On the molecular mechanism of thermal diffusion in liquids

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Abstract
A recently developed non-equilibrium molecular dynamics algorithm for heat conduction is used to compute the thermal conductivity, thermal diffusion factor, and heat of transfer in binary Lennard-Jones mixtures. An internal energy flux is established with local source and sink terms for kinetic energy. Simulations of isotope mixtures covering a range of densities and mass ratios show that the lighter component prefers the hot side of the system at stationary state. This implies a positive thermal diffusion factor in the definition we have adopted here. The molecular basis for the Soret effect is studied by analysing the energy flux through the system. In all cases we found that there is a difference in the relative contributions when we compare the hot and cold sides of the system. The contribution from the lighter component is predominantly flux of kinetic energy, and this contribution increases from the cold to the hot side. The contribution from the heavier component is predominantly energy transfer through molecular interactions, and it increases from the hot to the cold side. This explains why the thermal diffusion factor is positive; heal is conducted more effectively through the system if the lighter component is enriched at the hot side. Even for very large heat fluxes, we find a linear or almost linear temperature profile through the system, and a constant thermal conductivity. The entropy production per unit volume and unit time increases from the hot to the cold side.