Phase behavior of bidisperse ferrocolloids

Abstract

The phase behavior of bidisperse ferrocolloids consisting of binary mixtures of dipolar hard spheres (DHS's) with different particle diameters and different dipole moments is investigated using density-functional theory in a modified mean-field approximation. We focus on the fluid phase regime, where we consider both isotropic and anisotropic states. Depending on the parameter Gamma -measuring the asymmetry of the dipolar couplings-the systems display complex fluid-fluid phase behavior involving demixing transitions, as well as first- and second-order isotropic-to-ferromagnetic phase transitions. The topology of the resulting phase diagrams turns out to be similar to those corresponding to monodisperse DHS mixtures investigated previously by us [Phys. Rev. E 69, 041201 (2004)]]. However, additional size asymmetry has a strong impact on the relative importance of the various types of phase transitions. In particular, the demixing transition of bidisperse ferrocolloids is strongly destabilized compared to that of monodisperse DHS's in the sense that demixing critical points are significantly shifted towards lower temperatures.