Structure and dynamics of binary liquid mixtures near their continuous demixing transitions

TL;DR

This study computationally investigates the static and dynamic critical behavior of binary Lennard-Jones liquid mixtures near their demixing points, confirming universality class predictions and analyzing critical amplitudes and ratios.

Contribution

It provides comprehensive simulation data on static and dynamic critical phenomena in binary mixtures near demixing, validating theoretical universality class predictions and calculating critical amplitude ratios.

Findings

Static critical amplitudes agree with 3D Ising universality class.

Calculated universal amplitude ratios match theoretical and experimental values.

Evidence of inverse proportionality between pressure and compressibility at demixing.

Abstract

The dynamic and static critical behavior of five binary Lennard-Jones liquid mixtures, close to their continuous demixing points (belonging to the so-called model H' dynamic universality class), are studied computationally by combining semi-grand canonical Monte Carlo simulations and large-scale molecular dynamics (MD) simulations, accelerated by graphic processing units (GPU). The symmetric binary liquid mixtures considered cover a variety of densities, a wide range of compressibilities, and various interactions between the unlike particles. The static quantities studied here encompass the bulk phase diagram (including both the binodal and the $\lambda$-line), the correlation length, the concentration susceptibility, the compressibility of the finite-sized systems at the bulk critical temperature $T_c$, and the pressure. Concerning the collective transport properties, we focus on the Onsager coefficient and the shear viscosity. The critical power-law singularities of these quantities are analyzed in the mixed phase (above $T_c$) and non-universal critical amplitudes are extracted. Two universal amplitude ratios are calculated. The first one involves static amplitudes only and agrees well with the expectations for the three-dimensional Ising universality class. The second ratio includes also dynamic critical amplitudes and is related to the Einstein--Kawasaki relation for the interdiffusion constant. Precise estimates of this amplitude ratio are difficult to obtain from MD simulations, but within the error bars our results are compatible with theoretical predictions and experimental values for model H'. Evidence is reported for an inverse proportionality of the pressure and the isothermal compressibility at the demixing transition, upon varying either the number density or the repulsion strength between unlike particles.