Influence of Surfactants on the Motion of a Pair of Bubbles in Quiescent Liquids

We experimentally investigated the motion of a pair of bubbles initially positioned in-line configuration in ultrapure water or an aqueous surfactant solution. The bubble motion was observed by two high speed video cameras. The Reynolds number of bubbles were ranged from 50 to 200, and the bubbles hold a spherical shape in this range. In ultrapure water or small concentration of surfactant solution, initially the trailing bubble deviated from the vertical line on the leading bubble owing to the wake of the leading bubble. And then, the slight difference of the bubble radius changed the relative motion. When the trailing bubble slightly larger than the leading bubble, the trailing bubble approached to the leading bubble due to it' s buoyancy difference. The bubbles attracted and collided only when the bubbles rising approximately side by side configuration. On the other hand, the trailing bubble was drafted to the wake region of leading bubble when the bubbles were rising in high concentration of surfactant solution. In this case, the bubbles always collided in line configuration. We consider that these phenomena are caused by the changes of lift force direction in the wake region owing to the surface contamination. In addition, bubble coalescence was inhibited in surfactant solution, even the relative motion was similar to the clean bubble case.