Colloidal suspensions driven by external fields

Colloidal suspensions have been proven to play a pivotal role of model systems in order to understand the principles of equilibrium phase transitions such as freezing and fluid-fluid demixing. One of the main reasons for that is that real-space studies are possible thanks to the mesoscopic length scale of the particle size. The same model character of colloidal suspensions holds in non-equilibrium situations as e.g. represented by an external driving field (such as shear, gravity, an electric and/or magnetic field). In this paper some current examples of non-equilibrium transitions are reviewed where recent progress has been made by theory and computer simulation. In particular, we discuss the competition between phase separation and lane formation in driven colloidal mixtures, crystal nucleation in charged suspensions under shear and chain formation of two-dimensional superparamagnetic suspensions induced by an external magnetic field.