Glass is a solid material with such a disordered structure that it could be considered a liquid of extraordinarily high viscosity. It is found transparent and stained glass windows, in television screens and mobile devices, in fibre optics, in industrial plastic materials and also in the state of proteins, cellular structures and living tissues when frozen for cryopreservation.

Despite being so common, it is very difficult to develop theories and models that can explain its behaviour in detail. The mechanisms by which a liquid cools and transforms into a glass, and conversely, how a glass transforms into a liquid when heated, something known as “glass transition”, are still not fully understood. Physicists are still not sure whether this is a phase transition and glass can be considered as a thermodynamic state distinct from the liquid and solid states, or whether glass is simply a supercooled liquid—cooled below freezing temperature but retaining liquid properties—whose atoms or molecules have very little mobility. One of the major difficulties in understanding this process lies in the challenges of visualising it through the microscope with sufficient resolution, as the structures of the supercooled liquid and glass are virtually indistinguishable.

Researcher Jorge Alcalá, from the Department of Materials Science and Engineering of the Universitat Politècnica de Catalunya - BarcelonaTech (UPC), who is also a professor at the ETSEIB, is a member of the research group that has presented a new methodology that makes it possible to observe directly under the microscope what happens in a glass when it is heated above the glass transition temperature, the so-called “relaxation” process that transforms it into a liquid.

New methodology
Led by researchers from the Department of Physics of the Universitat Autònoma de Barcelona (UAB) and the Catalan Institute of Nanoscience and Nanotechnology (ICN2), also featuring researchers from the Institute of Microelectronics of Barcelona (IMB-CNM-CSIC) and the UPC, the research team has presented a new methodology that makes it possible to observe directly under the microscope what happens in a glass when it is heated above the glass transition temperature, the so-called “relaxation” process that transforms it into a liquid.

Researchers worked with ultrastable organic glass, which is prepared via thermal evaporation. It is denser and exhibits higher kinetic and thermodynamic stability than conventional glass obtained directly from liquids. Unlike conventional glass, which, as seen so far, transforms to the liquid state globally, without clear distinctions between different regions of the material, this ultrastable glass transitions to a supercooled liquid state in a similar way as crystalline solids do when they transition to the liquid state, with the formation of liquid-phase areas that grow progressively larger. This is a process that was already described indirectly by nanocalorimetry measurements and was observed only in computational models.

The new method developed to observe this transition consists of sandwiching the ultrastable glass between two layers of glass with a higher transition temperature. When the ultrastable glass layer is heated above its transition temperature, the instabilities that occur on the surface are transferred to the outer layers of the sandwich and can be observed directly with an atomic force microscope.

The work allows real-time monitoring of the devitrification of the glass. It allows quantifying the dynamics of the relaxation process in ultrastable crystals towards a supercooled liquid by directly measuring the distances between the liquid areas that appear, while observing the deformation of the surface and its evolution over time. In this way, it was possible to confirm how these distances between liquid areas are extraordinarily large in this type of glass, and the correlation of these distances with the time scales of the material, as predicted by computational models.

Published in Nature Physics, the work was led by professors Javier Rodríguez Viejo and Cristian Rodríguez Tinoco, researchers from the Universitat Autònoma de Barcelona and the Catalan Institute of Nanoscience and Nanotechnology (ICN2).

Reference paper.

• • Marta Ruiz-Ruiz, Ana Vila-Costa, Tapas Bar, Cristian Rodríguez-Tinoco, Marta González-Silveira, José Antonio Plaza, Jorge Alcalá, Jordi Fraxedas & Javier Rodríguez-Viejo, “Real-time microscopy of the relaxation of a glass”, Nature Physics (2023) https://doi.org/10.1038/s41567-023-02125-0