  Relativistic bubble walls from cosmological phase transitions (PT)
necessarily accumulate expanding shells of particles. We systematically
characterize shell properties, and identify and calculate the processes that
prevent them from free streaming: phase-space saturation effects,
out-of-equilibrium $2\to2$ and $3\to2$ shell-shell and shell-bath interactions,
and shell interactions with bubble walls. We find that shells do not free
stream in scenarios widely studied in the literature, where standard
predictions will need to be reevaluated, including those of bubble wall
velocities, gravitational waves (GW) and particle production. Our results
support the use of bulk-flow GW predictions in all regions where shells free
stream, irrespectively of whether or not the latent heat is mostly converted in
the scalar field gradient.
