The key role of interfacial non-bonding interactions in regulating lubricant viscosity using nanoparticles

The feasibility of using nanoparticles (NPs) as lubricant viscosity modifiers has attracted considerable attention as they can increase/decrease the lubricant viscosity. However, due to the lack of theoretical guidance for interface design, the practical application of NP viscosity modifiers is limited. In this study, molecular dynamics simulations are performed to analyze the non-bonding interactions of four typical lubricating oil/NP solid–liquid interfaces with different interface polarities. A model of the mechanism through which the NPs regulate the shear viscosity of the lubricating oil is proposed based on the solid–liquid interface slip behavior, which is verified experimentally. It is found that the shear resistance of the interface is determined by the "liquid all-atomic Coulomb potential energy" (LACPE) and is positively correlated with the “potential well” values of the LACPE distributed in the tangent plane parallel to the wall. Adjusting the characteristics of the solid surface and the "polar units", which are inside the lubricant molecules, as well as the linking modes between the polar units, is proposed as a means to regulate the interfacial shear resistance. This work could provide guidance for interface design and the R&D of nanoscale viscosity modifiers.