# Thermodynamic equilibrium of binary mixtures on curved surfaces

## TL;DR

This paper investigates how surface curvature influences the phase behavior of binary mixtures on closed surfaces, revealing curvature-dependent phenomena and stable inhomogeneous phases through theoretical modeling and numerical simulations.

## Contribution

It introduces a curvature-dependent free energy framework and a lattice-gas model to explain the impact of substrate curvature on binary mixture phase diagrams and equilibria.

## Key findings

- Curvature affects line tension and binodal concentrations.
- Phase diagrams split into critical sub-diagrams due to curvature.
- Stable inhomogeneous 'antimixed' phases can exist below critical temperature.

## Abstract

We study the global influence of curvature on the free energy landscape of two-dimensional binary mixtures confined on closed surfaces. Starting from a generic effective free energy, constructed on the basis of symmetry considerations and conservation laws, we identify several model-independent phenomena, such as a curvature-dependent line tension and local shifts in the binodal concentrations. To shed light on the origin of the phenomenological parameters appearing in the effective free energy, we further construct a lattice-gas model of binary mixtures on non-trivial substrates, based on the curved-space generalization of the two-dimensional Ising model. This allows us to decompose the interaction between the local concentration of the mixture and the substrate curvature into four distinct contributions, as a result of which the phase diagram splits into critical sub-diagrams. The resulting free energy landscape can admit, as stable equilibria, strongly inhomogeneous mixed phases, which we refer to as antimixed states below the critical temperature. We corroborate our semi-analytical findings with phase-field numerical simulations on realistic curved lattices. Despite this work being primarily motivated by recent experimental observations of multi-component lipid vesicles supported by colloidal scaffolds, our results are applicable to any binary mixture confined on closed surfaces of arbitrary geometry.