Granular Gas
Each collision steals a little energy — the gas cools unevenly and spontaneously condenses into clusters.
A granular gas is a collection of macroscopic particles (sand, seeds, marbles) that collide inelastically — each collision dissipates kinetic energy as heat and sound. Unlike molecular gases governed by thermodynamics, there is no bath to replenish lost energy.
The coefficient of restitution e (0 = perfectly inelastic, 1 = elastic) controls how much velocity is preserved in each collision. Below e = 1, the system inevitably cools. But cooling is not uniform: denser regions cool faster (more collisions per unit time), losing pressure and compressing further. This inelastic collapse creates dense clusters from an initially uniform gas — entropy appears to decrease locally, in defiance of intuition, driven by energy loss rather than thermal equilibration.