BIO In the literature: contested

What causes the super-Arrhenius slowdown of supercooled liquids

In plain words

As a glass-forming liquid is cooled, the time its molecules take to rearrange grows by about 14 powers of ten over a narrow temperature range, much faster than the simple Arrhenius law (a fixed energy barrier) predicts. Which microscopic mechanism produces this growth is unknown.

Precise statement

In molecular glass formers the structural relaxation time $\tau_\alpha(T)$ grows from about $1e-12\ \mathrm{s}$ in the high-temperature liquid to about $1e2\ \mathrm{s}$ at $T_g$, with an effective activation energy that increases on cooling; model liquids (Kob-Andersen Lennard-Jones mixture, polydisperse soft spheres) reproduce the onset of this growth. Competing explanations include random first-order transition theory (entropy-driven cooperative rearrangements), dynamic facilitation (kinetically constrained localized excitations) and elastic or frustration-based models. An answer is a theory that predicts $\tau_\alpha(T)$ and the associated growing length scales from the interaction potential and is singled out against the alternatives by equilibrium simulations near and below the experimental $T_g$. This is the root question of the topic; its working content is split into the sharper problems linked to it.

What would settle it

A theory whose quantitative predictions for $\tau_{\alpha}(T)$, $\xi(T)$ and their relation are confirmed, and those of rival theories refuted, in swap-Monte-Carlo-equilibrated simulations below the experimental $T_{g}$.

Status in the literature

Unverified note

Swap Monte Carlo has equilibrated model liquids below the experimental $T_{g}$ since about 2017, but as of 2026 no test has eliminated the main competing theories.