization of friction in machine components that operate in the elastohydrodynamic lubrication regime. Elastohydrodynamic lubrication involves friction pairs having elastic contact under very high pressure in nonformal contact. It describes the high-load regime for components like ball bearings and gears.

Kondratyuk et al. report on the applicability of molecular dynamics methods at conditions that appear in the elastohydrodynamic lubrication regime. In particular, the researchers computed the shear viscosity and self-diffusion coefficients for two model lubricants at pressures up to 400 MPa. The model lubricants represented paraffinic and aromatic oils.

They employed classical molecular dynamics approaches, such as the Green-Kubo and Einstein-Smoluchowski methods, to calculate the transport properties of the model lubricants. These methods are very sensitive to the choice of interatomic force field, which determines the accuracy and predictive power of the simulation. The researchers compared three different force fields: Transferable Potentials for Phase Equilibria United-Atom, Optimized Potentials for Liquid Simulations All-Atom and Condensed-phase Optimized Molecular Potentials for Atomistic Simulation Studies.

Overall, the results provide a better understanding of increasing viscosity of the model lubricants under high pressures. Interestingly, viscosity-diffusivity relations are found to be quantitatively the same for the three force fields, even in the pressure region where they deviate from simple Stokes-Einstein scaling. Thus, the pressure-viscosity curve can be computed with a computationally inexpensive united-atom potential and scaled using a more complex molecular model, which requires much less computation and cost.

Source: “Transport coefficients of model lubricants up to 400 MPa from molecular dynamics,” by N. Kondratyuk, D. Lenev, and V. Pisarev, Journal of Chemical Physics (2020). The article can be accessed at http://doi.org/10.1063/5.0008907 .