The "adiabatic" piston: And yet it moves

We perform molecular dynamic simulations of a system consisting of an insul ating piston, which separates two gases of hard disks, each initially at eq uilibrium at the same pressure but at different temperatures and densities. Upon releasing the piston, we monitor how the system returns to the global thermodynamic equilibrium with equal temperature and density on both sides . The approach of the piston to its final position is well described by an exponential law, while the heat flux is, to a good approximation, found to be proportional to the temperature difference.