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Abstract
Vapor 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 vapor-deposited glasses, but its microscopic kinetics have not been experimentally observed. Here we study the deposition growth kinetics of a two-dimensional colloidal glass at the single-particle level using video microscopy. We observe that newly deposited particles in the surface layer (depth, d < 14 particles) relax via out-of-cage diffusions of individual particles, while particles in the deeper middle layer (14 < d ≲ 100 particles) relax via activation of cooperative-rearrangement regions. These cooperative-rearrangement regions are much larger, more anisotropic and occur more frequently than cooperative-rearrangement regions in the bulk (d ≳ 100 particles) or after deposition. Cooperative-rearrangement regions move towards the surface and released free-volume bubbles at the surface, while the particles within cooperative-rearrangement regions move towards the bulk, resulting in a more compact bulk glass.
Highlights
Vapor deposition can directly produce ultrastable glasses which are similar to conventional glasses aged over thousands of years
Colloids are outstanding model systems for the study of glasses because the real-space trajectories of individual particles can be measured by video microscopy[18, 19]
The newly deposited particles underwent frequent out-of-cage motion in the surface layer until they were buried into the bulk, which experimentally confirms the efficient relaxations via a surface mobile layer in the deposition growth of glasses[1, 15, 16]
Summary
Vapor deposition can directly produce ultrastable glasses which are similar to conventional glasses aged over thousands of years. Vapor deposition can produce organic, polymeric and metallic glasses with extraordinary kinetic stability[1,2,3,4,5] Such ultrastable glasses can have highly uniform amorphous structures[2], unusually high[1, 2, 4] densities, enhanced elastic moduli[2, 3, 6] and highly anisotropic molecular orientations[5]. These properties are of significant interest in both practical material design and the theoretical understanding of the nature of glass transition Experimental techniques such as differential scanning calorimetry[1], neutron reflectivity[1], dielectric measurements[7], spectroscopic ellipsometry[5], and wide-angle X-ray scattering[8] have been applied to study vapor-deposited glasses. These CRRs propagated to the free surface, releasing free volumes to the vapor phase to give a more compact deposited glass
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