Molecular dynamics simulations of elastohydrodynamic lubrication oil film

Abstract

Highlights • A mechanism for the limited shear-stress transition of an elastohydrody-namic lubrication (EHL) oil film is proposed. • The spatial structure and velocity profile showed gradual changes. • Suppression of the fluctuation is found in the EHL regime. Abstract All-atom molecular dynamics simulations of an elastohydrodynamic lubrication oil film are performed to study the effect of pressure. Fluid molecules of n-hexane are confined between two solid plates under a constant normal force of 0.1-8.0 GPa. Traction simulations are performed by applying relative sliding motion to the solid plates. A transition in the traction behavior is observed around 0.5-2.0 GPa, which corresponds to the viscoelastic region to the plastic-elastic region, which are experimentally observed. This phase transition is related to the suppression of the fluctuation in molecular motion.

Molecular origin of limiting shear stress of elastohydrodynamic lubrication oil film studied by molecular dynamics

“…1. The highly layered fluid molecules are found in the vicinity of the wall which is already reported in our previous work [2,3]. In comparison with snapshots in other external pressures (they are shown in the graphical abstract), the film thickness is small and the density of the fluid molecules is high.…”

“…The solid atom layers move at a constant velocity so that shear is induced in the fluid film. Previous simulation of a submicron-thickness oil film showed quantitative agreement between experiment and simulation [2,3]. Here, we extend our study to examine the effect of an external pressure, in molecular level.…”

“…The fluid-solid interface is the (100) surface, and the vibrations of each solid atom are suppressed. The Lennard-Jones parameter for solid atoms is then set to 12.14 kcal/mol [2] in order to suppress boundary slip [3].…”

“…The molecular dynamic behavior of the oil film for EHL is not well-understood since the long trajectories of ensembles of a large number of fluid molecules are required to analyze the drastic phase transition induced by a high pressure. Typical practical EHL conditions include a film thickness on the order of a submicron and a shear rate less than 10 6 /s for a submicron thick film [2]. Although this film thickness exceeds the range of van der Waals structural forces of the solid plates, the behavior of the fluid layer differs from that of the bulk, i.e., the traction coefficients (the tangential traction force divided by the normal load) cannot be easily deduced from the bulk shear viscosity.…”

Structure of polyelectrolyte brushes studied by coarse grain simulations

“…Molecular simulations provide a suitable analytical tool to understand the soft nature of lubricants because ad-hoc parameters are not required, except for a set of interatom or molecular interactions. In a case of elastohydrodynamic lubrication, molecular dynamics simulations have been used to study the mechanism of momentum transfer related to the molecular structure of hydrocarbon fluids, the phase transition of the fluids under high pressure, and the submicron thickness behavior of an oil film [1]. For boundary lubrication, the mechanism behind the low friction of the diamond-like carbon containing silicon (DLC-Si) coating was understood by conducting a molecular dynamics study of an adsorbed boundary water film [2].…”