Thermal Shear Waves induced in Mesoscopic Liquids at Low Frequency Mechanical Deformation

TL;DR

This study demonstrates that mesoscopic liquids under low-frequency shear exhibit thermoelastic waves, revealing elastic-like behavior and non-equilibrium thermodynamics, which are not detectable through traditional stress measurements.

Contribution

It provides experimental evidence of thermoelastic effects in liquids, linking mechanical shear to thermal responses and suggesting elastic correlations at the molecular level.

Findings

Thermoelastic waves increase linearly with shear strain

Non-linear thermal behavior appears at higher strains

Thermoelastic effects are undetectable via stress measurements

Abstract

We show that a viscous liquid confined between two low thermal conductive surfaces (Al2O3) emit a thermal response upon applying a low frequency (Hz) shear excitation. Hot and cold thermal waves are observed in situ at atmospheric pressure and room temperature, in polypropylene glycol layers of various thicknesses ranging from 100 microns up to 340 microns, upon applying a mechanical oscillatory shear strain. The observed thermal effects, synchronous with the mechanical excitation, are inconsistent with a homogeneous viscous flow. It indicates that mesoscopic liquids are able to (partly) convert mechanical shear energy in non-equilibrium thermodynamic states. This effect called thermoelasticity is well known in solid materials. The observation of a thermal coupling to the mechanical shear deformation reinforces the assumption of elastically correlated liquid molecules. The amplitude of the thermoelastic waves increases linearly by increasing the shear strain amplitude up to a transition to a non-linear thermal behavior, similar in strain behavior to an elastic to plastic regime. The thermo-elastic effects are not detectable via stress measurements and thus appear as a complementary liquid dynamic characterization.