Micron-sized structure in a thin glycerol film revealed by fluorescent probes

We report on micrometer-sized structures in supercooled glycerol observed by imaging fluorescent probes at the temperatures close to, but above, the glass transition temperature (190 K). Two distinct heterogeneous patterns of the fluorescence intensity were detected, depending on how fast the sample was cooled down. In a slowly cooled sample, we observed a Swiss cheese-like pattern in which many micrometer-sized dark spots were nucleated in a bright background. A quickly cooled sample resulted in a spinodal decomposition pattern where many bright island-like features on micrometer scale were dispersed in a dark matrix. Similar patterns were seen earlier in triphenyl phosphite, another molecular liquid, which shows solid-like behavior at temperatures above its glass transition. Once the heterogeneous patterns are formed in the glycerol, they can persist for days, unless the samples are heated above 260 K for more than 10 h. Such heterogeneous patterns are ascribed to differential dye distributions in the glycerol film, pointing to long-lived and micrometer-scale density fluctuations in supercooled glycerol. The observation of such heterogeneity may provide additional understanding on how supercooled glycerol behaves before it turns into a glass.