  Relativistic, charged, superheated bubbles may play an important role in
neutron star mergers if first-order phase transitions are present in the phase
diagram of Quantum Chromodynamics. We describe the properties of these bubbles
in the hydrodynamic regime. We find two qualitative differences with
supercooled bubbles. First, the pressure inside an expanding superheated bubble
can be higher or lower than the pressure outside the bubble. Second, some fluid
flows develop metastable regions behind the bubble wall. We consider the
possible role of a conserved charge akin to baryon number. The fluid flow
profiles are unaffected by this charge if the speed of sound is constant in
each phase, but they are modified for more general equations of state. We
compute the efficiency factor relevant for gravitational wave production.
