Study On The Lubrication Mechanism Of Hydrous Nano Oil Film Based On Molecular Dynamics Simulations

Lubricants are often used on various vehicles to reduce friction,mainly playing the role of lubrication,cooling,sealing,buffer and so on.The water mixed in lubricants plays a serious negative influence on the lubrication performance,and even can destroy the lubricated parts.Thus,the research of theoretical explanation for the properties of the hydrous oil film from microscopic has important significances for safe utilization of lubricants and improving the theoretical system of nano lubrication.In this paper,the non-equilibrium molecular dynamics simulation method was applied to build Couette flow models of anhydrous and hydrous nano oil films confined between two solid walls through CHARMM potential and SPC/E water molecule potential.Based on the theory of statistic mechanics,the simulation results were statistical calculated and analyzed.Firstly,the structural properties of the thin oil film were analyzed including the spatial distribution pattern of the molecules in the oil film,the velocity distribution of the oil film and the global orientational order parameter calculated by chain hydrocarbons in the oil film.Then the shear viscosity of the thin film which was regarded as a crucial lubrication property was analyzed and the theoretical explanation was proposed based on the structure properties.Finally,the influence of water molecules on the structure and lubrication properties of the oil film was obtained by comparing the two conditions of the anhydrous and hydrous nano oil film.Aiming at the anhydrous nano oil film,three kinds of thicknesses of oil films models(mono-layer,bi-layer and tri-layer)were built,and the shear velocities were set as 5 m/s,10 m/s,15 m/s,20 m/s and 25 m/s respectively,to research the effects of film thickness and shear rate on the properties of oil film.The results showed that,when the oil film was sheared,it showed a solid-like structure or layered structure,and with the increase of oil film thickness and shear rate,the oil film had a certain probability to change from a solid-like structure to a layered structure.The calculation results of global orientational order parameter of chain hydrocarbons in the oil film showed that the ordering of the structure of chain hydrocarbons was the dominate factor of forming a solid-like structure or layered structure.The oil film was a typical non-Newtonian fluid with obvious properties of thixotropic and pseudoplastic.Besides,the shear viscosity of the oil film tended to decrease with the increasing of the thickness of the oil film.Aiming at the hydrous nano oil film,the water cuts were set as 5%,10%,15%and 20%and the shear velocity was set as 25 m/s.The effects of film thickness,water cut and shear rate on the properties of hydrous oil film were researched.The results showed that,when the film thickness of the hydrous oil film was relatively thin,the oil film formed a layered structure due to the force of water molecules.However,when the film thickness increased,the effect of water molecules on the oil film decreased and the oil film formed a solid-like structure.With the increased water cut the number of regions containing discrete water molecules decreased,and the large area of continuous water phase region appeared.The shear viscosity of the thin liquid film increased with the increased water cut,which indicated that the inherent characteristic of pseudoplastic of the oil film was weakened by water molecules.And the shear viscosity of the thin liquid film was decreased with the increase of the shear time,which indicated that the characteristic of thixotropic of the oil film still existed.