Surface melting of a colloidal glass

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. The melting process in glasses is not fully understood. Experiments with colloidal glasses now show that during melting, a liquid film develops at the surface, below which a region forms with highly mobile particles. This surface glassy layer reflects the properties of the surface and the underlying bulk material.

Automated summary

Despite the technological relevance of glasses, there is still a lack of complete microscopic understanding. This study experimentally investigates the surface of a two-dimensional glass and reveals the formation of a liquid film on the surface during the melting process. Additionally, a region with highly mobile particles is found underneath the liquid film. This surface glassy layer may provide insights into the properties of thin glassy films used in various technical applications.