Abstract
By combining, and modestly extending, a variety of theoretical concepts for the dynamics of liquids
in the supercooled regime, we formulate a simple analytic model for the temperature and wavevector
dependent collective density fluctuation relaxation time that is measurable using coherent dynamic
neutron scattering. Comparison with experiments on the ionic glass-forming liquid Ca–K–NO3 in the
lightly supercooled regime suggests the model captures the key physics in both the local cage and
mesoscopic regimes, including the unusual wavevector dependence of the collective structural relaxation
time. The model is consistent with the idea that the decoupling between diffusion and viscosity
is reflected in a different temperature dependence of the collective relaxation time at intermediate
wavevectors and near the main (cage) peak of the static structure factor. More generally, our analysis
provides support for the ideas that decoupling information and growing dynamic length scales can be
at least qualitatively deduced by analyzing the collective relaxation time as a function of temperature
and wavevector, and that there is a strong link between dynamic heterogeneity phenomena at the
single and many particle level. Though very simple, the model can be applied to other systems, such
as molecular liquids.
