  A variational principle for two-fluid mixtures is proposed. The Lagrangian is
constructed as the difference between the kinetic energy of the mixture and a
thermodynamic potential conjugated to the internal energy with respect to the
relative velocity of phases. The equations of motion and a set of
Rankine-Hugoniot conditions are obtained. It is proved also that the convexity
of the internal energy guarantees the hyperbolicity of the one-dimensional
equations of motion linearized at rest.
