Concentrated noncolloidal particles under nonuniform shear can migrate across streamlines even at low Reynolds numbers, breaking the well-known reversibility of Stokes flow. We conduct systematic experiments to study this phenomenon by oscillating a finite suspension plug comprising neutrally buoyant noncolloidal particles inside a cylindrical tube with varying strain amplitudes $\gamma$. We visualize the particles on the central axial plane of the tube via refractive index matching and track the motion of individual particles. Consistent with the previous findings, our experiments show that suspension plugs under oscillatory flows dilute to a critical volume fraction and become reversible in overall shape. However, our measurements reveal persistent particle motion even after this critical volume fraction threshold is reached, which points to a previously unreported disconnect between particle-scale and system-scale reversibility. In addition, we build a simplified two-dimensional model of particles under nonuniform shear and capture the gradual dispersion of the suspension plug, in qualitative agreement with the experiments.

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• Low Reynolds number flows
• Particle-laden flows