PHOTON SCIENCE

Even though glasses have been studied for a long time, surprisingly little is known on the glass formation. During the transition from the liquid to the solid state the special glassy state forms at the atomic level. It can be characterised as a special state of matter between a disordered liquid and a crystalline solid. A research team from DESY, led by Felix Lehmkühler, has now presented a new technique at PETRA III which shines light on the structure and dynamics of a liquid close to the glass transition. Their results have been published in The Journal of Physical Chemistry Letters.

The structure of a glass resembles the one of the liquid, but unlike a liquid the atoms in a glass are basically immobile. Normally, the dynamics or viscosity of these materials slows down by more than 12 orders of magnitude before crossing the glass transition, an effect which is still not understood. Especially the question why a material rather forms a glass and not a crystalline solid is unresolved. A common understanding from theory is that the local structure of the atoms in the (supercooled) liquid may play a key role. For instance, if some atoms tend to form icosahedral clusters, they cannot grow into an ordered crystal and thus disturb further nucleation as well as the growth of crystal nuclei. At supercooled conditions close to the glass transition the number of such locally favoured structures should increase. In this case, an amorphous solid state is energetically preferred and a ‘glassy’ material is formed. In fact, most plastics are glasses from a materials science point of view.

Studies of the glass transition need to address both structure and dynamics of the investigated system. The DESY research team has used the ‘Coherence Applications Beamline’ P10 of PETRA III, which enables investigating dynamics by means of X-ray photon correlation spectroscopy (XPCS). In the current work the researchers focused on a colloidal hard sphere system which is a frequently studied model system for phase and glass transition for theory, simulations and experiments. Here, the phase can be easily tuned by the volume fraction of the colloidal nanoparticles in the dispersion.
 
The researchers combined the XPCS technique with higher-order intensity correlation data from a so-called X-ray cross correlation analysis (XCCA). In doing so, they could track the dynamics not only of the single particles but also the lifetime of particle clusters via dynamics of higher-order correlation functions. Approaching the glass transition, these dynamics showed a stronger deceleration than the single-particle dynamics tracked only by XPCS. “This is an indication that the local order becomes more long-lived in the vicinity of the glass transition”, explains Nele Striker, the main author of the study. “This means that not only the number but also the lifetime of ordered clusters grows close to the glass transition.”

“This new experimental approach using coherent X-rays can be extended to detect structure-dynamics correlations on many length scales”, says Fabian Westermeier, scientist at the PETRA III beam line P10 and co-author of the study. Beside glassy samples, other phase transitions or lifetimes of transient structure in liquids such as water can be investigated. Most importantly, the correlation technique will benefit exceptionally from the increased brilliance of diffraction-limited storage rings such as PETRA IV at DESY.



Reference:
Dynamics and Timescales of Higher Order Correlations in Supercooled Colloidal Systems, Nele N. Striker, Irina Lokteva, Michael Dartsch, Francesco Dallari, Claudia Goy, Fabian Westermeier, Verena Markmann, Svenja C. Hövelmann, Gerhard Grübel, and Felix Lehmkühler, J. Phys. Chem. Lett. (2023), 10.1021/acs.jpclett.3c00631