Reversibility in the oscillatory motion of 2-D granular hydrogel packings

We are interested in reversibility, memory, and transitions between fluid-like and solid-like behavior in 2D packings of granular hydrogels. To investigate these phenomena, we pack a 2D tray with cm-sized granular hydrogels and move the tray in an oscillatory fashion past a bed of fixed obstacles. We vary the frequency and amplitude of the externally-driven motion, as well as the arrangement of obstacles, and perform two distinct types of experiments. In one, we continually oscillate the tray over many cycles, measuring particle positions and displacements at the end of each cycle to identify the limits between reversible and irreversible motion. We use a combination of particle tracking and image correlation methods to quantify particle positions, trajectories, coordination numbers, and other metrics to determine if and when the system reaches a steady, reversible state. In the other experiment type, we perform a series of 'fixed impulse' tests with pauses at the end of every half-cycle of oscillation to measure the time scale of residual motion and rearrangement in the particle configurations after the confining tray stops moving. This measurement provides us with an intrinsic time scale of motion in the system. We find this time scale depends on the details of the applied strain and we can use it to define an effective Peclet number for the oscillatory tests.