A combined molecular dynamics and Monte Carlo study of the approach towards phase separation in colloid-polymer mixtures

Abstract

A coarse-grained model for colloid-polymer mixtures is investigated where both colloids and polymer coils are represented as point-like particles interacting with spherically symmetric effective potentials. Colloid-colloid and colloid-polymer interactions are described by Weeks-Chandler-Andersen potentials, while the polymer-polymer interaction is very soft, of strength k(B)T/2 for maximum polymer-polymer overlap. This model can be efficiently simulated both by Monte Carlo and molecular dynamics methods, and its phase diagram closely resembles that of the well-known Asakura-Oosawa model. The static and dynamic properties of the model are presented for systems at critical colloid density, varying the polymer density in the one-phase region. Applying Lees-Edwards boundary conditions, colloid-polymer mixtures exposed to shear deformation are considered, and the resulting anisotropy of correlations is studied. Whereas for the considered shear rate, (gamma) over dot = 0.1, radial distribution functions and static structure factors indicate only small structural changes under shear, an appropriate projection of these correlation functions onto spherical harmonics 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 static structure 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. The anomalous dependence of the polymer's self-diffusion constant on polymer density is referred to the clustering of the colloid particles when approaching the critical point.