Suspension plug in an oscillatory pipe flow

We experimentally apply an oscillatory flow to a densely packed suspension plug inside a cylindrical tube in the limit of low Reynolds and high Peclet numbers. The plug becomes reversible in shape once it gradually expands and dilutes to reach a critical particle concentration, matching the previous experimental observations. Surprisingly, our particle-scale measurements reveal that above a critical strain amplitude, individual particles remain irreversible even after the plug shape has stabilized. We further explore this decoupling between particle-scale and system-scale reversibility with simplified two-dimensional simulations.