Two-dimensional fluid with competing interactions exhibiting microphase separation: theory for bulk and interfacial properties

TL;DR

This paper develops a density functional theory for a two-dimensional colloidal fluid with competing interactions, predicting various microphase separated structures and analyzing their bulk and confined behaviors.

Contribution

It introduces a simple theoretical model that captures the phase behavior and microstructure of 2D fluids with competing attractions and repulsions, aligning qualitatively with simulations.

Findings

Predicts cluster, stripe, and bubble phases in the bulk.

Provides approximate expressions for structure factors and modulation lengths.

Shows complex phase behavior under confinement.

Abstract

Colloidal particles that are confined to an interface such as the air-water interface are an example of a two-dimensional fluid. Such dispersions have been observed to spontaneously form cluster and stripe morphologies in certain systems with isotropic pair potentials between the particles, due to the fact that the pair interaction between the colloids has competing attraction and repulsion over different length scales. Here we present a simple density functional theory for a model of such a two-dimensional fluid. The theory predicts a bulk phase diagram exhibiting cluster, stripe and bubble modulated phases, in addition to homogeneous fluid phases. Comparing with simulation results for this model from the literature, we find that the theory is qualitatively reliable. The model allows for a detailed investigation of the structure of the fluid and we are able to obtain simple approximate expressions for the static structure factor and for the length scale characterising the modulations in the microphase separated phases. We also investigate the behaviour of the system under confinement between two parallel hard walls. We find that the confined fluid phase behaviour can be rather complex.