Surface Premelting and Melting of Colloidal Glasses

Abstract

The nature of liquid-to-glass transition is a major puzzle in science because little structural change causes drastic dynamic slowdown1. A similar challenge exists in glass-to-liquid transition, i.e. glass melting, but it has been poorly explored. In particular, the surface effects on glass are not well understood2, especially in melting3–6. Here, we assemble monolayer, bilayer and trilayer colloidal glasses composed of tunable attractive spheres by vapour deposition such that they are sufficiently stable to melt from the surface3, 5, in contrast to bulk melting of ordinary glasses3. The measured particles’ trajectories reveal that structural and dynamic parameters reach their bulk values at different depths, which define a surface liquid layer and an intermediate glassy layer. The thicknesses of both layers increase in power laws upon approaching the glass transition point, similar to surface liquid growth in crystal premelting. The power law exponents are robust for both layers under slow and fast temperature changes, and for monolayer and multilayer samples. The power-law temperature dependence of the structural parameters on the surface and their centrosymmetric profiles are similar to crystal premelting7. The profiles of dynamic parameters follow the prediction of the cooperative-string model for the surface mobile layer of glass8. Under the fast temperature change across the glass transition point, the centrosymmetric density profile and constant speed and width of the melting front are similar to crystal melting9. These results suggest that premelting and melting can be generalised from crystals to amorphous solids.

Surface melting of a colloidal glass

“…Similar to ξ dyn which is determined in the presence of a frozen interface and which characterizes the properties of the bulk glass 30 , the formation of a surface glassy layer during surface melting also reflects the properties of the underlying bulk material. Very recently, we became aware of another colloidal study which also observed that static and dynamic properties saturate at different depths near the surface of a glass 41 . Because in this study the attraction mechanism is different from ours, this strongly supports the idea the formation of surface glassy layers is expected to be a common phenomenon near the surface of glassy materials.…”