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Title: Why diffusion decouples from viscosity near the glass transition
Plain: In ordinary liquids, how fast a molecule wanders is inversely proportional to the viscosity, the Stokes-Einstein relation. Near the glass point molecules wander much faster than this rule predicts, and a quantitative explanation is missing.
Precise: Near $T_g$ in molecular liquids such as o-terphenyl, $D\eta/T$ exceeds its high-temperature value by roughly two to three orders of magnitude (approximate), and $D\sim \tau_{\alpha}^{-\xi}$ with a fractional exponent reported near 0.75 to 0.85. Predict the magnitude, temperature dependence and exponent $\xi$ from a theory of dynamic heterogeneity, and explain why rotational diffusion follows viscosity more closely than translational diffusion.
Settled by: A theory that predicts $D(T)$, $\eta(T)$ and the decoupling exponent for a model liquid and agrees with simulations and with measurements on molecular glass formers.
Kind: phenomenon
Answer type: mechanism
References: (empty)
Literature status: partially-resolved
Status note: Attribution of the decoupling to dynamic heterogeneity is widely accepted qualitatively, but a quantitative prediction of the exponent is lacking (2026).
Posed since: (empty)