Dynamic regimes of fluids simulated by multiparticle-collision dynamics

TL;DR

This paper explores the hydrodynamic behavior of fluids simulated with a mesoscopic model, identifying particle-like and fluid-like regimes, and examines how diffusion properties vary with particle interactions and concentrations.

Contribution

It introduces a detailed analysis of the regimes in multiparticle-collision dynamics and extends the model to study colloidal self-diffusion considering hydrodynamic correlations.

Findings

Analytical diffusion expressions fit well in the particle regime.

Deviations due to hydrodynamic correlations are significant in the collective regime.

Results align with Stokes and Smoluchowski theories in colloidal dispersions.

Abstract

We investigate the hydrodynamic properties of a fluid simulated with a mesoscopic solvent model. Two distinct regimes are identified, the `particle regime' in which the dynamics is gas-like, and the `collective regime' where the dynamics is fluid-like. This behavior can be characterized by the Schmidt number, which measures the ratio between viscous and diffusive transport. Analytical expressions for the tracer diffusion coefficient, which have been derived on the basis of a molecular-chaos assumption, are found to describe the simulation data very well in the particle regime, but important deviations are found in the collective regime. These deviations are due to hydrodynamic correlations. The model is then extended in order to investigate self-diffusion in colloidal dispersions. We study first the transport properties of heavy point-like particles in the mesoscopic solvent, as a function of their mass and number density. Second, we introduce excluded-volume interactions among the colloidal particles and determine the dependence of the diffusion coefficient on the colloidal volume fraction for different solvent mean-free paths. In the collective regime, the results are found to be in good agreement with previous theoretical predictions based on Stokes hydrodynamics and the Smoluchowski equation.