Fast and slow surfactants in turbulence-driven bubble breakup

When a large air cavity breaks in a turbulent flow, it goes through very large deformations and cascading events of new interface formation, including elongated filaments and bubbles over a wide range of scales, with their rate of formation controlled by turbulence and capillary processes. We experimentally investigate the effects of surfactants and salt on the fragmentation of a large air cavity, and observe a fivefold increase of the number of bubbles being produced in some cases. We characterise the bubble size distribution resulting from successive breakups in such a transient configuration. For bubbles larger than the Hinze scale d Subscript upper H d H $d_H$ (defined as the balance between surface tension and turbulence stresses), we observe that bubble size distributions remain unchanged for all solutions tested. For bubbles below d Subscript upper H d H $d_H$ , however, we observe an increase of the number of bubbles produced and an associated steepening of the bubble size distribution upon the addition of surfactant or salt. This later effect is only visible for surfactants with an adsorption time scale that is fast enough compared with the rate at which new interfaces are being generated by turbulence.