Velocity of the fourth sound in liquid helium IIvia extended thermodynamics

This work continues a study begun in previous works, where, using Extended Thermodynamics, a monofluid model of liquid helium II is formulated. The wave propagation inbulk liquid helium II is studied in the hypothesis that the thermal dilatation is not zero. Thepropagation of fourth sound, studied previously neglecting both the thermal dilatation and finitevolume of the powder, is studied without these simplified hypotheses: a scattering correction nis introduced to take into account the porosity. The model is more general than the standardtwo-fluid model because it allows that a small amount of entropy is associated with helium whenit flows through a very thin capillary or a porous medium. A comparison with experimentaldata is performed. From experimental values for velocities and attenuations of the two sounds inbulk liquid helium, the model provides the velocity of fourth sound in a porous medium. Thesevalues are determined at various temperatures and pressures and compared with fourth soundmeasurements in a packed powder.