Identification of Thermal Response of Mesoscopic Liquids under Mechanical Excitation: From Harmonic to Nonharmonic Thermal Wave

Abstract

In the conventional picture, the temperature of a liquid bath in the quiescent state is uniform down to thermal fluctuation length scales. Here we examine the impact of a low-frequency shear mechanical field (hertz) on the thermal equilibrium of polypropylene glycol and liquid water away from any phase transition confined between high-energy surfaces. We show the emergence of both cooling and heating shear waves of several tens of micrometers widths varying synchronously with the applied shear strain wave. The thermal wave is stable at low strain amplitude and low frequency while thermal harmonics develop by increasing the frequency or the strain amplitude. The liquid layer behaves as a dynamic thermoelastic medium challenging the extension of the fluctuation-dissipation theorem to nonequilibrium fluids. This view is in agreement with recent theoretical models predicting that liquids support shear elastic waves up to a finite propagation length scale of the order the thermal wave.