  The steady states of two vibrated granular gases separated by an adiabatic
piston are investigated. The system exhibits a non-equilibrium phase transition
with an spontaneous symmetry breaking. Even if the gases at both sides of the
piston have the same number of particles and are mechanically identical, their
steady volumes and temperatures can be rather different. The transition can be
explained by a simple kinetic theory model expressing mechanical equilibrium
and the energy balance occurring in the system. The model predictions are in
good agreement with molecular dynamics simulation results. The macroscopic
description of the steady states is discussed, as well as some physical
implications of the symmetry breaking.
