Unraveling the Landau's consistence criterion and the meaning of interpenetration in the "Two-Fluid" Model

TL;DR

This paper clarifies the physical basis of Landau's consistency criterion and the concept of interpenetration in the two-fluid model by emphasizing the importance of temporal coarse-graining in hydrodynamics.

Contribution

It provides a new interpretation of the two-fluid model's core concepts by linking them to time coarse-graining, without challenging the model's validity.

Findings

Landau's consistency criterion is rooted in temporal coarse-graining.

The meaning of interpenetration is clarified through the time fuzziness.

The validity of the two-fluid model remains unaffected.

Abstract

In this letter we show that it is possible to unravel both the physical origin of the Landau's consistence criterion and the specific and subtle meaning of interpenetration of the "two fluids" if one takes into account that in the hydrodynamic regime one needs a coarse-graining in time to bring the system into local equilibrium. That is, the fuzziness in time is relevant for the phenomenological Landau's consistency criterion and the meaning of interpenetration. Note also that we are not questioning the validity of the "Two-Fluid" Model.