Coalescence Preference Depends on Size Inequality

During bubble or droplet coalescence, there is a puzzling tendency for the coalesced bubble or droplet to be preferentially placed closer to the larger of its two parents. We confirm that this preference is a function of parent size ratio by directly visualizing coalescing air bubbles on an oil-water interface and coalescing water droplets immersed in oil. We find that the final position of the coalesced sphere is controlled by surface energy release and is related to the parent size ratio by a power-law relationship.