Structural and dynamical heterogeneity in a glass-forming liquid

Abstract

We use the "isoconfigurational ensemble" [Phys. Rev. Lett. 93, 135701 (2004)] to analyze both dynamical and structural properties in simulations of a glass forming molecular liquid. We show that spatially correlated clusters of low potential energy molecules are observable on the time scale of structural relaxation, despite the absence of spatial correlations of potential energy in the instantaneous structure of the system. We find that these structural heterogeneities correlate with dynamical heterogeneities in the form of clusters of low molecular mobility.PACS numbers: 64.70. Pf,05.60.Cd,61.43.Fs,81.05.Kf Over the last decade, the identification and study of dynamic heterogeneity (DH), especially in computer simulations, has added an important new dimension to our understanding of complex relaxation in glass forming liquids [1,2]. DH refers to the emergence and growth of spatially correlated domains of mobile and immobile molecules as temperature T approaches the glass transition temperature T g . A question that dominates research on DH concerns its connection to the structure of the liquid: What local configurational properties influence whether a given molecule is mobile or immobile?Recent work by Harrowell and Fynewever [3] has shown conclusively that a structuredynamics connection must exist at the molecular level.To do so, they use an "isoconfigurational (IC) ensemble" [3, 4, 5], a set of microcanonical molecular dynamics (MD) trajectories in which each run starts from the same initial equilibrium configuration, but with molecular momenta chosen randomly accordingly to a MaxwellBoltzmann distribution. The result is a set of trajectories lying on the same energy surface, and evolving away from their common initial point in configuration space. They then define and evaluate the "dynamic propensity": the average, in the IC ensemble, of the squared displacement of a molecule at a time comparable to the structural relaxation time. They show that DH is observed in this approach, in the form of increasing spatial correlations of the dynamic propensities in a glass forming liquid as T → T g . Since the influence of the initial momenta is averaged over, the observed spatial correlations must be configurational in origin.The strength of Ref.[3] is that it exposed the features of DH that are structural in origin, without needing to determine what structural properties are responsible. Other studies have worked towards explicitly identifying structural correlators to dynamics. A number of works over the past decades have identified relationships between average structural properties (especially free volume and measures of symmetry in the local molecular environment) and bulk dynamics; see e.g. Refs. [6,7,8,9]. More recently, several studies have sought a correlation at the microscopic level, e.g. between local free volume and local mobility, with more success in some systems [10,11] than in others [12]. A notable absence of correlation between the local volume and the local Debye-Waller factor has been report...

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