On the role of the surface rheology in film drainage between fluid particles

Coalescence in the presence of low surfactant concentrations is investigated via a film drainage model, where the interface is a Boussinesq surface fluid with surfactant concentration dependent physical properties. Three cases are considered representing the systems where water is the continuous phase and the dispersed phase is either high viscosity droplets, droplets of comparable viscosity to water or gas bubbles. In the former, the immobilization of the interface is due to high dispersed phase viscosity and surface viscosities, whereas in the latter two, the Marangoni flow plays an important role, too. When droplets of comparable viscosity to water or gas bubbles are considered, it is seen that both the Marangoni flow and the surface viscosities can change the coalescence time significantly for the experimentally encountered values of the initial surfactant concentration, and the Boussinesq and surface Peclet numbers. In all cases, the impact of the surface phenomena amplifies with the approach velocity, especially for the dimpled interfaces. A complete immobilization criterion that is independent of the dispersed phase viscosity is proposed as a function of the continuous phase and surface viscosities, and the particle radii. (C) 2019 The Authors. Published by Elsevier Ltd.