Glasses and glass transition
A number of liquids can be cooled to temperatures below the melting point without crystallization. In the supercooled liquid, the structural relaxation continuously slows down by about 15 orders of magnitude. Eventually, the structural relaxation ceases on the experimental time scale at the glass transition temperature – an amorphous solid forms. We use NMR experiments and MD simulations to improve our microscopic understanding of this glass-transition phenomenon.
For the structural relaxation of supercooled liquids, it is usually found that its time dependence deviates from single-exponential behavior and its temperature dependence differs from an Arrhenius law. Despite intense research in the last decades, a complete understanding of these characteristic behaviors of glass-forming liquids is still lacking.
NMR experiments on the glass transition
In general, non-exponential relaxation can be explained within two limiting scenarios, namely, homogeneous dynamics and heterogeneous dynamics. While it is not possible to distinguish different dynamical behaviors of the particles in the former case, a broad distribution of correlation times exists in the latter case. Using a sequence of seven radio-frequency pulses, we measure three-time correlation functions, which allow us to quantitatively determine the relevance of homogeneous and heterogeneous contributions in supercooled liquids. The results indicate that the existence of dynamical heterogeneities is an important feature, i.e., fast and slow molecules coexist at a given temperature. However, there are also rapid exchange processes between fast and slow molecules from the distribution of correlation times.
MD simulations on the glass transition
Our MD simulations show that particles, which exhibit a much higher mobility than an average particle, form clusters, indicating that the dynamics is spatially heterogeneous. Analyzing the dynamics in these clusters, we find that groups of highly mobile particles often show cooperative motion in “strings”. Thereby, the involved particles replace their respective neighbors along the direction of the string. Upon cooling, the size of the clusters increases and the motion in strings becomes more important. Possibly, these cooperative effects lead to a higher temperature dependence of the structural relaxation at lower temperatures and, hence, to the observed deviations from Arrhenius behavior. When structural relaxation evolves, different groups of particles become highly mobile. We find that particles in the neighborhood of previously mobile particles have a higher tendency to become mobile than other particles, i.e., there is an enhanced probability for continuous mobility propagation.