Colloidal hard spheres: Triumphs, challenges, and mysteries

The simplicity of hard spheres as a model system is deceptive. Although the particles interact solely through volume exclusion, that nevertheless suffices for a wealth of static and dynamical phenomena to emerge, making the model an important target for achieving a comprehensive understanding of matter. In addition, while real colloidal suspensions are typically governed by complex interactions, Pusey and Van Megen [Nature 320, 340 (1986)] demonstrated that suitably tuned suspensions result in hard-sphere-like behavior, thus bringing a valuable experimental complement to the renowned theoretical model. Colloidal hard spheres are thus both a material in their own right and a platform upon which phenomena exhibited by simple materials can be explored in great detail. The various purposes enable a particular synergy between experiment, theory, and computer simulation. The extensive body of work on colloidal hard spheres, which ranges from their equilibrium properties, such as phase behavior, interfaces, and confinement, to some of the nonequilibrium phenomena they exhibit, such as sedimentation, glass formation, and nucleation, is reviewed here.