Supercooled liquids are local equilibrium states where the features of glassy dynamics progressively emerge as we decrease the temperature. We have a broad understanding of what happens in a generic supercooled liquid: the dynamics become slow and highly heterogeneous, motion is gradually more cooperative, and some changes in the local spatial correlations occur accompanied by a reduction of the number of accessible configurations (the so-called configurational entropy). We have theoretical interpretations that emphasise one aspect or another of this phenomenological picture: some focus on the dynamical properties, others on the thermodynamical ones. Both schools of thought rely on intermediate concepts that vehiculate the message of the theories and link the microscopic constituents to the macroscopic relaxation behaviour: so-called dynamical facilitation relies on a hierarchy of excitations; thermodynamic theories have cooperatively rearranging regions as a critical ingredient for their description.
After many efforts by PhD student (and now Dr!) Levke Ortlieb, a quantitative analysis of how these concepts can be defined in practice at very low temperatures (where they should be more cogent) has just been published in Nature Communications, https://rdcu.be/dbtmK . The work, supervised by CP Royall and myself, leverages high-performance GPU molecular dynamics data produced by T Ingebrigtsen, from the Roskilde Glasss and Time group, and super-resolution colloidal experiments by J Hallett (Reading) to probe a supercooled regime well below conventional simulations. This allows us to revisit past definitions of excitations and cooperatively rearranging regions, to provide operative routes to measure these notions and compare one against the other. Furthermore, it allows us to show that the different theoretical approaches are sufficiently flexible to fit the data even in this low-temperature regime, making it difficult to rule out one or another mechanism. On the contrary, a complementarity between the two approaches appears to emerge: whether this is a physical or an epistemological reality is still yet to be investigated!
Ortlieb, L., Ingebrigtsen, T.S., Hallett, J.E., Turci, F., Royall, C.P, Probing excitations and cooperatively rearranging regions in deeply supercooled liquids. Nat Commun 14, 2621 (2023). https://doi.org/10.1038/s41467-023-37793-2