Puzzling Bubble Rise Speed Increase in Dense Granular Suspensions

We present an anomalous experimental observation on the rising speed of air bubbles in a Hele-Shaw cell containing a suspension of spherical, neutrally buoyant, non-Brownian particles. Strikingly, bubbles rise faster in suspensions as compared to particle-less liquids of the same effective viscosity. By carefully measuring this bubble speed increase at various particle volume fraction and via velocity field imaging, we demonstrate that this strange bubble dynamics is linked to a reduction in the bulk dissipation rate. A good match between our experimental data and computations based on a Suspension Balance Model (SBM) illustrates that the underlying mechanism for this dissipation-rate deficit is related to a nonuniform particle distribution in the direction perpendicular to the channel walls due to shear-induced particle migration.