Non-equilibrium molecular dynamics simulations of the transient Ludwig-Soret effect in a binary Lennard-Jones/spline mixture

Abstract

SummaryA binary isotope mixture of Lennard-Jones/spline particles at equilibrium was perturbed by a sudden change in the system's boundary temperatures. The system's response was determined by nonequilibrium molecular dynamics (NEMD). Three transient processes were studied: (1) The propagation of a pressure (shock) wave, (2) heat diffusivity and conduction, and (3) thermal diffusion (the LudwigSoret effect). These three processes occur at different time scales, which makes it possible to separate them in one single NEMD run. The system was studied in liquid, supercritical, and dense gas states with various forms and strengths of the thermal perturbation. The results show that heat was initially transported by two separate mechanisms; (1) heat diffusion as described by the transient heat equation and (2) as a consequence of a pressure wave. The pressure wave travelled faster than the speed of sound, generating a shock wave in the system. Local equilibrium was found in the transient phase, even with very strong perturbations and in the shock front. Although the mass separation due to the Ludwig-Soret effect developed much slower than the pressure and temperature fields in the system at large, it was found that the Soret coefficient could be accurately determined from the initial phase of the transient and close to the heat source. This opens the possibility of a new way to analyse results from transient experiments and thereby minimize effects of gravity and convection due to buoyancy.

Topical issue on non-isothermal transport in complex fluids

“…Indeed, during the IMT12 Conference there was a scientific session specifically devoted to molecular dynamic simulations. Such a research trend is also present in this topical issue, which includes a contribution by Hafskjold [13] who, using molecular dynamics, studies transient thermodiffusion in a binary Lennard-Jones fluid. In addition, Mutschler et al [14] present a CFD study of convection of a ternary mixture in a porous medium.…”

Ionic thermoelectric materials and devices

“…The ionic thermoelectric voltage can be combined with pyroelectric effect to enable fast response while keeping the stable response [120]. Theoretical studies also showing that the Soret coefficient could be accurately determined from the initial phase of the transient thermodiffusion [121,122].…”

A Study on Thermodiffusion in Ternary Liquid Mixtures Using Enhanced Molecular Dynamics Algorithm with Experimental Validation

“…Molecular dynamics is an excellent tool that can help us understand the thermodiffusive flows from the molecular perspective. Consequently, scientists in computational fields have tried to unfold the riddle of Soret effect mechanism in molecular level via molecular dynamics (MD) methods . MD techniques can be categorized into two main types: equilibrium‐molecular dynamics (EMD), and non‐equilibrium‐molecular dynamics NEMD .…”

“…Consequently, scientists in computational fields have tried to unfold the riddle of Soret effect mechanism in molecular level via molecular dynamics (MD) methods. [34][35][36][37][38] MD techniques can be categorized into two main types: equilibriummolecular dynamics (EMD), [39] and non-equilibrium-molecular dynamics NEMD. [40] EMD approaches anticipate the transport properties of a system through using the Green Kubo formula in the absence of any agitated fields wherein the dynamic properties are associated with the integral of autocorrelation of flow quantities.…”