Model system for classical fluids out of equilibrium

TL;DR

This paper introduces a DPD solid model for classical fluids out of equilibrium, analyzing heat transport via analytical and simulation methods, revealing a density-dependent diffusivity and a conductivity threshold linked to percolation phenomena.

Contribution

The study develops a comprehensive model combining collisional and kinetic transport mechanisms, extending kinetic theory to the generalized hydrodynamic regime, and elucidates the percolation-based conductivity threshold.

Findings

Heat diffusivity scales linearly with density at high densities.

A conductivity threshold exists at low densities due to percolation effects.

The model accurately describes transport properties across the full density range.

Abstract

A model system for classical fluids out of equilibrium, referred to as DPD solid (Dissipative Particles Dynamics), is studied by analytical and simulation methods. The time evolution of a DPD particle is described by a fluctuating heat equation. This DPD solid with transport based on collisional transfer (high density mechanism) is complementary to the Lorentz gas with only kinetic transport (low density mechanism). Combination of both models covers the qualitative behavior of transport properties of classical fluids over the full density range. The heat diffusivity is calculated using a mean field theory, leading to a linear density dependence of this transport coefficient, which is exact at high densities. Subleading density corrections are obtained as well. At lower densities the model has a conductivity threshold below which heat conduction is absent. The observed threshold is explained in terms of percolation diffusion on a random proximity network. The geometrical structure of this network is the same as in continuum percolation of completely overlapping spheres, but the dynamics on this network differs from continuum percolation diffusion. Furthermore, the kinetic theory for DPD is extended to the generalized hydrodynamic regime, where the wave number dependent decay rates of the Fourier modes of the energy and temperature fields are calculated.