Dielectric study of probe rotation in viscous liquids

Dielectric molecular probes are employed to assess the averaging required for a transition from heterogeneous to more hydrodynamic rotational behaviour in supercooled liquids. The degree of averaging is controlled by the size of the probe molecule relative to the liquid constituents. Larger solutes exhibit longer time constants and more exponential dynamics as a result of effective time averaging over fluctuating environments. With this approach, the time scale of rate exchange is measured across a range of eight decades, in which the characteristic relaxation time varies from 5s to 50ns. The sharp transition from dispersive to exponential probe rotation suggests that rate exchange occurs on the time scale of the slowest time constant of structural relaxation. No change in the environmental fluctuation to structural relaxation time ratio is observed across this wide range of frequencies.