Crossover from facilitation to hopping in a colloidal glass-former

Despite extensive research, it remains to be established whether glass formation is a fundamentally thermodynamic or dynamic phenomenon. In particular, it is not yet clear whether structural relaxation is dominated by the correlated motion of localized excitations, as postulated by the dynamical facilitation (DF) theory, or by the collective hopping of groups of particles, as envisioned by various thermodynamic approaches. Here, by analyzing data from experiments on dense colloidal suspensions, we critically compare the role of facilitation and hopping in governing structural relaxation in glass-forming liquids. In particular, we investigate the spatial organization of localized excitations within clusters of most mobile particles as well as their partitioning into shell-like and core-like regions. Our study reveals the existence of a dynamical crossover from a facilitation dominated regime at low area fractions to one dominated by collective hopping close to the glass transition. Our findings strongly suggest that glass formation is thermodynamic in origin.