Critical behaviour and interfacial fluctuations in a phase-separating model colloid–polymermixture: grand canonical Monte Carlo simulations

Abstract

By using Monte Carlo simulations in the grand canonical ensemble we investigate the bulkphase behaviour of a model colloid–polymer mixture, the so-called Asakura–Oosawa model.In this model the colloids and polymers are considered as spheres with a hard-spherecolloid–colloid and colloid–polymer interaction and a zero interaction between polymers. Inorder to circumvent the problem of low acceptance rates for colloid insertions, we introducea cluster move where a cluster of polymers is replaced by a colloid. We consider thetransition from a colloid-poor to colloid-rich phase which is analogous to the gas–liquidtransition in simple liquids. Successive umbrella sampling, recently introducedby Virnau and Müller (2003 Preprint cond-mat/0306678), is used to access thephase-separated regime. We calculate the demixing binodal and the interfacialtension, also in the region close to the critical point. Finite size scaling techniquesare used to accurately locate the critical point. Also investigated are the colloiddensity profiles in the phase-separated regime. We extract the interfacial thicknessw from the latter profiles and demonstrate that the interfaces aresubject to spatial fluctuations that can be understood by capillary wavetheory. In particular, we find that, as predicted by capillary wave theory,w2 diverges logarithmically with the size of the system parallel to the interface.