Simulations of Lubricant Behavior at the Interface with Bearing Solids

Molecular dynamics simulations are used to study how solid walls affect the structure and dynamics of a contacting fluid. We first describe the changes in structure induced by a single solid wall. Both layering and in-plane order are found within 4 to 10 molecular diameters of the solid surface. The degree of in-plane order is directly correlated with the transfer of shear stress, and thus with the effective boundary condition for fluid flow. A range of boundary conditions from partial slip to entrainment of fluid layers is found. We next consider fluid films confined between two walls. As the film thickness decreases, the ordering influences of the two walls begin to overlap. This may ultimately lead to a phase transition in the film. Spherical molecules crystallize, while chain molecules enter a glassy state. The onset of the glass transition is heralded by dramatic increases in viscosity and by universal non-Newtonian behavior. Crystalline or glassy films have a non-zero yield stress. When this is exceeded the film undergoes a transition to a fluid state and shears easily. This releases stress in the film, which may then refreeze. We show that this is the origin of stick-slip motion at low velocities. © 1993, Elsevier Science & Technology.