Mixing-Demixing Phase Diagram for Simple Liquids in Non-Uniform Electric Fields

TL;DR

This paper derives a phase diagram for binary liquid mixtures in non-uniform electric fields, revealing how electric parameters influence phase separation and providing analytic tools for experimental guidance.

Contribution

It introduces analytic expressions for the phase boundaries and predicts the existence of electrostatic spinodal and binodal lines, advancing understanding of electric field effects on liquid mixtures.

Findings

Liquid separation is limited to a finite potential window.

Larger potentials can suppress phase separation, causing mixing.

The phase diagram depends strongly on dielectric relations.

Abstract

We deduce the mixing-demixing phase diagram for binary liquid mixtures in an electric field for various electrode geometries and arbitrary constitutive relation for the dielectric constant. By focusing on the behavior of the liquid-liquid interface, we produce simple analytic expressions for the dependence of the interface location on experimental parameters. We also show that the phase diagram contains regions where liquid separation cannot occur under any applied field. The analytic expression for the boundary "electrostatic binodal" line reveals that the regions' size and shape depend strongly on the dielectric relation between the liquids. Moreover, we predict the existence of an "electrostatic spinodal" line that identifies conditions where the liquids are in a metastable state. We finally construct the phase diagram for closed systems by mapping solutions onto those of an open system via an effective liquid composition. For closed systems at a fixed temperature and mixture composition, liquid separation occurs in a finite "window" of surface potential (or charge density). Larger potentials or charge densities counterintuitively destroy the interface, leading to liquid mixing. These results give valuable guides for experiments by providing easily testable predictions for how liquids behave in non-uniform electric fields.