Molecular dynamics of flow in micropores

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Abstract
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.

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