Abstract
Glass transition is characterised by drastic dynamical slowing down upon cooling, accompanied by growing spatial heterogeneity. Its rationalisation by subtle changes in the liquid structure has been long debated but remains elusive, due to intrinsic difficulty in detecting the underlying complex structural ordering. Here we report that structural order parameter characterising local packing capability can well describe the glassy dynamics not only macroscopically but also microscopically, no matter whether it is driven by temperature or density. A Vogel-Fulcher-Tammann (VFT)-like relation is universally identified between the structural relaxation time and the order parameter for supercooled liquids with isotropic interactions. More importantly, we find such an intriguing VFT-like relation to be statistically valid even at a particle level, between spatially coarse-grained structural order and microscopic particle-level dynamics. Such a unified description of glassy dynamics based solely on structural order is expected to contribute to the ultimate understanding of the long-standing glass-transition problem.
Highlights
Glass transition is characterised by drastic dynamical slowing down upon cooling, accompanied by growing spatial heterogeneity
We perform molecular dynamics simulations of sixteen glass formers, covering the degrees of freedom in terms of spatial dimensions, interaction potentials, compositions, and state points in the phase space controlled by temperature T or density ρ
The fact that we have the local VFT-like relation between structural order parameter and dynamics only after spatial coarse-graining of the former suggests that the cooperativity of dynamics is a direct consequence of the spatial correlation of structural order
Summary
Glass transition is characterised by drastic dynamical slowing down upon cooling, accompanied by growing spatial heterogeneity. The fundamental question of interest is whether the macroscopic slowing down and the microscopic dynamic heterogeneity in apparently different glass formers, either driven by temperature or density, can be understood in a unified manner from a structural perspective To this end, we construct structural order parameters X detecting sterically favoured structures, namely Θ in 2D and Ω in 3D, in the instantaneous liquid states (see Methods section). We further establish a direct quantitative relation between structural order parameters X and structure relaxation time τα in the Vogel-Fulcher-Tammann (VFT) form, indicating that ταðT; ρÞ is a universal function of XðT; ρÞ alone [the ðT; ρÞ-dependence of the former is through the ðT; ρÞ-dependence of the latter] This further suggests the structural/entropic origin of slow glassy dynamics in line with the Adam-Gibbs scenario[12]. Our results may provide an essential piece in the microscopic theoretical description of the long-standing glass transition problem from a structural perspective
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