Topological lifetimes of polydisperse colloidal hard spheres at a wall

Confocal scanning laser microscopy was used to study the behavior of dense suspensions of model colloidal hard spheres at a single wall. Due to the slight polydispersity, our system shows a reentrant melting transition at high densities involving a hexatic structure [Phys. Rev. Lett 92, 195702 (2004)]]. The reentrant melting transition is accompanied by an increase in the mean-squared displacement. The correlation between structure and dynamics was quantitatively analyzed on a single-particle level. In particular, the topological lifetime, being the average time that a particle spends having the same coordination number, is determined for all coordination numbers and as a function of volume fraction. The defective (non-sixfold-coordinated) particles exhibit shorter lifetimes than sixfold-coordinated particles, indicating that the mobility of the system is larger at or close to defective particles. The lifetime itself is a strong function of volume fraction. In particular, the global behavior of the mean-squared displacement is proportional to the hopping frequency (the inverse of the lifetime), showing that particles changing their coordination number contribute most to the local mobility.