Gradient generalization to internal state variables and a theory of superfluidity

Main properties of liquid helium regarded as starting points of the model developed in the paper are presented. Thermomechanical balance equations together with the second law of thermodynamics are discussed in the framework of a gradient generalization of the internal state variable approach. A comparison is made between the present model and the Landau's one; moreover some main physical properties of helium II are analyzed in the framework of the present approach.