Change in the mechanical energy of particles with different coefficients of restitution versus time. Panels (a) and (b) correspond to the global change and the local change, respectively. The local change refers to the enlarged view of the part surrounded by the black dash-lined box in panel (a). The red dash-lined box marks the collision site, and the blue dash-lined box marks the range of time that coefficients of restitution keep monotonicity versus mechanical energy. The mechanical energy for different ε is presented with different colors as indicated by the legend.