Liquids are normally considered to be thermodynamically stable. However, rapidly cooled liquids attain a so-called metastable state — the supercooled or undercooled liquid. As we decrease the temperature, these liquids become more and more viscous and structural relaxation becomes slower and slower. One could naively infer that, as the slowing down proceeds, nothing happens in such systems since the motion of the constituents (atoms or molecules) is so severely hindered. Actually, the mystery of supercooled liquids resides precisely in the origin of such slowing down. Ultimately, this form of dynamic arrest leads to the formation of an amorphous solid, i.e. a solid that is not crystalline: we call this a glass.
This longstanding open problem in thermodynamics and statistical mechanics has prompted several theoretical approaches. These are inspired by different facets characterising glassy physics: glasses present a wide and heterogeneous distribution of relaxation timescales; glasses have signatures of reordering on very small lengths; glass-forming liquids display a reduction of the number of distinct configurations that the constituents can attain. Several conflicting theories have emerged over time, attempting to provide a unified picture.
We have just published a Perspective (a Featured Article in the Journal of Chemical Physics) retracing the connections existing between the theory of dynamical phase transitions and the structural and thermodynamical approaches to the glass transitions. We show that the theory of dynamical phase transitions allows us to identify metastable states in an operative sense. The glassy phenomenology can then be re-interpreted in an extended phase space. Here dynamical transitions between metastable states are coupled to structural features and configurational entropy reduction. This suggests a close relationship between microscopic structural arrangement, mesoscopic kinetic rules and thermodynamic phase transitions.
You can read the full Perspective here
C. Patrick Royall, Francesco Turci and Thomas Speck J. Chem. Phys. 153, 090901 (2020)