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color="#000000"><b>Seminar on</b></font></div>
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<div align="center"><font face="Times New Roman" size="+1"
color="#000000"><b>Modern Optics and Spectroscopy</b></font></div>
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<div align="center"><font face="Times New Roman" size="+1"
color="#000000"><i><b>Role of the probe in single-molecule experiments
on supercooled liquids</b></i></font></div>
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color="#000000"><i><b><br></b></i></font></div>
<div align="center"><font face="Times New Roman" size="+1"
color="#000000"><b>Laura Kaufman</b>,</font></div>
<div align="center"><font face="Times New Roman" size="+1"
color="#000000">Columbia University</font></div>
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<div align="center"><font face="Times New Roman" size="+1"
color="#000000">Tuesday, March 17, 2009</font></div>
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<div align="center"><font face="Times New Roman" size="+1"
color="#000000">12:00 - 1:00 p.m.</font></div>
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<div align="center"><font face="Times New Roman" size="+1"
color="#000000">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.</font></div>
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<div align="center"><font face="Times New Roman" size="+1"
color="#000000">Grier Room, MIT Bldg 34-401</font></div>
<div align="center"><font face="Times New Roman" size="+1"
color="#000000">Refreshments served after the lecture</font></div>
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