Surfactant effects on bubble breakup in turbulent flow through laboratory experiments

Bubbles entrained by breaking waves are trapped beneath the ocean surface and fragment within the intense turbulence the waves generate, a process that strongly modulates air–sea gas exchange. The breakup of bubble in bulk turbulence is governed by the Kolmogorov–Hinze scale: below this critical diameter surface tension stabilizes the interface and prevents breaking, whereas above it inertial stresses can still induce rupture. To elucidate the influence of ocean chemistry on bubble breakup, we consider three solutions of increasing surfactant concentration: sodium dodecyl sulfate (SDS) in deionized water, SDS in artificial seawater, and Tween-20 in deionized water. The corresponding static surface tensions of these solutions decrease progressively. A tank was filled with each solution, and a large air cavity was injected at its center and released into turbulence generated by four symmetrically arranged pumps. Bubble populations were quantified with two high-speed cameras with two lenses of different magnifications, which provides an overall bubble distribution and resolves bubble diameters down to 200 µm. We demonstrate that for SDS in both deionized water and artificial seawater, reducing surface tension by increasing surfactant concentration produced a pronounced rise in bubbles smaller than the Hinze scale (sub-Hinze bubbles), whereas the larger bubbles (super-Hinze bubbles) were unchanged. In contrast, the slow-adsorbing surfactant Tween-20 left both sub- and super-Hinze populations unaffected.