Surface relaxation of vapour-depositing colloidal glasses

Abstract

Vapour deposition can directly produce ultrastable glasses, which are similar to conventional glasses aged over thousands of years. The highly mobile surface layer is believed to accelerate the ageing process of vapour-deposited glasses, but its microscopic kinetics has not been experimentally observed. Here we studied the deposition growth kinetics of a two-dimensional colloidal glass at the single-particle level using video microscopy. We found that newly deposited particles in the surface layer (depth $d<14$ particles) relaxed via frequent out-of-cage motions, while particles in the deeper middle layer ($14<d\lesssim 100$ particles) relaxed via activation of cooperative rearrangement regions (CRRs). These CRRs were much larger, more anisotropic and occurred more frequently than CRRs in the bulk ($d\gtrsim 100$ particles) or after deposition. Their centers of mass moved towards the surface, while the particles within moved towards the bulk, causing free-volume "bubbles" to move towards the surface to give a more compact bulk glass. This two-step relaxation in two surface layers is distinct from the previously assumed relaxation in one surface mobile layer

arXiv comment14 pages

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