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Molecular dynamics simulation for the Soret effect of air near the critical point
Seiji Fujiwara, Seichi Tanaka and Kanji Kunimine
Supercritical fluid sometimes shows unusual singular behavior such as the inverse Soret effect. Non-equilibrium molecular dynamics simulations have been performed for artificial air near the critical point in order to examine behavior of molecule diffusion. Classical molecular dynamics was applied to reduce the computational load. At first, Lennard-Jones 6–12 potential for spherical atoms was adopted as intermolecular potential of a diatomic molecule such as nitrogen and oxygen. Thermal diffusion ratio kT was used to evaluate the Soret effect. Simulation results, however, show the general Soret effect and the inverse Soret effect does not observed. Furthermore, the simulation results of thermal diffusion rate agree well with referenced predictive equation for general gas. It is known that density fluctuation of fluid at the critical point is caused by long-range interaction of molecules. Therefore, simulations of large cut off length of intermolecular potential are performed. However, the inverse Soret effect was not observed. Moreover, in order to investigate the influence of rotational motion of rod-like molecule, the simulations using two-center Lennard-Jones potential for polyatomic molecule were carried out. Nevertheless, the inverse Soret effect did not occurred. These simulation results show that the classical molecular dynamics can not mimic the inverse Soret effect of air near the critical point.
Keywords: Molecular dynamics, Soret effect, thermal diffusion ratio, air, critical point, two-center Lennard-Jones potential