Entropic patchiness drives multi-phase coexistence in discotic colloid-depletant mixtures

TL;DR

This paper demonstrates that entropic patchiness in discotic colloid-depletant mixtures leads to complex multi-phase coexistence, including triple and quadruple phase regions, explained through free volume theory.

Contribution

It introduces a simple, generic free volume theory framework to understand entropic patchiness effects in anisotropic particle mixtures, explaining complex phase behaviors.

Findings

Mixtures of platelets and depletants show multi-phase coexistence including triple and quadruple regions.

Free volume theory effectively rationalizes experimental phase behaviors of anisotropic colloids.

Entropic patchiness induces orientation-dependent attractions, leading to rich phase diagrams.

Abstract

Entropy-driven equilibrium phase behaviour of hard particle dispersions can be understood from excluded volume arguments only. While monodisperse hard spheres only exhibit a fluid-solid phase transition, anisotropic hard particles such as rods, discs, cuboids or boards exhibit various multi-phase equilibria. Ordering of such anisotropic particles increases the free volume entropy by reducing the excluded volume between them. The addition of depletants gives rise to an entropic patchiness represented by orientation-dependent attraction resulting in non-trivial phase behaviour. We show that free volume theory is a simple, generic and tractable framework that enables to incorporate these effects and rationalise various experimental findings. Plate-shaped particles constitute the main building blocks of clays, asphaltenes and chromonic liquid crystals that find widespread use in the food, cosmetics and oil industry. We demonstrate that mixtures of platelets and ideal depletants exhibit a strikingly rich phase behaviour containing several types of three-phase coexistence areas and even a quadruple region with four coexisting phases.