Materials Science

Glassy Dynamics

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Science  20 Dec 2013:
Vol. 342, Issue 6165, pp. 1418
DOI: 10.1126/science.342.6165.1418-d

In the glassy state, atomic or molecular motion is limited to localized regions within an overall disordered structure. As a material is cooled toward its glass transition temperature, there is a rapid increase in the viscosity, which is marked by a slowdown of the atomic or molecular motions that is stronger than one would predict from a simple Arrhenius law. One theory is that as the glassy material is cooled, there is an increase in the number of correlated molecules that need to move together. This leads to a temperature-dependent activation energy, E(T) = exp(Δ/T), where Δ can be thought of as an energy barrier. Bauer et al. measured the third-order nonlinear dielectric susceptibility for four materials that included one “strong” glass former and two “fragile” ones. The latter are of particular interest, because the energy barrier Δ itself shows an excess component that is also temperature-dependent. Surprisingly, they find that there is a simple correlation between E(T) and the number of correlated molecules, largely independent of the molecular interactions within each material. The formation of the glassy state and the rapid viscosity rise are thus primarily due to an increase in the number of atoms or molecules, whose motions couple together as the temperature is lowered.

Phys. Rev. Lett. 111, 225702 (2013).

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