Spatio-temporal dynamics in a train of rising bubbles

It has been suggested that rising bubbles in dense fluids resemble an inverted dripping faucet and that they undergo analogous perioddoubling bifurcations to chaos. We present experimental results that demonstrate that this analogy is weak because the dominant source of instability in the bubble train is inherently different - mutual interactions between spatially separated bubbles as opposed to nozzle dynamics. Unlike the dripping faucet, the initial instability in a bubble train develops at a location far from the injection nozzle and progresses toward the nozzle with increasing gas flow. From qualitative and rigorous quantitative observations, we conclude that rising-bubble dynamics are best described as 'small-box spatio-temporal chaos' with a flow instability. Such dynamics can superficially appear to be simple temporal chaos when considering spatially localized measurements. We show similarity between our experimental results and a bubble-interaction model that accounts for drag and coalescence effects without considering any nozzle dynamics.