Space and time dynamical heterogeneity in glassy relaxation. The role of democratic clusters

In this work we review recent computational advances in the understanding of the relaxationdynamics of supercooled glass-forming liquids. In such a supercooled regime these systemsexperience a striking dynamical slowing down which can be rationalized in terms of the pictureof dynamical heterogeneities, wherein the dynamics can vary by orders of magnitude from oneregion of the sample to another and where the sizes and timescales of such slowly relaxingregions are expected to increase considerably as the temperature is decreased. We shall focus onthe relaxation events at a microscopic level and describe the finding of the collective motions ofparticles responsible for the dynamical heterogeneities. In so doing, we shall demonstrate thatthe dynamics in different regions of the system is not only heterogeneous in space but also intime. In particular, we shall be interested in the events relevant to the long-time structuralrelaxation or α relaxation. In this regard, we shall focus on the discovery of cooperativelyrelaxing units involving the collective motion of relatively compact clusters of particles, called?democratic clusters? or d-clusters. These events have been shown to trigger transitions betweenmetabasins of the potential energy landscape (collections of similar configurations or structures)and to consist of the main steps in the α relaxation. Such events emerge in systems quitedifferent in nature such as simple model glass formers and supercooled amorphous water.Additionally, another relevant issue in this context consists in the determination of a linkbetween structure and dynamics. In this context, we describe the relationship between thed-cluster events and the constraints that the local structure poses on the relaxation dynamics,thus revealing their role in reformulating structural constraints.Fil: Appignanesi, Gustavo Adrian. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Instituto de Química del Sur. Universidad Nacional del Sur. Departamento de Química. Instituto de Química del Sur; ArgentinaFil: Rodriguez Fris, Jorge Ariel. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Instituto de Química del Sur. Universidad Nacional del Sur. Departamento de Química. Instituto de Química del Sur; Argentin