Spatial and temporal heterogeneity in supercooled glycerol: Evidence from wide field single molecule imaging

Abstract

We quantify spatial and temporal heterogeneity in supercooled glycerol at T=T(g)+14 K employing a widefield detection scheme and using rubrene as the probe molecule. We describe how microscopy configuration affects measured intensity, linear dichroism, and the resulting autocorrelation function. Rotational relaxation times tau(c) of 241 probe molecules are measured, and we find spatial heterogeneity over almost three orders of magnitude present at this temperature. An approach for detecting temporally heterogeneous molecules and quantifying exchange times is introduced. Of molecules that can be assessed, approximately 15% display evidence of temporal heterogeneity-changes of tau(c) during the measurement-that are detected with the analysis technique employed. Exchanges between dynamically disparate environments occur rarely but in the proportion expected given the rarity of very slowly rotating molecules present. Heterogeneous molecules are characterized by persistence and exchange times. Median exchange and persistence times of the molecules identified as heterogeneous relative to glycerol's structural relaxation time tau(alpha) are found to be tau(ex)/tau(alpha)=202 and tau(pers)/tau(alpha)=405, respectively. These results are discussed in the context of values of exchange times that have been determined in other experiments.