Microphase structuring in two-dimensional magnetic colloid mixtures

Suspensions of super-paramagnetic colloids trapped in the water - air interface are an excellent model system for the study of two-dimensional systems. Here, we investigate the structural behaviour of two-component mixtures of such particles. The interparticle interactions are tunable through the application of a magnetic field perpendicular to the air - water surface. Further, they can be influenced by the choice of the relative magnetic susceptibilities of the two colloidal species. We have performed integral equation theory and computer simulations to study the static properties of such a binary superparamagnetic system. For all susceptibility ratios studied, nomacroscopic phase separation takes place; the fluid phase remains macroscopically homogeneous but microphase structuring occurs: the interactions with the large particles lead to a clustering of the smaller particles. We combine structural information in reciprocal space together with morphological measures in real space to characterize the ordering of the two species in the binary mixture.