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Experimental evidence of mosaic structure in strongly supercooled molecular liquids

The molecules undergoing the βJG-relaxation (red spheres) are highly mobile and, after a time of the order of τDSβJG, perform larger spatial excursions than the rest of the molecules (white spheres). These spatial excursions are, on average, of the order of those prescribed by the Lindemann criterion for crystal instability19. These "un-caged" molecules form a (close to) percolating cluster. The sketch is based on an experimentally measured configuration for a colloidal glass of hard spheres (volume fraction 0.61) reported in ref. 44 but has no direct connection to the discussion reported in that study. The cluster of mobile molecules was obtained by randomly selecting particles with a probability of 0.25, according to the here discussed estimate for the fraction of molecules participating in the βJG-relaxation at τDSβJG (relaxation strength).

When a liquid is cooled to produce a glass its dynamics, dominated by the structural relaxation, become very slow, and at the glass-transition temperature Tg its characteristic relaxation time is about 100s. At slightly elevated temperatures (~1.2 Tg) however, a second process known as the Johari-Goldstein relaxation, βJG, decouples from the structural one and remains much faster than it down to Tg. While it is known that the βJG-process is strongly coupled to the structural relaxation, its dedicated role in the glass-transition remains under debate. Here we use an experimental technique that permits us to investigate the spatial and temporal properties of the βJG relaxation, and give evidence that the molecules participating in it are highly mobile and spatially connected in a system-spanning, percolating cluster. This correlation of structural and dynamical properties provides strong experimental support for a picture, drawn from theoretical studies, of an intermittent mosaic structure in the deeply supercooled liquid phase.

The data that support the findings of this study are available from the corresponding authors upon reasonable request. The data used to produce the cover figure have been taken from here.

These data has been addressed in a paper published today in the international scientific journal Nature Communications (share this article). The study reveals the mosaic structure in strongly supercooled molecular liquids. The study was conducetd by Prof. Simone Capaccioli of the Dipartimento di Fisica and the Centro per l’Integrazione della Strumentazione dell’Universtà di Pisa (CISUP), in collaboration with national and international researchers.

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