Structure and dynamics during training of memory in jammed packings

Jammed packings can be trained by intermediate-amplitude quasistatic oscillatory shear to fall into periodic trajectories in which the same multiple local minima are explored in each period. Here we ask how the rearrangement dynamics and stroboscopic snapshots of packing structure evolve during the training process. We use persistent homology to characterize the structure. We characterize rearrangements in terms of T1 events and classify them as reversible if they return to their initial configuration and irreversible if they do not. At the end of the training process, irreversible T1 events must vanish when the system falls into a periodic trajectory (develops a memory). We find that the structure varies continuously during the training, but that the nature and number of T1 events remains nearly constant until the last cycle or two before the system develops memory. Thus, observable changes in structure have relatively little effect on dynamics until near the end of the training process, in contrast with glassy dynamics, in which extremely subtle changes in structure lead to enormous changes in dynamics.