Simulation of lubricant rheology in thin film lubrication Part I: simulation of Poiseuille flow

Abstract

In this two part study, molecular dynamics simulation of Poiseuille and Couette flow has been performed for a fluid of spherical molecules to examine its rheological properties in thin film lubrication. As the first part of the study, the present paper describes the method and geometry of the simulation and presents simulation results of Poiseuille flow. The results show that the equivalent viscosity of confined fluid increases and goes toward divergence as the two walls are brought close together continuously. The phase transition pressure is found declining with diminishing film thickness, which implies that the lubricant in thin films may undergo a confinement-induced phase transition at much lower pressure than in the bulk. As the film becomes thicker, the transition pressure rises and tends to an asymptotic value—the bulk transition pressure of the fluid. The spatial probability distributions of fluid molecules show the in-plane order formed near solid walls. The ordering structure originates from the wall-fluid interface and grows towards the middle of the film as the system pressure increases. It is concluded that the rheological performance of lubricant may deviate from its bulk behaviour and become film thickness dependent if the film is molecularly thin, and lubricant solidification may be induced in many tribological systems with such a thin film.