Discontinuous shear modulus determines the glass transition temperature

TL;DR

This study experimentally demonstrates a discontinuous change in shear modulus at the glass transition in colloidal systems, linking elastic properties to vitrification and validating continuum elasticity theory at low frequencies.

Contribution

It provides direct experimental evidence of a discontinuous shear modulus at the glass transition, clarifying elastic behavior during vitrification.

Findings

Shear modulus drops discontinuously at T_G

Continuum elasticity describes low-frequency excitations

Elastic properties determine the glass transition temperature

Abstract

A solid - amorphous or crystalline - is defined by a finite shear modulus while a fluid lacks such. We thus experimentally investigate the elastic properties of a colloidal glass former near the glass transition: spectroscopy of vibrational excitations yields the dispersion relations of longitudinal and transverse phonons in the glassy state. From the long wavelength limit of the dispersion relation we extract the bulk and the shear modulus. As expected, the latter disappear in a fluid and we measure a clearly resolved discontinuous behaviour of the elastic moduli at the glass transition. This not only determines the transition temperature T_G of the system but also directly addresses recent discussions about elasticity during vitrification. We show that low frequency excitations in our system are plane waves such that continuum elasticity theory can be used to describe the macroscopic behaviour.