  We discovered an out-of-equilibrium transition in the ideal gas between two
walls, divided by an inner, adiabatic, movable wall. The system is driven
out-of-equilibrium by supplying energy directly into the volume of the fluid.
At critical heat flux, we have found a continuous transition to the state with
a low-density, hot gas on one side of the movable wall and a dense, cold gas on
the other side. Molecular dynamic simulations of the soft-sphere fluid confirm
the existence of the transition in the interacting system. We introduce a
stationary state Helmholtz-like function whose minimum determines the stable
positions of the internal wall. This transition can be used as a paradigm of
transitions in stationary states and the Helmholtz-like function as a paradigm
of the thermodynamic description of these states.
