Vapour-deposited Colloidal Glass and the Microscopic Friction

Vapour deposition can directly form ultra-stable glasses which are similar to conventional glasses aged over thousands years after quenching from the liquid. It is believed that the highly mobile surface layer accelerates 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 with frequent out-of-cage motions, while particles in the deeper middle layer (14 < d < 100 particles) relaxed via activations of cooperative-rearrangement regions (CRRs). These CRRs were much larger, anisotropic and occurred more frequently than those in the bulk (d > 100 particles) or after deposition. The centers of masses of CRRs moved toward the surface, while particles in CRRs moved toward the bulk, resulting free-volume “bubbles” moving toward the surface and making the bulk glass more compact. Such two-step relaxation in two surface layers is distinct to the previously assumed relaxation in one surface mobile layer. The frictional dynamics of monolayer crystal on a tilted commensurate energy landscape was studied at single particle level using video microscope. For small tilting angle q < 0:08 rad, the crystal remains motionless due to a static frictional force. For an intermediate tilting angle 0:08 < q < 0:012 rad, pairs of kinks and antikinks started to nucleate and propagated along the symmetry direction of the crystal. Further increase of tilting angle q > 0:12 rad leads to the formation of clusters of kinks, within which the atoms sliding freely, which dramatically increased the system mobility. Our research illustrates a way of studying the Frankel-Kontorova model by using colloids.