Adiabatic and isothermal sound waves: The case of supercritical nitrogen

The acoustic sound dispersion of nitrogen in its liquid and supercritical phase has been studied by Inelastic X-Ray Scattering (IXS). Approaching supercritical conditions, the gradual disappearance of the positive sound dispersion, characteristic of the low-temperature liquid, is observed. In the supercritical state, evidence for a crossover between adiabatic and isothermal sound propagation regimes is inferred by an analysis of the dynamic structure factor based on generalized hydrodynamics.