Variational multi-fluid dynamics and causal heat conductivity

Understand

We discuss heat conductivity from the point of view of a variational multi-fluid model, treating entropy as a dynamical entity.

• We demonstrate that a two-fluid model with a massive fluid component and a massless entropy can reproduce a number of key results from extended irreversible thermodynamics.
• In particular, we show that the entropy entrainment is intimately linked to the thermal relaxation time that is required to make heat propagation in solids causal.
• We also discuss non-local terms that arise naturally in a dissipative multi-fluid model, and relate these terms to those of phonon hydrodynamics.