Molecular dynamics studies are reported for a 6-12 Lennard–Jones liquid in pore channels ranging from about 2–12 molecules wide. The pore walls are modeled as flat surfaces interacting with the fluid molecules via a continuous potential varying only with perpendicular distance from the wall. Liquid density profiles, solvation forces, interfacial tensions, and self-diffusion coefficients along the pore axis were computed. The density profiles indicate multilayer adsorption in the pore, whereas the locally defined diffusion coefficients do not vary significantly across the pore. The pore-averaged diffusivity as well as the solvation force oscillate with varying pore width at constant chemical potential. For pore widths greater than ten molecular diameters, the average diffusion coefficient is almost equal to its bulk value, and the solvation force equals the bulk pressure. In the smaller pores the mean square displacement normal to the pore walls never achieves linearity in time, and thus does not reach a diffusive limit. Thermodynamic equations relating the solvation force to the interfacial tension are derived, and the appropriate mechanical expressions for these quantities are identified. Simulation results are shown to be consistent with these thermodynamic equations. The simulations presented here will be useful in the development of the theory of fluid structure and transport in the tight pores occurring in such materials as vicor glass, clay dispersions, and biological pores and membranes.

The method of nonequilibrium molecular dynamics is used to study the viscosity and flow properties of strongly inhomogeneous liquids, a particular case of which is a liquid confined in a micropore only a few molecular diameters wide. Fluid inhomogeneity is introduced by imposing an external potential that in one case simulates flat solid walls and in the other case causes density peaks in the middle of a thin liquid film. For comparison a homogeneous fluid is also simulated. In both types of inhomogeneous fluid, the shear stress and effective viscosity are smaller than in the homogeneous fluid. The density profiles and the diffusivities in the micropore were found to be independent of flow, even at the extremely high rates, 10¹⁰–10¹¹ s⁻¹ of the simulation. The Green–Kubo relation is found to be valid for the diffusivity under the flow studied. We propose a local average density model (LADM) of viscosity and diffusivity, in which the local transport coefficients are those of homogeneous fluid at a mean density obtained by averaging the local density over a molecular volume. LADM predicts qualitatively correct velocity profiles, effective viscosities, and shear stresses using only equilibrium density profiles and molecular diameters. An analogous local equilibrium version of Enskog’s theory of diffusivity agrees well with the simulated pore diffusivities. Recently Vanderlick and Davis generalized Enskog’s theory of diffusivity to strongly inhomogeneous fluids. Their theoretical pore diffusion coefficient is also in good agreement with simulation results.