Local viscosity of supercooled glycerol near T g probed by rotational diffusion of ensembles and single dye molecules

We probe the rotational diffusion of a perylene dye in supercooled glycerol, 5–25 K above the glass-transition temperature ( T g = 190 K) at the ensemble and the single-molecule level. The single-molecule results point to a broad distribution of local viscosities that vary by a factor of five or more for different individual fluorophores at a given temperature. By following the same single molecules at various temperatures, we find that the distribution of local viscosities itself broadens upon approaching T g . This spatial heterogeneity is found to relax extremely slowly, persisting over hours or even days. These results convey a picture of heterogeneous liquid pockets separated by solid-like walls, which exist already well above the viscosimetric glass transition.