Peculiarities of Surfactant Extraction by a Droplet from its Homogeneous Solution

The process of surfactant extraction by a droplet from the surrounding homogeneous solution was studied experimentally via selecting the system of liquids in which the surfactant concentration in the droplet (upon completion of the diffusion) is many times higher than its initial concentration in the environment. It has been found that, under normal gravity, a significant concentration gradient is formed in the droplet, which results in the development of Marangoni convection, although an external gradient is absent. Visualization of flow patterns and concentration fields in the droplet and the environment made it possible to trace their evolution and to determine the conditions for occurrence of an oscillatory mass transfer regime. The results obtained demonstrate how the surfactant concentration and its vertical distribution vary vs. the droplet diameter, the initial surfactant concentration in the surrounding liquid and the time elapsed from the droplet formation.