Drift by air bubbles crossing an interface of a stratified medium at moderate Reynolds number

The dynamics of a single air bubble rising through a stably-stratified sharp interface, separating two Newtonian miscible liquids, are studied experimentally. Both liquids were water-glycerin mixtures; salt was added to the lower fluid to make its density higher than the upper one. The size of the bubbles was varied to span a range of terminal velocities and shapes. The rising bubbles, crossing the interface, drag along denser fluid into the upper lighter fluid as a drift volume. The Planar Laser-Induced Fluorescence (PLIF) technique was used to quantify this drift volume. For small bubbles, rising in a straight trajectory, the lower, denser fluid returns to the lower layer. For this case, the drift volume is stable and its maximum was found to be inversely proportional to the bubble Reynolds number, but proportional to the Froude number. A heuristic model is proposed to predict the size and evolution of the drift volume in the stable case. The model predictions showed good agreement with the experimental results. For larger bubbles, rising with a zig-zag or spiral trajectory, the drift volume becomes unstable and detaches from the bubble, leaving a trail of heavier fluid blobs in the lighter phase. This locally unstable configuration could potentially overturn, leading to fluid mixing.