Microscopic dynamics of fluids confined between smooth and atomically structured solid surfaces

Abstract

Molecular dynamics and the grand canonical Monte Carlo techniques are employed to simulate the structure and dynamics of a fluid in a slit micropore at equilibrium and under Couette flow. Calculated quantities include the fluid’s density profiles, pair correlation functions, diffusion coefficients, normal pressure, stress tensor, and velocity profiles. Simulation results for fluids in equilibrium with the same bulk fluid, but confined by either atomically smooth or structured face centered cubic pore walls are compared. At the conditions considered, fluid in the contact layer next to structured walls exhibits enhanced fluid order which is not altered by flow for pores capable of accommodating two fully developed fluid layers (i.e., for pores wider than 2.5 molecular diameters across). At narrower pore widths, the equilibrium fluid structure is changed by flow and the fluid is more sensitive to shear-induced changes in the diffusivity and normal pressure. The layer average density profiles of the confined fluid are similar in the structured and smooth pores. However, in the fluid layers adjacent to the structured pore walls, the local density distributions are ordered with a periodicity reflecting that of the atomic structure of the pore walls. At the strongest fluid–wall interaction considered, the confined fluid does not solidify although the diffusion coefficient of the fluid confined by the structured walls is reduced by as much as a factor of 87 compared to the bulk phase.