Terminal stage of highly viscous flow

TL;DR

This paper presents a theoretical model for the terminal stage of highly viscous flow based on Eshelby relaxations, successfully predicting dielectric spectra features and relating shear and density fluctuations with experimental validation in various substances.

Contribution

It extends the shear misfit model to density fluctuations, deriving a new relation between adiabatic and isothermal compressibility jumps at the glass transition.

Findings

The model predicts a Debye peak in dielectric spectra consistent with experiments.

Adiabatic compressibility equilibrates earlier than isothermal in studied materials.

Adiabatic density fluctuations are observed in thermal expansion measurements.

Abstract

The shear misfit model for the highly viscous flow is based upon a theoretical prediction for its terminal stage in terms of irreversible Eshelby relaxations in the five-dimensional shear space. The model is shown to predict a small delta-function (Debye peak) in the dielectric spectrum, in agreement with experimental evidence. It is extended from shear fluctuations to density fluctuations, a new relation between adiabatic and isothermal compressibility jumps at the glass transition is derived, and the model is brought into a form which requires only three temperature-dependent parameters for the fit of shear relaxation data of a glass former with a secondary relaxation peak. The model is applied to high precision measurements of the shear, dielectric and bulk relaxation data in two vacuum pump oils and in squalane, a short chain polymer with a strong secondary relaxation peak. In all three substances, the adiabatic compressibility equilibrates much earlier than the isothermal one. The terminal stage of aging data in squalane demonstrate that one sees also the adiabatic density fluctuations in the thermal expansion, explaining why it seems to equilibrate a bit faster than the dynamic heat capacity.