A structural route for dynamical arrest: locally favoured structures in colloidal gels and glasses

The mechanism by which a liquid may become arrested, forming a glass or gel, is a long standing physical problem. While possible dynamic mechanisms have received considerable attention, direct experimental evidence of structural mechanisms has proved elusive. The connection between long-lived (energetically) locally favoured structures (LFS), whose geometry may prevent the system relaxing to its equilibrium state, and dynamical arrest dates back at least to F.C. Frank in the 1950s. In a similar spirit, we propose a much broader definition of LFS which we identify with a novel topological method and combine these with experiments at the single particle level on a colloidal liquid-gel transition, which we confirm with Brownian dynamics computer simulations. The population and lifetime of the LFS is a strong function of (effective) temperature in the ergodic liquid phase, rising sharply approaching dynamical arrest, and indeed the LFS form a percolating network which become the 'arms' of the gel. Due to the LFS, the gel is unable to reach equilibrium, crystal-gas coexistence. Our results form the first direct experimental observation of a link between local structure and dynamical arrest, and open a new perspective on a wide range of metastable materials.