  Quantum Chromodynamics (QCD) is expected to have a first order phase
transition between the confined hadron gas and the deconfined quark gluon
plasma at high baryon densities. This will result in phase boundary effects in
the metastable and unstable regions. It is important to include these effects
in phenomenological models of heavy ion collisions to identify experimental
signatures of a phase transition. This requires building intuition on phase
separation in rapidly expanding fluids. In this work we present the equations
of relativistic hydrodynamics with a phase boundary, provide prescriptions to
extend the equation of state to metastable and unstable regions, and show the
effects of spinodal separation in a Bjorken flow.
