title: Relaxation in glassy systems - Light-scattering study of in and out of equilibrium dynamics
Creators: name: Böhmer, Till Lukas
Publisher: name: UNSPECIFIED
Year: 2024

description: Supercooled liquids and glasses are universally characterized by the non-Arrhenius temperature-dependence of relaxation times, the non-exponential shape of the α-process and the non-linear response to temperature changes. Despite decades of research, a comprehensive theory of the glass-transition phenomenon is still pending. The present work attempts to contribute to the understanding of two of the three aspects by employing dynamic light scattering experiments. 
In the frequency domain, the non-exponential shape of the α-process manifests as an asymmetrically broadened relaxation peak with ν
^−β high-frequency behavior. For molecular supercooled liquids, the common belief based on results from dielectric spectroscopy has been that values of β depend on the molecular structure and are distributed between 0.3 and 1. On the contrary, it is shown for various supercooled liquids that the relaxation shape of the α-process probed by dynamic light scattering shows a generic ν^−1/2 high-frequency behavior.
The discrepancy with regard to results from dielectric spectroscopy is resolved by showing that dipolar cross-correlation contributions influence the relaxation shape considerably and lead to larger values of β. Building on this, a quantitative relation between β and the degree of static dipolar cross-correlations quantified in terms of the Kirkwood correlation factor gK is established. It resolves the previously identified empirical correlation between β and the dielectric relaxation strength. The developed concepts are applied to disentangle the relaxation spectra of hydrogen-bonding supercooled liquids, which are complex to interpret due to containing different contributions associated with structure-formation. Cross-correlation effects also contribute considerably to relaxation spectra of asymmetric binary mixtures. In this regard it is shown that solute-induced solvent-solvent cross-correlations are the origin of apparent slow solvent relaxation, which are commonly observed in binary mixtures.
The non-linear response to temperature changes is universal for physical aging, i.e. the slow and gradual evolution of material properties. One formalism for describing physical aging is the concept of material time, which can be thought of as time measured on a clock whose rate changes as the glass ages. Material time, however, has never been determined experimentally. Here, multispeckle dynamic light scattering is used to probe time-resolved intensity autocorrelation functions of an aging molecular glass. It is demonstrated that the material time can be extracted from these data. Going further, it is shown that time-irreversible thermal fluctuations during aging become reversible when replacing time with material time. Also other aging materials are shown to obey material-time reversibility, i.e. a physically aging colloidal glass, a chemically aging polymerizing epoxy and a physically aging computer-simulated glass. Thus, the present work confirms aging of various different systems being controlled by a material time. Finally, the evolution of the material-time clock rate is explored after small temperature changes and quenches deep into the glassy state. The former can be described in terms of a simple logistic differential equation, while a sub-linear aging behavior is observed for the latter, thus challenging current theories of physical aging.