Lifetime of bubble clusters and their rise velocities in a quiescent liquid

We use experiments to study the evolution of bubble clusters in a swarm of freely rising, deformable bubbles. A new machine learning-aided algorithm allows us to identify and track

bubbles in clusters and measure the cluster lifetimes. Our results show that 2-bubble clusters are the most common, however, 3 and 4-bubble clusters also often occur. The clusters persist on average for a time of order d_b/U_b (with d_b the bubble diameter and U_b the slip velocity), although rare clusters persisting for an order of magnitude longer are also observed. Furthermore, surfactants in the carrier liquid are observed to enhance the formation of bubble clusters and prolong the cluster lifetimes. The mean bubble rise velocities conditioned on the bubble cluster size are also explored, and we find a positive correlation between the cluster size and the rise speed of the bubbles in the cluster, with clustered bubbles rising up to 20% faster than unclustered bubbles.