Twin satellites and ring bubbles from coalescing magnetically levitated air bubbles in water

We investigate the ejection of satellite bubbles during the coalescence of two air bubbles in water.Using magnetic levitation we suspend the bubbles, with diameters in the range 3--15 mm, in a stable, neutrally buoyant state, free from mechanical constraints.Our experiments reveal the simultaneous ejection of twin satellite bubbles from the poles of the bubble formed from the coalescence of equal-sized precursors.More generally, we find that the number and size of the ejected satellites are dependent on the initial size ratio of the precursor bubble pair. Numerical simulations, which agree well with our experiments, show that satellite production is governed by the combined effect of two distinct processes: the arrival of a capillary wave crest at a pole and the rapid retraction of that polar region. We propose a timing model that successfully predicts the range of size ratios for which single, double, or no satellites are formed.Our analysis also offers insight into the absence of satellite ejection in similarly-sized liquid drop coalescence.Furthermore, we report the experimental observation of toroidal bubble ejection from precursor bubbles with approximately 2:1 diameter ratios, and we show that the formation of these toroidal bubbles is also governed by our timing model.