Coarse-Grained Molecular Dynamics Simulation of Fatty Acid Additives in Lubricating Oil Sheared by Corrugated Solid Surfaces

A thorough understanding of the dynamic behavior of friction modifier additives added to lubricating oil is crucial for improving the performance of boundary lubrication. However, such understanding is still lacking due to difficulty in measurements. In this study, we used coarse-grained (CG) molecular dynamics (MD) simulations to gain insight into the behavior of stearic acid additives physically adsorbed on corrugated or smooth hematite surfaces which were separated by lubricating oil of dodecane at a nanometric distance and sheared relative to each other. Compared with all-atom MD simulations, our CG MD simulations provided comparable accuracy and 40 times faster computational speed. The simulation results showed that, in contrast to the case of smooth solid surfaces, shear-induced desorption of stearic acid molecules and re-adsorption during rest occurred at the convex portions of the corrugated solid surfaces. This demonstrates that roughness of sliding surfaces largely influences the dynamic behavior of additives in lubricating oil.