Molecular dynamics simulation of thermodiffusion and mass diffusion in structureless and atomistic micropores

Abstract

In this work, we have studied the effect of surface roughness on thermodiffusion in simple “isotopic” mixtures confined in a slit nanopore. To do so, we have performed non-equilibrium molecular dynamics simulations of Lennard–Jones binary equimolar mixtures confined in structureless (in which the interaction with the fluid is described by a Lennard–Jones 9–3 potential) and atomistic walls for various widths, from 5 to 35 times the size of a molecule, in the NP//T ensemble. For that purpose, a new algorithm is proposed in atomistic pore. Different super-critical conditions have been explored, ranging from low to moderate densities. In addition to the thermal diffusion factor, we have also estimated the mass diffusion and thermodiffusion coefficients separately. The results show that the two types of walls lead to noticeably different results. The thermal diffusion factor tends to increase in atomistic wall and slightly decrease in structureless wall when the pore width is decreasing, this being related to the average density behaviour. More precisely, both mass and thermodiffusion coefficients are weakly affected by the pore width for structureless walls, whereas both quantities largely decrease (up to 70% and 55% respectively compared to bulk fluid) when pore size decreases in the case of a rough solid surface because of the friction on the walls.