Categories: Chemistry and Chemical Biology (CCB)
Speaker: Laura Kaufman, Asst. Professor, Department of Chemistry, Columbia University
Date & Time: December 16, 2008 - 11:00am
Location: Chem. 260
Towards Elucidating the Role of the Probe in Single Molecule Experiments in Supercooled Liquids
Chemistry and Chemical Biology
Supercooled liquids display a range of unusual behaviors that can be detected through bulk experiments. The results from most such experiments are consistent with the presence of heterogeneous dynamics in these systems. Indeed, it is suspected that, at any given time, some molecules in a supercooled liquid are moving orders of magnitude faster than others in the same system. Because the sets of molecules exhibiting a given set of dynamics are expected to be small (and themselves changing dynamical behavior over time), only techniques that avoid ensemble averaging can directly reveal the length and time scales associated with these heterogeneities. Of techniques that limit ensemble averaging, perhaps the most straightforward to perform and understand are single molecule (SM) fluorescence measurements that can directly follow the rotations of fluorescent probes embedded in a supercooled liquid. Previous SM experiments on small molecule glass formers near their glass transition temperature (Tg) have reached different conclusions on the breadth of spatial heterogeneities present, the temperature dependence of this breadth of heterogeneities, and the timescales on which slow molecules tend to stay slow and fast molecules tend to stay fast. In addition, discrepancies between SM experiments and bulk experiments regarding the lifetime of heterogeneous regions remain stark. These discrepancies call into question whether SM probes embedded in supercooled liquids experience and report on the heterogeneous dynamics in these systems in an unbiased manner. We present complementary simulations and experimental results that suggest the probe can affect local dynamics in supercooled liquids and that careful data analysis is necessary to use SM experiments to elucidate the size and lifetimes of heterogeneous dynamics in these systems.
Host: Edward Castner