Unexpected Effect of Internal Degrees of Freedom on Transverse Phonons in Supercooled Liquids

TL;DR

This study reveals that internal molecular modes in supercooled liquids influence transverse phonons and shear viscosity, causing additional broadening of Brillouin lines, a phenomenon not previously understood.

Contribution

It demonstrates experimentally that internal degrees of freedom affect transverse phonons and shear viscosity, highlighting the need for a new theoretical framework.

Findings

Internal modes contribute to shear viscosity and phonon damping.

Internal processes cause additional broadening of Brillouin lines.

Structural relaxation and internal processes effects can be separated by high-pressure measurements.

Abstract

We show experimentally that in a supercooled liquid composed of molecules with internal degrees of freedom the internal modes contribute to the frequency dependent shear viscosity and damping of transverse phonons, which results in an additional broadening of the transverse Brillouin lines. Earlier, only the effect of internal modes on the frequency dependent bulk viscosity and damping of longitudinal phonons was observed and explained theoretically in the limit of weak coupling of internal degrees of freedom to translational motion. A new theory is needed to describe this new effect. We also demonstrate, that the contributions of structural relaxation and internal processes to the width of the Brillouin lines can be separated by measurements under high pressure.