Experimental observations of sub-Hinze bubble production in turbulence with surfactant

Knowledge of the spectrum of sizes of bubbles existing in turbulent dispersions of gases in liquids is essential to understanding many natural and industrial processes. However, the distribution of bubbles that are smaller than the Hinze scale d_H (at which turbulent stresses to bubbles are balanced by surface tension) is not well understood. We approach the problem with two experiments involving air bubbles injected into forced water turbulence: in the first, the bubble size distribution is described statistically using a range of parent bubble sizes and concentrations of surfactant; in the second, break-up events are resolved dynamically and in three dimensions using multiple high-speed cameras. Consistent with recent numerical observations, the sizes of the sub-Hinze bubbles produced are set by the capillary pinching dynamics instigated by turbulent deformations (of scales larger than d_H) to the parent bubble. Increased concentration of surfactant promotes greater production of sub-Hinze scale bubbles, pointing to distinct mechanisms for sub- and super-Hinze bubble production and an important role of surface tension in the former.