  We use holography to study collisions of phase domains formed in a
four-dimensional, strongly-coupled gauge theory with a first-order, thermal
phase transition. We find three qualitatively different dynamical regimes
depending on the collision velocity. For low velocities the domains slow down
before the collision and subsequently merge and relax to equilibrium. For
intermediate velocities no slow down is present before the merger. For high
enough velocities the domains can collide and break apart several times before
they finally merge. These features leave an imprint on the time evolution of
the entropy of the system, which we compute from the area of the dual horizon
on the gravity side.
