We probe the mechanical response of two supercooled liquids, glycerol and ortho-terphenyl, by conducting rheological experiments at very weak stresses. We find a complex fluid behavior suggesting the gradual emergence of an extended, delicate solidlike network in both materials in the supercooled state-i.e., above the glass transition. This network stiffens as it ages, and very early in this process it already extends over macroscopic distances, conferring all well known features of soft glassy rheology (yieldstress, shear thinning, aging) to the supercooled liquids. Such viscoelastic behavior of supercooled molecular glass formers is difficult to observe because the large stresses in conventional rheology can easily shear-melt the solid-like structure. glass transition ͉ glycerol ͉ ortho-terphenyl ͉ yield-stress

Optische Signale von Einzelmolekülen liefern Informationen über die Struktur und Dynamik ihrer nanoskaligen Umgebung, frei von zeitlichen und räumlichen Mittelungen. Die so erhaltenen Daten sind von Nutzen, wenn komplexe Strukturen oder Dynamiken auftreten, wie bei Polymeren oder porösen Oxiden, aber auch bei anderen Materialien, deren Heterogenität zunächst weniger offensichtlich ist. Der Aufsatz gibt einen Überblick über die wichtigsten aktuellen Anwendungen von Einzelmolekülen in Untersuchungen an kondensierter Materie auf Nanometerebene, mit Betonung auf weicher Materie und Materialwissenschaften. Beispiele umfassen die Orientierungsverteilung von Molekülen in Kristallen, Rotationsdiffusion in glasbildenden molekularen Flüssigkeiten, Studien an Polymeren mit Sonden und markierten Ketten, poröse und heterogene Oxidmaterialien, das Blinken einzelner Moleküle und Nanokristalle sowie das Potenzial oberfächenverstärkter Raman‐Streuung für die Analyse lokaler chemischer Eigenschaften. Die Beispiele zeigen, dass statische und dynamische Heterogenität und die Breiten der Verteilungen molekularer Parameter wesentlich größer sind, als bisher angenommen.

The optical signals of single molecules provide information about structure and dynamics of their nanoscale environment, free from space and time averaging. These new data are particularly useful whenever complex structures or dynamics are present, as in polymers or in porous oxides, but also in many other classes of materials, where heterogeneity is less obvious. We review the main uses of single molecules in studies of condensed matter at nanometer scales, especially in the fields of soft matter and materials science. We discuss several examples, including the orientation distribution of molecules in crystals, rotational diffusion in glass-forming molecular liquids, polymer studies with probes and labeled chains, porous and heterogeneous oxide materials, blinking of single molecules and nanocrystals, and the potential of surface-enhanced Raman scattering for local chemical analysis. All these examples show that static and dynamic heterogeneities and the spread of molecular parameters are much larger than previously imagined.

We experimentally investigate the solidification of supercooled glycerol during aging that has recently been observed by Zondervan et al. We find that a slow cooling at 5 K/h prior to the aging is required for solidification to take place. Furthermore, we show that the time of onset depends strongly on the aging temperature which we varied between 220 and 240 K. The nature of the solid phase remains unclear. The experiments show that upon heating the solid glycerol melts at the crystal melting point. However, rheology experiments in the plate-plate geometry revealed the growth of a soft, slushlike phase that is distinct from a crystal grown by seeding at the same aging temperature. The slushlike glycerol grows from a nucleation point at almost the same speed as a seeded crystal quenched to the same temperature, but its shear modulus is almost 2 orders of magnitude lower than the crystal phase, which we measure independently. While solidification was reproducible in the Couette geometry, it was not in the plate-plate geometry.