Transport properties of highly asymmetric hard sphere mixtures

TL;DR

This study investigates the static and dynamic properties of highly asymmetric binary hard sphere mixtures using molecular dynamics, comparing simulation results with theoretical predictions, and revealing a 'fines effect' on viscosity at high packing fractions.

Contribution

It provides detailed simulation data on asymmetric mixtures and evaluates the accuracy of revised Enskog theory, highlighting its limitations in certain conditions.

Findings

Enskog theory generally agrees with simulation data.

A 'fines effect' reduces viscosity when adding smaller spheres at high packing fractions.

Contact values of pair correlation functions match recent theoretical expressions.

Abstract

The static and dynamic properties of binary mixtures of hard spheres with a diameter ratio of $\sigma_B/\sigma_A=0.1$ and a mass ratio of $m_B/m_A=0.001$ are investigated using event driven molecular dynamics. The contact value of the pair correlation functions are found to compare favourably with recently proposed theoretical expressions. The transport coefficients of the mixture, determined from simulation, are compared to the predictions of revised Enskog theory, using both a third-order Sonine expansion and direct simulation Monte Carlo. Overall, Enskog theory provides a fairly good description of the simulation data, with the exception of systems at the smallest mole fraction of larger spheres ($x_A=0.01$) examined. A "fines effect" was observed at higher packing fractions, where adding smaller spheres to a system of large spheres decreases the viscosity of the mixture; this effect is not captured by Enskog theory.