Lifetime of Rate Domains: Comparison of Simulations and Single-Molecule Experiments in <i>o</i>-Terphenyl

Supercooled liquids have domains with different relaxation rates. The lifetime of those domains is not well understood. The usefulness of single-molecule measurements has been limited by conventional, molecule-by-molecule analysis, which suffers from excessive noise. An alternative, ensemble-based analysis using multidimensional correlation functions is applied to single-molecule dichroism results in o-terphenyl near T_g (T_g+4.5–1.5 K, T_rot = 3–15 s). It yields a detailed rate-exchange correlation function, which is biphasic. The slow, main component is 22 times slower than the alpha-relaxation time, implying that single-particle relaxation does not fully equilibrate the liquid. A small, fast phase of rate exchange is attributed to molecules on the boundaries of rate domains. The same analysis was applied to a ms-long, all-atom simulation of o-terphenyl from the D. E. Shaw group [J. Phys. Chem. B 117, 12898 (2013)]. It is at a higher temperature (272.5 K, T_rot = 16 μs), but it is still well below the mode-coupling temperature (290 K). There are clear differences from the results near T_g, suggesting that time–temperature superposition does not hold for rate exchange.