We have just published in the Journal of Chemical Physics the simulation work of a brilliant former master student of the School of Physics at the University of Bristol (Thomas Jenkinson) on the local structural changes occurring during ageing in two atomistic glass formers with Lennard-Jones interactions (the Kob-Andersen and Wahnström mixtures).
We find some expected results (local order steadily increases as the out-of-equilibrium liquid ages) and some more surprising ones (for example, the rate at which such increase occurs changes weakly for temperatures above the apparent dynamical divergence of viscosities, T0). We also investigate the effect of transient deep quenches, finding very moderate traces of so-called rejuvenation.
The reference to the work is J. Chem. Phys. 147, 054501, (2017).
One central piece of the problem of dynamic arrest is whether the phenomenology of slow relaxation, increasing dynamical length scales, mild (or dramatic) structural changes are somewhat related to the existence of a zero entropy amorphous state emerging at a non-zero temperature.
A comprehensive theory would need on the one hand to take into account of the well established phenomenon of dynamical heterogeneities, i.e. the non-homogenous patterns of diffusion that emerge together with the glassy dynamics itself; on the other hand, it should also rationalise the many findings that point (for several model systems) to a dramatic reduction of the so-called configurational entropy as one approaches a finite temperature (sometimes termed Kauzmann temperature) at which also the relaxation times appear to diverge.
In our recent work (Physical Review X 7, 031028) Thomas Speck, C. Patrick Royall and I discuss a unified scenario that combines dynamical aspects to structural ones in order to sample very low energy and entropy states, employing dynamical large deviations.
We find that the equilibrium supercooled liquid competes with a secondary metastable amorphous liquid rich in long-lived structural motifs, hidden in the tails of probability distributions in trajectory space. We also show that sampling the tails of such probabilities at a single moderate temperature allows us to retrieve the thermodynamic properties of the ordinary liquid in much wider range of temperatures, down to very low temperatures. We can then draw a diagram for the stable and metastable phase, pointing towards critical-like fluctuations in the region where the Kauzmann temperature is normally located, and allowing us to review currently proposed scenarios from an alternative point of view, rooted in the large deviation theory of metastability.
Rattachai Pinchaipat is a crafty experimentalist (PhD student in Bristol) that I have had the pleasure to work with within a Bristol-Mainz collaboration aimed at demonstrating in experiments the existence of phase transitions in trajectory space for supercooled liquids. Our work is going to appear in Physical Review Letters. Here is the preprint.
In Mainz, preliminary simulations on hard spheres in trajectory space (by Matteo Campo) have sampled the tails of the probability distribution of time-integrated structural observables and predicted long non-gaussian tails (signature of a phase transition). In experiments, Rattachai managed to find an analogous signature via subsampling the trajectories of a rather large system.
The result demonstrates that the dynamical heterogeneities that characterise fragile glass forming liquids can be read as the coexistence, in trajectory space, of different long-lived (metastable) stationary states: some are structure-less, while others show the presence of extraordinary extended and long-lived motifs. Crucially, the phase transition between the two states is not accessible in current experiments, and could eventually never be accessible, if it is always buried in the tails of the probability distribution.
But this is another story… (actually, this story).
Our work on curved space and frustration with Gilles Tarjus at the Université Pierre et Marie Curie in Paris is
going to appear published in Physical Review Letters, 118, 215501.
In it, we provide a test of one of the competing theories for the origins of the glass transition: this is geometric frustration, i.e. the idea that the slowing down observed in glass forming liquids goes hand in hand with the formation of particular non-cristalline geometric motifs, that increase in size as the liquids are cooled.
We test this on the most favourable ground for the theory, which is a curved manifold. We do this for the first time in three dimensions, observing the structural evolution of a glass former on the surface of a sphere embedded in four dimensions (This is a funny space to work in. A beautiful way to visualise such a hypersurface is to use the so-called two-ball construction, see image above, which nicely matches with the vision of the universe that Dante and his teacher Brunetto Latini had).
What we find is that geometrical motifs become gradually unfrustrated as the curvature increases (which is compatible with the basic assumptions of geometric frustration) and ordered phases (with some tricky defects, that we discuss in the Supplemental Material) spontaneously form for low enough temperatures. However, the size of the domains in such motifs is tightly coupled with the slowing down only for very strong curvatures, making geometric frustration just one of the mechanisms that eventually play a role in realistic glass-forming fluids (that exist in our ordinary Euclidean space).