[MOS] March 17, 2009

[Zina Queen]

Seminar on

Modern Optics and Spectroscopy

Role of the probe in single-molecule experiments on supercooled liquids

Laura Kaufman,
Columbia University

Tuesday, March 17, 2009

12:00 - 1:00 p.m.

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.


Grier Room, MIT Bldg 34-401
Refreshments served after the lecture
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