A study conducted at the European Synchrotron Radiation Facility (ESRF) in Grenoble by researchers from the Universities of Brussels, Padua and Pisa, together with ESRF, and published in Nature Physics under the title “At the Crossover Between Quasi-Localized Dynamics and Diffusion in Deeply Supercooled Liquids”, reshapes our understanding of the molecular dynamics that govern the transition of liquids towards the glassy state. The research reveals that the “dance of molecules” is not made up of separate and independent motions, but rather of a single coherent mechanism.
What separates a flowing liquid from a solid, brittle glass? At first glance, the answer seems straightforward: in the former, atoms move freely; in the latter, they are locked in place. Yet for condensed matter physicists, the so-called glass transition remains one of nature’s most challenging puzzles. As a liquid is cooled towards the glassy state, the time required for molecules to reorganise increases exponentially, while their atomic structure remains almost unchanged.
How can a system become rigid without altering the arrangement of its components? For decades, physicists have interpreted this phenomenon as the superposition of different types of motion. On long timescales, there is structural relaxation, the main process through which molecules eventually escape the “molecular cages” formed by their neighbours, allowing the liquid to flow. At the opposite extreme, on short timescales, are the frenetic vibrations of atoms trapped within those same cages.
Between these two extremes lies the mysterious Johari–Goldstein relaxation, first identified in the 1970s. For fifty years, scientists have debated whether it represents an autonomous local motion or whether it is instead connected to the liquid’s main relaxation process.
“Johari–Goldstein relaxation plays a crucial role in glasses because it is responsible for their residual mobility,” explains Giulio Monaco of the Department of Physics and Astronomy at the University of Padua. “It is linked to important but hard-to-predict properties such as ductility and resistance to crystallisation. Until now, its microscopic origin remained poorly understood.”
Today, this international research team has used the ESRF’s advanced facilities to investigate this long-standing mystery. The researchers employed a state-of-the-art technique known as X-ray time-domain interferometry, which made it possible both to observe molecular motion at the atomic scale within a time window ranging from 10 nanoseconds to 10 microseconds — previously almost inaccessible — and to isolate the signature of Johari–Goldstein relaxation.
“Our experiment provides information about Johari–Goldstein relaxation in reciprocal space, and this information must then be transformed into real space,” explains Federico Caporaletti of the Experimental Soft Matter and Thermal Physics group at the Université libre de Bruxelles. “The image of this relaxation in real space became progressively clearer as more information became available in reciprocal space, until, at a certain point, everything fell into place.”
The results, published in Nature Physics, overturn previous assumptions: beta relaxation is not an isolated and independent event. Instead, it is the direct precursor of structural relaxation — the first sign of the breakdown of molecular cages that ultimately leads to the material becoming fluid.
Although the study does not invalidate previous models, it suggests that the distinction between different types of relaxation may be more formal than substantial. At the microscopic level, the dynamics are deeply interconnected.
“After almost ten years of research in our laboratories and at ESRF’s ID18 beamline, we have finally understood the microscopic origin of some very important properties of glasses,” concludes Simone Capaccioli of the Department of Physics at the University of Pisa. “These results could contribute to the design of glasses with properties better tailored to society’s needs.”
Research link: https://www.nature.com/articles/s41567-026-03320-5
Title: At the Crossover Between Quasi-Localized Dynamics and Diffusion in Deeply Supercooled Liquids (Nature Physics, 2026)
Authors: Federico Caporaletti, Simone Capaccioli, Dimitrios Bessas, Aleksandr I. Chumakov, Alessandro Martinelli, Francesco Dallari and Giulio Monaco.
