Glassy behavior in hard-sphere colloids

Hard spheres have long been the reference model system for studying nucleation and glass formation. The experimental realization of colloidal hard spheres paved the way for detailed investigations and comparison with current theories, including the Mode Coupling Theory (MCT) for the ideal glass transition. Early works by Pusey and van Megen put forward the evidence that hard spheres form a glass at a packing fraction ?~ 0.58--0.59. However, this view has not been always fully accepted and others claim that a glass transition of hard spheres below random close packing does not take place. One crucial ingredient that is often overlooked is the effect of polydispersity in size of the measured samples. I will discuss its role on the dynamics and interplay with crystallization from results of computer simulations. I will report on the findings obtained using a particle size distribution measured experimentally by transmission electron microscopy (TEM), show that polydispersity may often play a central role, giving rise to two populations of large and small particles with markedly different dynamics. Indeed, at high enough polydispersity, large particles undergo an ideal glass transition, compatible with MCT predictions, at a packing fraction close to 0.59 while the small particles remain mobile. In addition, I will also discuss recent experimental and numerical results on binary hard-sphere mixtures. We observe the emergence of anomalous, logarithmic relaxation of the small particles in the quasi-arrested matrix of the large ones at a critical value of the size ratio between large and small ones. While this is reminiscent of higher-order singularities predicted by MCT, it turns out that, in this case, the observed phenomenon arises from the competition between localisation due to confinement and caging, which can be interpreted in terms of a percolative framework of the voids outside the large particles matrix.