Fig. 4.

Randomly distributed particles develop into a long train of particles through a series of interactions. (A) A single particle joins a particle pair. (B) Two particle pairs join to make a four-particle train. (C) Train elongation. First, a faster single particle catches up to a three particle train, then, a slower particle joins the other particles. (D) A first particle (black arrow) joins a three-particle train. The impact is transferred down the train. (E) Particle acceleration from (D). Accelerations of the last three particles (red, blue, green) are nearly identical but translated in time displaying an aspect of wave propagation.