Direct Observation of Heterogeneous Translational Motion at Tg

Recent experiments have provided direct evidence of heterogeneous translational motion in a supercooled small molecule organic liquid, tris-naphthylbenzene. The early stages of diffusion were measured using neutron reflectivity, and indicate that translational motion near the glass transition temperature T$_{g}$ is qualitatively different than diffusion in ``normal'' liquids. The diffusion coefficient was found to be wave-vector dependent, D(q) $\propto $ q$^{-2}$, with a crossover to a q-independent value, D(q $\to $ 0), at a length scale of $\sim $22 nm at T$_{g}$. These results demonstrate that translational motion on the nanometer length scale can be extremely heterogeneous in a single component system near T$_{g}$, giving rise to large jumps of roughly 20 molecular diameters. This observation explains the unusually fast diffusion coefficients found in many materials near T$_{g}$, and also the unusually rapid crystallization of supercooled liquids.