Microscopic origins of the viscosity of a Lennard-Jones liquid

TL;DR

This study uses molecular dynamics simulations to evaluate two theoretical models explaining the microscopic origins of viscosity in Lennard-Jones liquids, finding that the free volume model aligns well with simulation data and can be parameterized from local structure.

Contribution

The paper demonstrates that the free volume model accurately describes the temperature dependence of viscosity and diffusion in Lennard-Jones liquids and links its parameters to local structural features.

Findings

Both models fit the simulation data across various conditions.

Parameters of the free volume model can be derived from local structural parameters.

The free volume model provides a credible microscopic explanation for liquid transport properties.

Abstract

Unlike crystalline solids or ideal gases, transport properties remain difficult to describe from a microscopic point of view in liquids, whose dynamics result from complex energetic and entropic contributions at the atomic scale. Two scenarios are generally proposed: one represents the dynamics in a fluid as a series of energy barrier crossings, leading to Arrhenius-like laws, while the other assumes that atoms rearrange themselves by collisions, as exemplified by the free volume model. To assess the validity of these two views, we computed, using molecular dynamics simulations, the transport properties of the Lennard-Jones fluid and tested to what extent the Arrhenius equation and the free volume model describe the temperature dependence of the viscosity and of the diffusion coefficient at fixed pressure. Although both models reproduce the simulation results over a wide range of pressure and temperature covering the liquid and supercritical states of the Lennard-Jones fluid, we found that the parameters of the free volume model can be estimated directly from local structural parameters, also obtained in the simulations. This consistency of the results gives more credibility to the free volume description of transport properties in liquids.