Bulk Phase Properties Of Alkane Lubricating Oil Investigated By The Molecular Simulation

Lubrication is the effective measure to reduce and control the friction, and the viscosity of lubrication is an important factor to determine its lubrication performance. While, under the actual working condition, the viscosity of lubrication will be changed with the change of surrounding temperature and shear rate. So, understanding the changing rule and the change mechanism of the lubrication viscosity are of great importance. In this work, molecular dynamics simulations were adopted to study the shear thining behavior under shear flow field and viscosity-temperature characteristics with the change of temperature for n-hexadecane fluid, and the microscopic mechanism of such changes are also revealed.As the all-atomic potential model was used in this article for the first time, so the rationality of the potential energy model is verified. Results indicate that COMPASS potential energy model can well simulate the rheological behavior of n-hexadecane fluid, and the zero shear rate viscosity gained in this work is more accurate than the result gained with United Atoms(UA) potential energy model which is used before. Therefore, this article continues to use COMPASS potential energy model to study the bulk properties of lubrication.The results of shear thinned behavior showed that the viscosity of n-hexadecane fluid will keep constant over shear rate, which is called Newtonian fluid state, when the shear rate is small. When the shear rate exceeds the critical shear rate, the viscosity of n-hexadecane fluid will decrease with the rise of shear rate, which is called shear thining. The shearing thinning phenomenon is a result of molecules’ orientation arrangement along the flow direction, which cause the flow resistance reduced, so the viscosity of n-hexadecane fluid decreased. In addition, there is an entanglement structure in molecular fluid. Under high shear rate, the entanglement structure would be destroyed, which result the decline of viscosity. When disentanglement is started, the n-hexadecane molecule would become curled spontaneously, which lead to the reduction of radius of gyration and end-to-end distance. Meanwhile, the trans conFiguration of n-hexadecane molecule would become other conFigurations.The research results of viscosity-temperature characteristics show that the law of the change of viscosity with temperature for n-hexadecane fluid follows Arrhenius equation, and the viscosity increased with the rise of temperature. The viscosity-temperature characteristic of n-hexadecane fluid is related to the molecular motion ability, the space structure of fluid and the molecular flexibility.At higher temperatures, molecular kinetic energy increases, so that molecular can overcome energy barrier obstacles and start to flow. Furthermore, the rise of temperature will increase the free volume fraction of fluid, which indicates that fluid void structure become rich, so the jump and flow of molecules become more easily in the internal fluid. Finally, the molecular flexibility increased with the rise of temperature, and molecules can more easily adjust their conformation according to the change of fluid space structure, so that flow become more likely.