Is the secret of glass in a rare leap? How do atoms move a split second before solidifying?
Is the secret of glass in a rare leap? How do atoms move a split second before solidifying?
The answer comes from a study by Federico II University in collaboration with the University of Montpellier, published in *Physical Review X*
Glass is a solidified paradox, a liquid that has forgotten how to flow: glass is, in fact, a rigid solid, yet at the atomic level it retains the disorder typical of fluids. Typically, glass is produced by cooling a liquid below its solidification temperature so rapidly that the atoms do not have time to arrange themselves into the perfect order of a crystal; as it cools, the motion of the atoms slows dramatically, until they freeze completely at the so-called glass transition temperature. Before reaching this temperature, the liquid is said to be “supercooled.”
How individual particles move in this peculiar supercooled state is one of the most fascinating and persistent questions in condensed matter physics. While it has been known for decades that at a certain critical temperature T_c—also predicted by classical theories of the glass transition—the microscopic dynamics change in nature, exactly what happens to the particles “below” T_c has long remained an open question: just a few degrees Celsius from the formation of real glass, where everything comes to a standstill, the relaxation times of the particles become simply too long to be tracked, effectively making the ultra-cooled regime a territory that has remained inaccessible to both experiments and numerical simulations until now .
It is precisely this uncharted territory that is the subject of the study by Francesco Rusciano, Raffaele Pastore, and Francesco Greco (Department of Chemical, Materials, and Industrial Production Engineering, Federico II) in collaboration with Walter Kob (University of Montpellier). The study, which combines large-scale molecular dynamics simulations (requiring over 50 CPU-years of computation) with a statistical analysis of the motion of each individual particle, was published in the prestigious journal *Physical Review X*, a leading open-access journal of the American Physical Society, which selects only research of exceptional significance and innovation from the global physics community.
The study’s discovery is as simple as it is unexpected: at these extreme temperatures, the relaxation of the entire material is governed by extremely rare events, yet with far-reaching consequences: each particle remains trapped for a long time within the “cage” formed by its neighbors and, from time to time, escapes with a leap as wide as it is sudden. From these rare leaps emerge universal scaling laws of surprising simplicity. The same mechanism also provides a microscopic explanation for the so-called “excess wing” phenomenon—an anomaly in the response of glassy materials to external stresses, observed everywhere but for which there has been no agreed-upon explanation until now.
The joint Naples-Montpellier study brings together long-debated phenomena into a single, microscopically grounded framework.The potential implications of this discovery extend to our understanding of transport phenomena in a broad family of amorphous materials, ranging from everyday glasses to those of technological interest.
F. Rusciano, R. Pastore, F. Greco, and W. Kob, “Rare cage escapes drive relaxation in deeply supercooled liquids, ” Phys. Rev. X (2026).
Link to the Open Access article.