A model colloidal fluid with competing interactions: bulk and interfacial properties

TL;DR

This paper uses density functional theory to analyze a colloidal fluid with competing attractive and repulsive interactions, revealing complex phase behaviors, correlation decay patterns, and interfacial structures influenced by a nearby lambda transition.

Contribution

It introduces a detailed DFT analysis of a colloidal model with competing interactions, highlighting rich crossover behaviors and the impact of lambda transitions on structure and phase stability.

Findings

Rich crossover in correlation decay from monotonic to oscillatory.

Identification of a lambda-line indicating instability to density fluctuations.

Pronounced long-wavelength oscillations in interfacial density profiles near the lambda transition.

Abstract

Using a simple mean-field density functional theory theory (DFT), we investigate the structure and phase behaviour of a model colloidal fluid composed of particles interacting via a pair potential which has a hard core of diameter $\sigma$, is attractive Yukawa at intermediate separations and repulsive Yukawa at large separations. We analyse the form of the asymptotic decay of the bulk fluid correlation functions, comparing results from our DFT with those from the self consistent Ornstein-Zernike approximation (SCOZA). In both theories we find rich crossover behaviour, whereby the ultimate decay of correlation functions changes from monotonic to long-wavelength damped oscillatory decay on crossing certain lines in the phase diagram, or sometimes from oscillatory to oscillatory with a longer wavelength. For some choices of potential parameters we find, within the DFT, a $\lambda$-line at which the fluid becomes unstable with respect to periodic density fluctuations. SCOZA fails to yield solutions for state points near such a $\lambda$-line. The propensity to clustering of particles, which is reflected by the presence of a long wavelength $\gg \sigma$, slowly decaying oscillatory pair correlation function, and a structure factor that exhibits a very sharp maximum at small but non zero wavenumbers, is enhanced in states near the $\lambda$-line. We present density profiles for the planar liquid-gas interface and for fluids adsorbed at a planar hard wall. The presence of a nearby $\lambda$-transition gives rise to pronounced long-wavelength oscillations in the one-body densities at both types of interface.