Nonisothermal gravitational segregation by molecular dynamics simulations.

In this work, a molecular dynamics algorithm is proposed to study the transient and the stationary state of gravitational segregation in simple fluid mixtures. Both isothermal and stable nonisothermal (where thermodiffusion occurs) cases have been studied. This approach is applied extensively on a simple fluid model: Lennard-Jones mixtures composed of species differing only in their masses. First, using isothermal binary equimolar mixtures, it is shown that the molecular dynamics simulations provide stationary results consistent with the thermodynamic modeling in various thermodynamic conditions and for different gravity fields. Next, in stable nonisothermal mixtures heated from below, it is shown that the gravitational segregation and the thermodiffusion process (Soret effect) have an opposite effect on the concentration profiles along the fluid column. Then, molecular dynamics simulations are performed on ternary and ten-component mixtures. For these multicomponent nonisothermal mixtures, results obtained emphasize the fact that the way the thermodiffusion is estimated should be done with care. In addition, for all nonisothermal configurations, the simulation results confirm that the thermodiffusion may have a non-negligible influence on the concentration profile in a petroleum reservoir. Finally, by analyzing the transient behavior during the molecular dynamics simulations, it is shown that the dynamic of the gravitational segregation is unambiguously controlled by the mass diffusion.