Microflow: Thermal changes suggest a phonon-like liquid transport

We knew that classical thermal assumptions, such as Fourier’s law, were no longer valid at very small scales. Here, we highlight thermal effects occurring in the absence of a heat source and at a much larger scale (sub-millimeter scale) during the flow of Newtonian fluids such as water. The emergence of cold and hot non-equilibrium temperatures is the strong indication that non-diffusive processes govern microflows. We relate this mechano-caloric conversion to the excitation of shear elastic modes existing in confined liquids. We establish a linear relationship between flow-induced temperatures and the applied pressure gradient within a low pressure range (<1 bar). These results are consistent with non-extensive models incorporating parameters that allow for a spatiotemporal mesoscopic description, suggesting a phonon-like propagation of thermal waves in mesoscopic liquids.