A combined molecular dynamics and Monte Carlo study of the approach towards phaseseparation in colloid–polymer mixtures

Abstract

A coarse-grained model for colloid–polymer mixtures is investigated whereboth colloids and polymer coils are represented as point-like particlesinteracting with spherically symmetric effective potentials. Colloid–colloidand colloid–polymer interactions are described by Weeks–Chandler–Andersenpotentials, while the polymer–polymer interaction is very soft, of strengthkBT/2 for maximum polymer–polymer overlap. This model can be efficiently simulated both byMonte Carlo and molecular dynamics methods, and its phase diagram closely resemblesthat of the well-known Asakura–Oosawa model. The static and dynamic properties of themodel are presented for systems at critical colloid density, varying the polymer density inthe one-phase region. Applying Lees-Edwards boundary conditions, colloid–polymermixtures exposed to shear deformation are considered, and the resulting anisotropy ofcorrelations is studied. Whereas for the considered shear rate, , radial distribution functions and static structure factors indicate only small structuralchanges under shear, an appropriate projection of these correlation functions onto sphericalharmonics is presented that allows us to directly quantify the structural anisotropies.However, the considered shear rate is probably not high enough to see anisotropies in staticstructure factors at small wavenumbers that have been predicted by Onuki and Kawasaki(1979 Ann. Phys. 121 456) for the critical behavior of systems under shear. Theanomalous dependence of the polymer’s self-diffusion constant on polymer density isreferred to the clustering of the colloid particles when approaching the criticalpoint.