Phase transitions in simple and not so simple binary fluids

TL;DR

This paper explores the complex phase behavior of binary fluid mixtures using a microscopic renormalization group approach, revealing how interaction parameters influence phase boundaries and transitions, including demixing and coexistence phenomena.

Contribution

It introduces a microscopic implementation of the renormalization group to analyze phase diagrams of simple binary mixtures, highlighting the impact of interaction strength on phase topology.

Findings

Interaction strength drastically alters phase boundary topology.

Homogeneous phase separation can be driven by repulsive interactions.

Demixing transition occurs at high density in soft particle models.

Abstract

Compared to pure fluids, binary mixtures display a very diverse phase behavior, which depends sensitively on the parameters of the microscopic potential. Here we investigate the phase diagrams of simple model mixtures by use of a microscopic implementation of the renormalization group technique. First, we consider a symmetric mixture with attractive interactions, possibly relevant for describing fluids of molecules with internal degrees of freedom. Despite the simplicity of the model, slightly tuning the strength of the interactions between unlike species drastically changes the topology of the phase boundary, forcing or inhibiting demixing, and brings about several interesting features such as double critical points, tricritical points, and coexistence domains enclosing `islands' of homogeneous, mixed fluid. Homogeneous phase separation in mixtures can be driven also by purely repulsive interactions. As an example, we consider a model of soft particles which has been adopted to describe binary polymer solutions. This is shown to display demixing (fluid-fluid) transition at sufficiently high density. The nature and the physical properties of the corresponding phase transition are investigated.