Bubble–bubble interactions in two-dimensional bubble-chain flows

We experimentally investigate the bubble-bubble interactions and associated flow structures in a two-dimensional bubble chain while varying the bubble generation frequency up to 21Hz. The considered bubbles are characterized by a Bond number of around 1.0, and Archimedes and Reynolds numbers of O(102), high enough to have the shape and path instabilities. Near the nozzle (developing region), bubble trajectories and spacing between adjacent bubbles are retained (bubble-in-line arrangement), driven by the upward inertia. As the flow develops, the bubble wakes become organized and thus their interactions govern the flow. With increasing the bubbling frequency, it can be further classified into the bubble pairing, grouping, and coalescence-dominant regime. In the in-line arrangement, the trailing bubble is acceleratingly dragged to the wake behind the leading bubble, which are rearranged in tandem by the asymmetric wake. With increasing the bubbling frequency, the bubbles form a group, rapidly diffusing the jet-like liquid flow horizontally. As the vertical inertia (inviscid mechanism) dominates, the coalescence occurs frequently and the horizontal spreading of the liquid flow is suppressed substantially. By analyzing the time-averaged and fluctuating liquid velocity fields, we explain the bubble-bubble interactions in relation to the consequent modulation of the liquid-phase flow.