Collective Effects in Ionic Liquid [emim][Tf2N] and Ionic Paramagnetic Nitrate Solutions without Long-Range Structuring.

Mesoscopic shear elasticity has been revealed in ordinary liquids both experimentally by reinforcing the liquid/surface interfacial energy and theoretically by nonextensive models. The elastic effects are here examined in the frame of small molecules with strong electrostatic interactions such as room temperature ionic liquids [emim][Tf2N] and nitrate solutions exhibiting paramagnetic properties. We first show that these charged fluids also exhibit a nonzero low-frequency shear elasticity at the submillimeter scale, highlighting their resistance to shear stress. A neutron scattering study completes the dynamic mechanical analysis of the paramagnetic nitrate solution, evidencing that the magnetic properties do not induce the formation of a structure in the solution. We conclude that the elastic correlations contained in liquids usually considered as viscous away from any phase transition contribute in an effective way to collective effects under external stress whether mechanical or magnetic fields.

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

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.

Highlighting Thermo-Elastic Effects in Confined Fluids.

The recent identification of a finite shear elasticity in mesoscopic fluids has motivated the search of other solid-like properties of liquids. We present an innovative thermal approach of liquids. We identify a dynamic thermo-elastic mesoscopic behavior by building the thermal image produced by different liquids upon applying a low frequency mechanical shear field. We selected three fluids: a low molecular weight polybutylacrylate (PBuA), polypropyleneglycol (PPG), and glycerol. We demonstrate that a part of the energy of the shear strain is converted in cold and hot shear bands varying synchronously with the applied shear field. This thermodynamic change suggests a coupling to shear elastic modes in agreement with the low frequency shear elasticity theoretically foreseen and experimentally demonstrated.

Strain-induced violation of temperature uniformity in mesoscale liquids.

Thermo-elasticity couples the deformation of an elastic (solid) body to its temperature and vice-versa. It is a solid-like property. Highlighting such property in liquids is a paradigm shift: it requires long-range collective interactions that are not considered in current liquid descriptions. The present microthermal studies provide evidence for such solid-like correlations. It is shown that ordinary liquids emit a modulated thermal signal when applying a low frequency (Hz) mechanical shear stress. The liquid splits in several tenths microns wide hot and cold thermal bands, all varying synchronously and separately with the applied stress wave reaching a sizable amplitude of ± 0.2 °C. Thermomechanical coupling challenges fluid dynamics: it reveals that the liquid does not dissipate the energy of shear waves at low frequency, but converts it in non-uniform thermodynamic states. The dynamic thermal changes work in an adiabatic way supporting the hypothesis of the excitation of macroscopic elastic correlations whose range is limited to several tens of microns, in accordance with recent non-extensive theoretical models. The proof of thermomechanical coupling opens the way to a new generation of energy-efficient temperature converters.