Enhanced cooperativity below the caging temperature of o-terphenyl

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The utility of a cooperative length scale for describing the dynamics of glass-forming liquids is shown using literature data on o-terphenyl. Molecular dynamics and Monte Carlo simulations reveal a distribution of cooperative fractal events below the caging temperature Tₐ. Guided by these results, we show how to extract the size of slow regions in any glass-forming liquid from probe rotation/diffusion measurements, which agrees quantitatively with 4-D NMR and grows steadily as temperature is lowered below Tₐ. We clarify why this length must also be the size of the largest cooperative events.