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Glasses are non-equilibrium amorphous solids with liquid-like structure and solid-like strength. Unlike the ordered crystalline phase, glasses are spatially homogeneous from macro to nano scale. These characteristics are at the basis of the importance of these materials in modern technologies, from the well-known windows glasses, to metallic glasses used in transformers, but also in fibre optics for fast data transmission or in thin films used as organic diodes for the fabrication of screens for electronic devices. And in spite of the high impact of glass science and technology in modern times and the efforts of the scientific community, many aspects of glass physics including the transformation of a glass into a liquid  by increasing the temperature (’the melting of a glass’) are poorly understood.

The field of disordered matter or glassy systems is one of those with many intriguing questions and few certain answers. Researchers from the Unitat de Física de Materials I of the Physics Department at the Universitat Autònoma de Barcelona have revealed the nature of the heterogeneous mechanism by which highly stable glasses transform into the liquid state. In their study, they have used glasses grown by physical vapour deposition, a preparation technique that has recently emerged as a powerful tool to prepare glasses with tuneable thermodynamic stability. These glasses, prepared in few minutes, can attain thermodynamic stabilities comparable to amber, a material that has been naturally stabilized during geological time-scales.

According to this research, glasses can transform via front-like propagation from the surface as a consequence of the difference in dynamics between the glass and the corresponding liquid phase. In particular, the work shows how the geometrical mean of the relaxation time of the glass and liquid phases (a measurement of their dynamics) controls the transformation process. This work also points towards ordinary thin film glasses transforming via a frontlike transformation mechanism if heated sufficiently fast, establishing a close connection between vapor-deposited and liquid-cooled thin film glasses. These findings may be applied to make more efficient and stable electronic devices, such as organic light emitting devices, where interfaces plays a strong role due to the confined geometry.

The results published in Physical Review Letters and highlighted by the editors as an Editors’ Suggestion demonstrate that both vapor-deposited ultrastable and ordinary glasses show front propagation on melting, with an intermediate liquid layer between the glass and the liquid.