  Terminal velocity reached by bubble walls in first order phase transitions is
an important parameter determining both primordial gravitational-wave spectrum
and production of baryon asymmetry in models of electroweak baryogenesis. We
developed a numerical code to study the real-time evolution of expanding
bubbles and investigate how their walls reach stationary states. Our results
agree with profiles obtained within the so-called bag model with very good
accuracy, however, not all such solutions are stable and realised in dynamical
systems. Depending on the exact shape of the potential there is always a range
of wall velocities where no steady state solutions exist. This behaviour in
deflagrations was explained by hydrodynamical obstruction where solutions that
would heat the plasma outside the wall above the critical temperature and cause
local symmetry restoration are forbidden. For even more affected hybrid
solutions causes are less straight forward, however, we provide a simple
numerical fit allowing one to verify if a solution with a given velocity is
allowed simply by computing the ratio of the nucleation temperature to the
critical one for the potential in question.
