When two objects collide, the coefficient of restitution, or COR, is the ratio of the speed of separation after the collision to the initial speed of approach. A perfectly elastic collision will have a restitution coefficient of one, but almost all macroscopic collisions are inelastic, with coefficients less than one. A golf ball dropped on a concrete floor, for example, will bounce with a COR of around 0.8, and it can never be greater than one–despite golfers’ dreams–because some of the kinetic energy always goes into heating the ball.
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[B]elow a certain threshold of cohesiveness some of the simulated events displayed a COR greater than one and as high as 1.05. The fraction of these anomalous rebounds increased with the cluster temperature. “When the temperature increases, more and more vibrating modes are excited,” Kuninaka says. These vibrations can sometimes give an extra kick to the collision, like a gymnast pushing off the pummel horse to get a greater lift.
IOW, when the particles are really small, you are more likely to see when they are expanding or contracting, a behavior that will get averaged out for larger particles. If you have a collision when the expansion happens, that internal vibration energy gets transferred to translational energy, and a COR > 1.
I don’t like how they say that the second law of thermodynamics doesn’t necessarily hold, because later on they explain how it actually does. There will an increase in entropy for the system, but you have to look at this stochastically rather than for an individual particle. It’s like a compressed spring which is at maximum compression just as it hits the ground, and snaps open on the impact (you can sometimes do this with a retractable pen) — it will rebound higher than the point from which it was dropped, because you convert the potential energy of the spring into kinetic energy, and this doesn’t violate any physical laws. But for an ensemble of springs for which the spring motion is random, the average rebound will be lower than the release point.