Roles of icosahedral and crystal-like order in the hard spheres glass transition

A link between structural ordering and slow dynamics has recently attracted much attention from the context of the origin of glassy slow dynamics. Candidates for such structural order are icosahedral, exotic amorphous and crystal-like. Each type of order is linked to a different scenario of glass transition. Here we experimentally access local structural order in polydisperse hard spheres by particle-level confocal microscopy. We identify the key structures as icosahedral and FCC-like order, both statistically associated with slow particles. However, when approaching the glass transition, the icosahedral order does not grow in size, whereas crystal-like order grows. It is the latter that governs the dynamics and is linked to dynamic heterogeneity. This questions the direct role of the local icosahedral ordering in glassy slow dynamics and suggests that the growing length scale of structural order is essential for the slowing down of dynamics and the non-local cooperativity in particle motion. The dynamics and structure of the glass transition in liquids is still debated. Using particle-level confocal microscopy, Leocmach and Tanaka investigate supercooled colloidal liquids and distinguish different scenarios for glassy slow dynamics, suggesting that local ordering may only play a minor role.