  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
Suspension Balance Model illustrates that the underlying mechanism for this
dissipation-rate-deficit is related to a non-uniform particle distribution in
the direction perpendicular to the channel walls due to shear-induced particle
migration.
