Critical-like behaviour of glass-forming liquids

Abstract

Recently it has been revealed that when approaching the glass-transition temperature, T(g), the dynamics of a liquid not only drastically slows down, but also becomes progressively more heterogeneous. From our simulations and experiments of six different glass-forming liquids, we find that the heterogeneous dynamics is a result of critical-like fluctuations of static structural order, contrary to a common belief that it is purely of dynamic origin. The static correlation length and susceptibility of a structural order parameter show Ising-like power-law divergence towards the ideal glass-transition point. However, this structural ordering accompanies little density change, which explains why it has not been detected by the static structure factor so far. Our results suggest a far more direct link than thought before between glass transition and critical phenomena. Indeed, the glass transition may be a new type of critical phenomenon where a structural order parameter is directly linked to slowness.

Supercooled liquids with enhanced orientational order

“…Hereafter, this particular liquid phase will be denoted as medium-range order enriched liquid (MROL). Concurrently, we observed structural relaxation times 4-5-fold longer than the relaxation time of the OL, corresponding to a shift in T g of B4 K. This finding is in line with the quantitative predictions proposed both for Lennard-Jones liquids 28 and for model polymeric systems 29 .…”

“…Moreover, such an increase rules out the presence of long-range order and crystal-like BOO, which would instead cause a decrease in the dielectric response due to the suppression of the rotational and translational modes in these clusters. Furthermore, as a consequence of a larger correlation length, the MRO regions are expected to have slower structural dynamics (corresponding to B2-10 t) with respect to the isotropic liquid 28,29 .…”

“…Such a trend is in further agreement with the idea that the more fragile glass formers (m xylitol 4m threitol 4m glycerol , (ref. 32) with m the fragility) are characterized by lower frustration, that is, by a lower concentration of (locally) ordered structures 11,28 , which explains the smaller increase in polarization with increasing molecular size. Lifetime and length scale of the enhancement in polarization.…”

“…The MRO enrichment should persist as long as the LPSs have not recovered their equilibrium concentration at a given temperature. Until the achievement of this condition, the MRO regions continuously destroy and nucleate, driven by the minimization of local free energy, achieved via spatial reorganization at the time scale of t (refs 11,28). This long-lasting behaviour permitted us to detect the differences in the orientational polarization.…”

A robust comparison of dynamical scenarios in a glass-forming liquid

“…In doing so, we address two specific questions about the dynamics of liquid glycerol, a prototypical glass-forming liquid: the first one concerns how many distinct dynamical processes are present in glycerol in the sub-nanosecond (ns) regime. The second one concerns whether a HomoGeneous (dynamically driven, hereafter HG) 1,2 or HeTerogeneous (structurally driven, HT) [3][4][5] scenario better describes the QENS data. To avoid the common pitfalls associated with ''classical'' data-analysis procedures (e.g., use of additional parameters not justified by the information content of the data), we make extensive use of Bayesian inference methods.…”