A Hidden phase Between Liquid And Solid May Have Been Found in 2d materials

Amazing stuff!

"Anything made out of plastic or glass is known as an amorphous material. Unlike many materials that freeze into crystalline solids, the atoms and molecules in amorphous materials never stack together to form crystals when cooled. In fact, although we commonly think of plastic and glass as “solids,” they instead remain in a state that is more accurately described as a supercooled liquid that flows extremely slowly. And although these “glassy dynamic” materials are ubiquitous in our daily lives, how they become rigid at the microscopic scale has long eluded scientists. ...

Specifically, using theory, computer simulations, and previous experiments, the scientists explained why the molecules in these materials, when cooled, remain disordered like a liquid until taking a sharp turn toward a solid-like state at a certain temperature called the onset temperature – effectively becoming so viscous that they barely move. This onset of rigidity – a previously unknown phase transition – is what separates supercooled from normal liquids. ...

Any supercooled liquid continuously jumps between multiple configurations of molecules, resulting in localized particle movements known as excitations. In their proposed theory ... treated the excitations in a 2D supercooled liquid as though they were defects in a crystalline solid. As the supercooled liquid’s temperature increased to the onset temperature, they propose that every instance of a bound pair of defects broke apart into an unbounded pair. At precisely this temperature, the unbinding of defects is what made the system lose its rigidity and begin to behave like a normal liquid. ..."

From the significance and the abstract:
"Significance

The dynamics of glass formers exhibit dramatic slowdown below an onset temperature that delineates the high-temperature and supercooled regimes. For two-dimensional (2D) glass formers, we propose that the onset temperature is described by a Kosterlitz–Thouless transition driven by the elastic excitations underlying the relaxation mechanism for glassy dynamics. Analogous to dislocation-mediated melting in 2D solids, the excitations exist as a bound dipole–dipole state in the supercooled regime and as free dipoles above the onset temperature. The Kosterlitz–Thouless scenario explains the elastic behavior of 2D supercooled liquids at intermediate timescales and thus, the Mermin–Wagner fluctuations observed in experiments and simulations of 2D glass formers. The present work reveals the exotic nature of 2D glass formers relevant to systems under extreme confinement.

Abstract

Below the onset temperature To, the equilibrium relaxation time of most glass-forming liquids exhibits glassy dynamics characterized by a super-Arrhenius temperature dependence. In this supercooled regime, the relaxation dynamics also proceeds through localized elastic excitations corresponding to hopping events between inherent states, i.e., potential-energy-minimizing configurations of the liquid. Despite its importance in distinguishing the supercooled regime from the high-temperature regime, the microscopic origin of To is not yet known. Here, we construct a theory for the onset temperature in two dimensions and find that an inherent-state melting transition, described by the binding–unbinding transition of dipolar elastic excitations, delineates the supercooled regime from the high-temperature regime. The corresponding melting transition temperature is in good agreement with the onset temperature found in various two-dimensional (2D) atomistic models of glass formers and an experimental binary colloidal system confined to a water–air interface. Additionally, we find the predictions for the renormalized elastic moduli to agree with the experimentally observed values for the latter 2D colloidal system. We further discuss the predictions of our theory on the displacement and density correlations at supercooled conditions, which are consistent with observations of the Mermin–Wagner fluctuations in experiments and molecular simulations."

A Hidden State Between Liquid And Solid May Have Been Found : ScienceAlert

Scientists Theorize a Hidden Phase Transition Between Liquid and a Solid Improved understanding of glassy dynamics could help scientists explain why a liquid behaves like a solid, and develop useful new materials

Inherent-state melting and the onset of glassy dynamics in two-dimensional supercooled liquids (open access)

(Left) Above an onset temperature, a 2D material exhibits normal liquid behavior with all particles similarly mobile (yellow). (Right) Below that temperature, it becomes supercooled, with the onset of rigidity leading to just some mobile particles (yellow) amongst solid-like ‘frozen’ regions (blue).