(a) The probability distribution function of the curvature radius R/a of the particle trajectories. The inset shows the ensemble average of the radius as a function of the area fraction ϕ. (b) The mechanism of the increase in the inter-particle distances. The figure shows a particle trajectory that is surrounded by six rotating particles that are fixed in space. Due to the collision events, the inter-particle distance gradually increases and results in a distance r_ij/a = 2.