Density relaxations across the glass-transition under equilibrium and non-equilibrium conditions

Cooling down a liquid below its melting temperature can lead to an amorphous frozen system commonly known as a glass. Despite glasses are common in everyday life, the nature of the glass transition still remains one of the most intriguing unsolved problems of condensed matter physics. In this Thesis the atomic length-scale rearrangements of glassy systems close to the glass transition temperature are explored under equilibrium and non-equilibrium conditions with X-Ray Photon Correlation Spectroscopy (XPCS). In particular, we explore the role of directional stresses on the dynamics in a colloidal glass of silica nanoparticles dispersed in a binary solvent. Our results show that the macroscopic stress stored in this glass relaxes via the cooperative ballistic motion of groups of particles with a characteristic size of the order of ten particle diameters. The role of stresses is further investigated in borate-based glasses, where the dynamics well below the glass transition are dictated by the recently discovered X-ray beam-induced dynamics. We show that these dynamics are related to the topology of the network, with peculiar similarities with the stress-phenomenology observed in colloids and metallic glasses.