| This thesis discusses several problems where microscopic phenomena at solid-fluid interfaces control macroscopic behavior. Continuum theories have been unable to address such phenomena. The difficulties inherent in such approaches were circumvented through non-equilibrium molecular dynamics simulations of simple fluids. Velocity fields, boundary conditions, and fluid structure were studied for both single and two fluid systems confined between solid walls.; For single fluid systems, a wide variety of boundary conditions was observed including slip, no-slip, and locking. The degree of slip was directly correlated with the amount of structure induced in the fluid by the adjacent solid. Strong wall-fluid couplings and equal wall and fluid densities induced epitaxial ordering in the first layers, and led to locked boundary conditions. For weaker couplings and unequal densities, there was little order and slip boundary conditions were observed.; The ordering tendency was enhanced when the wall separation decreased below approximately 5 molecular spacings. In this regime, crystalline order could be induced across the entire film, and a generic stick-slip dynamics occurred when the walls were sheared. The simulations revealed that stick-slip motion involves periodic shear-melting and recrystallization of the film. The origin of stick-slip motion is the thermodynamic instability of the sliding state rather than a dynamic instability, as usually assumed. The results clarify many details of recent boundary lubrication experiments.; The two fluid simulations shed new light on the spreading of fluids on solid surfaces. Conventional hydrodynamics with the no-slip boundary condition predicts a divergent energy dissipation when fluids spread. Simulations presented here indicate that this singularity is removed by slip. The slip occurs within about 2 molecular spacings of the contact line and appears to be associated with a breakdown of local hydrodynamics at atomic scales. Outside the slip region, changes with capillary number in the interface shape were consistent with macroscopic theory and experiment. Simulations also revealed that the amount of dissipation near the contact line is highly sensitive to the amount of structure induced in the fluid by the periodic potential of the wall. |
