Fast and slow surfactants in turbulent bubble breakup

TL;DR

This study experimentally examines how surfactants and salt influence turbulent bubble breakup, revealing increased bubble production and size distribution changes for smaller bubbles, depending on surfactant adsorption dynamics.

Contribution

It provides new insights into the effects of surfactants and salt on bubble fragmentation in turbulence, especially regarding size distribution and the role of surfactant adsorption timescales.

Findings

Surfactants and salt increase the number of smaller bubbles produced.

Bubble size distributions remain unchanged for bubbles larger than the Hinze scale.

Fast-adsorbing surfactants cause notable changes in bubble size distribution below the Hinze scale.

Abstract

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, and observe an order of magnitude increase of the number of bubbles being produced in some cases. For bubbles larger than the Hinze scale $d_H$ (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 $d_H$, 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 some of the surfactants tested when their adsorption timescale is fast enough compared to the rate at which new interfaces are being generated by turbulence.