Nonequilibrium molecular dynamics simulations of confined fluids in contact with the bulk

A nonequilibrium molecular dynamics (MD) simulation study is reported of the structural and rheological properties of confined n-decane between two Au(111) surfaces in contact with its bulk under constant normal loads or constant heights. In the constant-load MD simulations, it was observed that fluid molecules were squeezed out of the pore continuously in a single simulation upon compression, whereas fluid molecules in the bulk were soaked into the pore when applied normal load was released. Pore separation depends on bulk pressure under the same normal load and approaches a steady value as normal load increases. In the constant-height simulations, density, velocity, and orientational profiles of the confined film were accumulated along the Z (perpendicular to the walls) and Y (parallel to the walls and finite due to the bulk) directions. These distributions are not uniform not only along the Z direction but also along the Y direction, particularly for weak fluid–wall interactions. The shear-thinning behavior and “slip” boundary conditions were also studied in this work. Even though the shear-thinning behavior was reported by several previous studies, the number of particles was fixed and the bulk condition was unknown in those simulations. The simulation geometry employed in this work is closer to that of surface-force apparatus (SFA) experiments and of engine lubricating systems where confined liquid is in contact with its bulk.