Classification of the reversible-irreversible transitions in particle trajectories across the jamming transition point

Abstract: The reversible-irreversible (RI) transition of particle trajectories in athermal colloidal suspensions under cyclic shear deformation is an archetypal nonequilibrium phase transition which attracts much attention recently. In the low-density limit, the RI transition is predicted to belong to a universality class of the absorbing state transitions, whereas at the high densities well above the jamming transition density, $\varphi_{\rm J}$, the transition is discontinuous and is closely related to the yielding transition. The transition between the two limiting cases is largely unexplored. In this paper, we study the RI transition of athermal frictionless colloidal particles over a wide range of densities, with emphasis on the region below $\varphi_{\rm J}$, by using oscillatory sheared molecular dynamics simulation. We reveal that the nature of the RI transitions in the intermediate densities is very rich. As demonstrated by the previous work by Schreck $et\ al.$ [Phys. Rev. E. ${\bf 88}$, 052205 (2013)], there exist the point-reversible and the loop-reversible phases depending on the density and the shear strain amplitude. We find that, between the two reversible phases, a quasi-irreversible phase where the particles' trajectories are highly non-affine and diffusive. The averaged number of contacts of particles is found to characterize the phase boundaries. We also find that the system undergoes the yielding transition below but in the vicinity of $\varphi_{\rm J}$ when the strain with a small but finite strain rate is applied. This yielding transition line matches with the RI transition line separating the loop-reversible from the irreversible phases. Surprisingly, the nonlinear rheological response called "softening" has been observed even below $\varphi_{\rm J}$. These findings imply that geometrical properties encoded in the sheared configurations control the dynamical transitions.