Structural relaxation and aging in complex systems studied by x-ray photon correlation spectroscopy

Orateur : Beatrice RUTA (ESRF)

A large class of materials, ranging from molecular glasses to soft matter and biological systems, undergo structural arrest and physical aging. Following different experimental routes, these materials can be driven in an out-of-equilibrium configuration, where the dynamics slows down enormously, and depends on the sample age or waiting time tw. This field of research has greatly developed in the last years thanks to a strong synergy between theoretical advancements and experimental work, and is at the focus of a large community of scientists in these days. After a brief discussion on the dynamical process occurring at the single particle level in different soft materials, I will report on the first experiments that follow at the atomic scale the relaxation and aging dynamics in a molecular glass former, a metallic melt, as the material gets frozen into a glass. Existing studies only probe length scales of the order of hundreds of nanometers, which correspond to the relevant structural length scale for many soft matter systems, but are definitively too large for molecular materials. In order to accomplish this goal, we have exploited the unique possibilities offered today by the X-ray photon correlation spectroscopy at third generation synchrotron sources. Specifically, we show that the glass transition is accompanied by a dynamical crossover which leads to a complex hierarchy of aging behaviours in the glassy state. These findings seem to be universal in structural glasses and hold the promise to strongly improve the knowledge on the structural arrest and to allow discriminating among the several proposed theoretical models. In addition, we find clear similarities between our results in structural glasses and previous investigations on soft materials such as polymers, colloidal suspensions, emulsions and gels. Collectively, these findings suggest the notion of internal stress relaxation as a universal physical mechanism driving the dynamics of amorphous materials out of equilibrium.

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