  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.
