Abstract
Many systems, including polymers and molecular liquids, when adequately cooled and/or compressed, solidify into a disordered solid, i.e., a glass. The transition is not abrupt, featuring progressive decrease of the microscopic mobility and huge slowing down of the relaxation. A distinctive aspect of glass-forming materials is the microscopic dynamical heterogeneity (DH), i.e., the presence of regions with almost immobile particles coexisting with others where highly mobile ones are located. Following the first compelling evidence of a strong correlation between vibrational dynamics and ultraslow relaxation, we posed the question if the vibrational dynamics encodes predictive information on DH. Here, we review our results, drawn from molecular-dynamics numerical simulation of polymeric and molecular glass-formers, with a special focus on both the breakdown of the Stokes–Einstein relation between diffusion and viscosity, and the size of the regions with correlated displacements.
Highlights
When polymers, liquids, biomaterials, metals and molten salts are cooled or compressed, if the crystallization is avoided, they freeze into a microscopically disordered solid-like state, a glass [1,2,3]
There is wide experimental, numerical and theoretical evidence that the fast vibrational dynamics, as sensed by the Debye–Waller factor u2, and the time scale τα of the slow microscopic reorganisation of a liquid close to the transition to the glassy state are correlated in an universal way
Less attention has been paid to a series of numerical MD simulation studies concluding in favour of strong correlations between the vibrational dynamics and the dynamical heterogeneity, the spatial distribution of long-time mobility developing when approaching the disordered solid state
Summary
Liquids, biomaterials, metals and molten salts are cooled or compressed, if the crystallization is avoided, they freeze into a microscopically disordered solid-like state, a glass [1,2,3]. Despite the huge difference in time scales, earlier [33] and later theoretical studies [5,34,35,36,37,38,39,40], and experimental ones [41], addressed the rattling process within the cage to understand the slow dynamics, rising a growing interest on the DW factor [8,29,30,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61] Within this context, most interest has been devoted to the correlations between DW factor and the structural relaxation time τα, which are found to be strong and encompassed by a universal master curve [47]: τα = F ( u2 ). Even if it incorporates some consequence of DH, i.e., the presence of a wide distribution of relaxation times p(τ), it does not cover any spatial aspect related to DH, which instead has been revealed by the simulations, as we will see in Sections 5.1 and 7, and accounted for by Equation (3)
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