On the coalescence of dissolving bubbles in surfactant presence

Abstract

In any real system with dispersed bubbles, dissolution and impurity/surfactant presence will affect the bubble dynamics to a certain extent. This work aims to determine the quantitative conditions under which these mechanisms significantly change the coalescence behavior and time in a dispersed system. To achieve this, coalescence of two gently colliding, deformable, dissolving bubbles is investigated through a film drainage study in the surfactant presence. The drainage model is analyzed in the thin film limit and coupled with the boundary integral method. The dissolution is found to slow down the drainage and increase the coalescence time. When the film saturation is relatively fast, i.e., low solubility and high collision velocity, however, the change in the coalescence time appears to be insignificant. The non-uniform distribution of the surfactants along the interface creates Marangoni stresses, which in turn delays the coalescence by immobilizing the particle interfaces. The contribution of the immobilization in delaying the coalescence is found to be much bigger than that of the dissolution, even for very low surfactant concentrations. The conditions under which the interfaces are completely immobilized due to Marangoni stresses are presented. However, these conditions do not seem to be easily achievable in typical systems, indicating that other factors, such as the interfacial viscosity, may play significant roles in immobilization.