Summary: Broader aspects and grand challenges of the slow dynamics and glass transition problem.
C. A. Angell,
Dept. of Chemistry and Biochemistry,
Arizona State U., Tempe, AZ 85287, U.S.A.
The usual viscous liquid slow-down phenomenon, ending in the glass transition, is a popular problem
with many aspects to challenge the theoretician and experimentalist alike. However it is only one aspect of a
broader problem in which disordered elements of a condensed phase system slow down and finally grind to a
halt. While the familiar liquid problem is associated in most people's thinking with metastability, there are
systems with such slow dynamics that are not metastable, though their ergodicity-breaking may have all the
hysterisis and time-dependent features of the glass transition1
. In some systems the relaxation time behavior is
more interesting than others, being super-Arrhenius in character ("fragile"). In some systems, usually the same
ones, the relaxation process is more interesting than in the others, being stretched exponential rather than
exponential. In the same systems the dynamics are "heterogeneous"2
, both in space and time3
but this is mainly
because the dynamics are exponentially sensitive to subtle changes in the structure or thermodynamics. One of
the biggest challenges is to understand what it is that makes these systems "heterogeneous", causing
breakdowns in such familiar relations as the Stokes-Einstein equation.
Some of the most pronounced breakdowns4