Bulk phase behaviour of binary hard platelet mixtures from density functional theory

TL;DR

This study uses density functional theory to analyze phase behavior in binary mixtures of circular platelets, revealing how size ratios influence phase coexistence and demixing, with comparisons to Onsager theory.

Contribution

It introduces a fundamental measure density functional theory for binary platelet mixtures and compares its predictions with Onsager theory, highlighting differences in phase transition predictions.

Findings

Isotropic-nematic coexistence widens with increasing size ratio.

Nematic-nematic demixing becomes stable for size ratios ≥ 2.

Fundamental measure theory predicts smaller biphasic regions than Onsager theory.

Abstract

We investigate isotropic-isotropic, isotropic-nematic and nematic-nematic phase coexistence in binary mixtures of circular platelets with vanishing thickness, continuous rotational degrees of freedom and radial size ratios $\lambda$ up to 5. A fundamental measure density functional theory, previously used for the one-component model, is proposed and results are compared against those from Onsager theory as a benchmark. For $\lambda \leq 1.7$ the system displays isotropic-nematic phase coexistence with a widening of the biphasic region for increasing values of $\lambda$. For size ratios $\lambda \geq 2$, we find demixing into two nematic states becomes stable and an isotropic-nematic-nematic triple point can occur. Fundamental measure theory gives a smaller isotropic-nematic biphasic region than Onsager theory and locates the transition at lower densities. Furthermore, nematic-nematic demixing occurs over a larger range of compositions at a given value of $\lambda$ than found in Onsager theory. Both theories predict the same topologies of the phase diagrams. The partial nematic order parameters vary strongly with composition and indicate that the larger particles are more strongly ordered than the smaller particles.