  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.
