Surface melting of a colloidal glass

TL;DR

This study experimentally demonstrates surface melting in a two-dimensional colloidal glass, revealing a liquid-like surface layer and a highly mobile interfacial region that extends deep into the material, enhancing understanding of glass surfaces.

Contribution

It provides the first direct experimental observation of surface melting in colloidal glasses and uncovers a mobile interfacial layer with significant depth, advancing microscopic understanding of glass surfaces.

Findings

Surface melting occurs at the glass surface below the bulk glass transition temperature.

A highly mobile, percolating layer forms beneath the surface, extending tens of particle diameters.

The mobile layer's properties may explain behaviors of thin glassy films in applications.

Abstract

Despite their technological relevance, a full microscopic understanding of glasses is still lacking. This applies even more to their surfaces whose properties largely differ from that of the bulk material. Here, we experimentally investigate the surface of a two-dimensional glass as a function of the effective temperature. To yield a free surface, we use an attractive colloidal suspension of micron-sized particles interacting via tunable critical Casimir forces. Similar to crystals, we observe surface melting of the glass, i.e., the formation of a liquid film at the surface well below the glass temperature. Underneath, however, we find an unexpected region with bulk density but much faster particle dynamics. It results from connected clusters of highly mobile particles which are formed near the surface and deeply percolate into the underlying material. Because its thickness can reach several tens of particle diameters, this layer may elucidate the poorly understood properties of thin glassy films which find use in many technical applications