Effects of surfactants on bubble-induced turbulence

TL;DR

This study investigates how surfactants influence bubble-induced turbulence in water, revealing that increased surfactant levels amplify turbulence strength and flow anisotropy, with effects varying based on bubble size and gas volume fraction.

Contribution

It provides experimental insights into the impact of surfactants on bubble-induced turbulence, highlighting changes in flow anisotropy and extreme event probability across different bubble sizes and concentrations.

Findings

Surfactants increase the strength of bubble-induced turbulence.

Flow anisotropy is enhanced with higher surfactant concentrations.

Extreme velocity fluctuations are more probable at lower surfactant levels for small bubbles.

Abstract

We use experiments to explore the effect of surfactants on bubble-induced turbulence (BIT) at different scales, considering how the bubbles affect the flow kinetic energy, anisotropy and extreme events. To this end, high-resolution Particle Shadow Velocimetry measurements are carried out in a bubble column in which the flow is generated by bubble swarms rising in water for two different bubble diameters ($3$ mm $\&$ $4$ mm) and moderate gas volume fractions ($0.5\%\sim1.3\%$). To contaminate the flow, different amounts of 1-Pentanol were added to the flow, leading to different bubble shapes and surface boundary conditions. The results reveal that with increasing surfactant concentration, the BIT generated increases in strength, even though bubbles of a given size rise more slowly with surfactants. We also find that the level of anisotropy in the flow is enhanced with increasing surfactant concentration for bubbles of the same size, and that for the same surfactant concentration, smaller bubbles generate stronger anisotropy in the flow. Concerning the intermittency quantified by the normalized probability density functions of the fluid velocity increments, our results indicate that extreme values in the velocity increments become more probable with decreasing surfactant concentration for cases with smaller bubbles and low gas void fraction, while the effect of the surfactant is much weaker for cases with larger bubble and higher void fractions.