Transport of a passive scalar around confined bubbles submitted to reactive mass transfer

We present new experiments about the transport of a passive constituent around a bubble rising at large Reynolds number in a vertical thin-gap cell filled with a reactive liquid. A chemical reaction occurs at the interface and the constituents are transported in the liquid. First, 4D-Lagrangian particle tracking (4D-LPT) measurements are performed to scrutinise the three-dimensional structure of the flow in the vicinity of a large bubble rising steadily. They reveal that two horseshoe vortices, superposed in the direction perpendicular to the walls, precede the bubble. The attached wake also consists of two superposed recirculations and two transverse vortices stacked to the rear interface. The horseshoe vortices are also observed for oscillating bubbles, and their radius scales with the bubble diameter, as explained by inviscid arguments. Then, time-resolved concentration fields of a non-diffusive constituent are obtained around the bubbles by a colorimetry method. A comparative discussion with 4D-LPT results or bubble kinematics provides new insights into the mode of transport of this constituent transferred at the interface. Regardless of the size of the bubble, mass transfer occurs predominantly in the liquid films located between the bubble and the walls, resulting in a direct deposit of mass near the walls. On another hand, the high-Reynolds number flow surrounding the bubble combines transport in the horseshoe vortices, intermittent detachment of mass from the equator of the bubble synchronised with vortex shedding and bubble shape contraction and finally transport in a strongly stretched and dampened von Kármán array leading to final deposits of mass.