Phase behaviour of binary mixtures of diamagnetic colloidal platelets in an external magnetic field

TL;DR

This study uses density functional theory to explore how external magnetic fields influence phase coexistence and demixing in binary mixtures of diamagnetic colloidal platelets with different diameters, revealing complex phase behaviors including critical points and immiscibility loops.

Contribution

It provides a detailed theoretical analysis of phase behavior in binary platelet mixtures under magnetic fields, highlighting the effects of particle size ratio and field strength on phase coexistence and demixing.

Findings

Phase coexistence depends on particle size ratio and magnetic field strength.

Increasing field strength can eliminate phase coexistence regions.

Complex phase diagrams with critical points and immiscibility loops are identified.

Abstract

Using fundamental measure density functional theory we investigate paranematic-nematic and nematic-nematic phase coexistence in binary mixtures of circular platelets with vanishing thicknesses. An external magnetic field induces uniaxial alignment and acts on the platelets with a strength that is taken to scale with the platelet area. At particle diameter ratio lambda=1.5 the system displays paranematic-nematic coexistence. For lambda=2, demixing into two nematic states with different compositions also occurs, between an upper critical point and a paranematic-nematic-nematic triple point. Increasing the field strength leads to shrinking of the coexistence regions. At high enough field strength a closed loop of immiscibility is induced and phase coexistence vanishes at a double critical point above which the system is homogeneously nematic. For lambda=2.5, besides paranematic-nematic coexistence, there is nematic-nematic coexistence which persists and hence does not end in a critical point. The partial orientational order parameters along the binodals vary strongly with composition and connect smoothly for each species when closed loops of immiscibility are present in the corresponding phase diagram.