Effect of surfactant on bubble collisions on a free surface

We report on the coefficient of restitution of bubble collision on a free surface in the presence of surfactants. In pure fluids, the collision process is well described by a competition between thin film drainage and interfacial tension. When surfactants are introduced in pure water, they generate Marangoni stresses on both the bubble interface and free surface, which provides an additional mechanism affecting the collision process. We investigate this mechanism for the bubble collision process in surfactant solutions through a combination of experimental and numerical approaches, with results showing a reduced rebound velocity during the collision process in surfactant solutions compared with that in pure water. Furthermore, by varying both bubble size and surfactant concentration, our experiments show that bubbles experience elastic, partially inelastic, and perfectly inelastic collisions. We identify the Langmuir number, the ratio between absorption and desorption rates, as the fundamental parameter that quantifies the Marangoni effect on the collision process. The effect of Marangoni stress on the bubble's coefficient of restitution is nonmonotonic, where the coefficient of restitution first decreases with Langmuir number and then increases.