On the nature of Thermal Diffusion in binary Lennard-Jones liquids

TL;DR

This study uses reverse nonequilibrium molecular dynamics to systematically analyze how mass, size, and interaction strength ratios influence thermal diffusion (Soret effect) in binary Lennard-Jones liquids near the triple point.

Contribution

It provides a detailed microscopic understanding of the factors affecting the Soret coefficient and demonstrates the nearly additive influence of different parameters, highlighting mass ratio as dominant.

Findings

Soret coefficient magnitude depends on mass, size, and interaction strength ratios.

Heavier, smaller, or strongly interacting species tend to migrate to colder regions.

Mass ratio is often the most influential factor in thermal diffusion behavior.

Abstract

The aim of this study is to understand deeper the thermal diffusion transport process (Ludwig-Soret effect) at the microscopic level. For that purpose, the recently developed reverse nonequilibrium molecular dynamics method was used to calculate Soret coefficients of various systems in a systematic fashion. We studied binary Lennard-Jones (LJ) fluids near the triple point (of one of the components) in which we separately changed the ratio of one of the LJ parameters mass, atomic diameter and interaction strength while keeping all other parameters fixed and identical. We observed that the magnitude of the Soret coefficient depends on all three ratios. Concerning its sign we found that heavier species, smaller species and species with higher interaction strengths tend to accumulate in the cold region whereas the other ones (lighter, bigger or weaker bound) migrate to the hot region of our simulation cell. Additionally, the superposition of the influence of the various parameters was investigated as well as more realistic mixtures. We found that in the experimentally relevant parameter range the contributions are nearly additive and that the mass ratio often is the dominating factor.