Fluctuations in the relaxation of structural glasse

This dissertation focuses on the study of the dynamics of structural glasses. The presence of dynamical heterogeneity has been invoked to explain the non-exponential nature of relaxation in glassy systems and the presence of anomalies in transport properties near the glass transition. Although recent experiments have provided direct evidence for its presence, the origin of this phenomenon is currently poorly understood. One of its possible theoretical explanations involves the presence of local fluctuations in the time variable. In this work, we test predictions coming from this theoretical explanation. We study fluctuations in the relaxation of structural glasses by means of molecular dynamics simulations of various structural glass models. The focus is mostly on the aging regime, but we also study the equilibrium supercooled regime.;For a simple aging Lennard-Jones binary glass former, we find that the probability distributions of local fluctuations collapse for a fixed value of the global correlation C(t, tw). We also find that both the generalized dynamic susceptibility associated with particle density fluctuations and the corresponding correlation length increase as a power of the waiting time. We observe that the generalized susceptibilities and the dynamic correlation length can be scaled and fall onto master curves as functions of C(t, tw).;We also analyze data from a structural glass where the interactions between the particles are via the purely repulsive Weeks-Chandler-Anderson potential. We compare the behavior of fluctuations in the aging and equilibrium regimes. We find that the dynamic susceptibility grows and saturates as the system reaches equilibrium and that the distributions of local fluctuations related to the aging and equilibrium regimes collapse with each other and show scaling. The results put a condition on theoretical explanations for dynamical heterogeneity: they should predict that the behavior of the fluctuations is similar in the aging and the equilibrium regime.;Finally, we present simulation results for two polymer glass systems and a strong glass SiO2 system. We compare the results of simulations in small molecule fragile and strong glassy systems along with polymer systems to check whether universality applies for the behavior of fluctuations. We find universality in the behavior of coarse-grained local fluctuations and also, to some extent, in the behavior of the dynamic spatial correlations.;The results of these simulations, i.e. the scaling behavior in the local fluctuations and growth of a dynamic correlation length as the system relaxes, are consistent with the predictions that emerge from assuming the presence of local fluctuations in the time variable. Some of the results are consistent with the presence of universality, although some non-universal aspects remain to be investigated.