The first direct nanoscale observations of how glass turns into a liquid at increased temperatures

July 13, 2023

(Nanowerk News) Researchers from the Universitat Autònoma de Barcelona and ICN2 have developed a methodology that makes it possible for the first time to observe under a microscope, in real time, what happens when glass is heated and turns into an extremely cold liquid phase, known as the “glass transition”.

Research published in Natural Physics (“Real-time microscope of glass relaxation”), is essential for the cryopreservation of proteins, living cells and tissues, for the manufacture of pharmaceuticals and electronic devices, and for tissue engineering, where this glass-to-liquid transition plays a key role. Surface wrinkle images caused by the glass transition process were obtained for the first time by atomic force microscopy. (Image: UAB/ICN2)

Glass is a solid material with a disordered structure that can be considered a liquid with an unusually high viscosity. It is found in transparent and stained glass windows, in television screens and mobile devices, in optical fibers, in industrial plastic materials, and also in the protein state, cellular structures, and living tissues when frozen for cryopreservation.

Despite their generality, it is very difficult to develop theories and models that can explain their behavior in detail. The mechanics of how liquids cool and turn into glass, and vice versa, how glass turns into liquid when heated, something known as the “glass transition”, is still not fully understood.

Physicists are still not sure whether this is a phase transition and glass can be thought of as a thermodynamically distinct state from the liquid and solid states; or is glass simply a very cold liquid – cooled below freezing temperatures but retaining liquid properties – whose atoms or molecules have very little mobility.

One of the main difficulties in understanding this process lies in the challenge of visualizing it through a microscope with sufficient resolution, because the supercooled liquid and glass structures are almost indistinguishable.

A team led by researchers from the Physics Department of the Universitat Autònoma de Barcelona (UAB) and the Catalan Institute of Nanoscience and Nanotechnology (ICN2), with the involvement of the UPC and IMB-CNM-CSIC, have presented a new methodology that makes it possible to observe directly under a microscope what happens to glass when heated above the glass transition temperature, known as the “relaxation” process that turns it into a liquid.

The researchers worked with ultra-stable organic glass, which is prepared via thermal evaporation. They are denser and exhibit higher kinetic and thermodynamic stability than conventional glass obtained directly from a liquid. Unlike conventional glass which, as seen so far, shifts into a liquid state globally, with no discernible distinction between the various parts of the material, this ultra-stable glass transitions to a very cold liquid state in the same way that a crystalline solid transitions to a liquid state. liquid, with the formation of a growing region of the liquid phase. This is a process that has been indirectly described by nanocalorimetric measurements and has only been observed in computational models.

“It has previously been inferred from these models that the resulting liquid phase areas have extraordinary separation between them when it comes to ultra-stable glass, but this has never been directly observed,” said Cristian Rodriguez Tinoco, researcher at UAB and ICN2. .

A newly developed method for observing this transition consists of sandwiching an ultra-stable glass between two layers of glass with a higher transition temperature. When the ultrastable glass layer is heated above its transition temperature, the surface instability is transferred to the outer layer of the sandwich and can be observed directly with an atomic force microscope.

“These are movements and compressions that are very small, on the order of a few nanometers when the transformation begins, but large enough to be measured precisely with this type of microscope, which monitors where surface deformations appear above the transition temperature,” explains PhD student Marta Ruiz Ruiz. .

Work allows glass devitrification to be followed in real time. This allows quantification of the dynamics of relaxation processes in ultra-stable crystals towards supercooled fluids by directly measuring the distances between emerging fluid domains, while observing surface deformation and its evolution over time. In this way, it is possible to confirm how the distances between these molten areas are unusually large in this type of glass, and the correlation of these distances to the time scale of the material, as predicted by computational models.

“The microscopic description we have achieved for the first time allows direct comparisons between computational models and physical reality. We believe that this technique will also be very useful in exploring glass transitions on smaller space and time scales, which will enable a better understanding of transitions. in the less stable glass produced from cooled liquids,” concludes Javier Rodríguez Viejo, researcher at UAB and ICN2.

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