Do Surfactants Prevent the Partial Coalescence of Droplets?

When a droplet approaches its homophase, the two liquid bodies will coalesce upon contact. Under certain conditions, the droplet does not fully join the homophase and partial coalescence occurs, where a smaller droplet, forms. In this work, we investigate the partial coalescence of aqueous droplets with a liquid/liquid interface in the presence of surfactants. The experiments were carried out with 5 CST Silicone oil as organic liquid and pure water as the aqueous phase. Span80 was added to the oil with a mass ratio to silicon oil from 0 to 5e-5. It was found that for high surfactant concentrations over 5e-5, surfactants prevented partial coalescence by significantly deforming the interface. For low surfactant concentrations of 1e-5, partial coalescence occurs. At medium concentrations of 2e-5, however, a new phenomenon occurs, where multiple secondary droplets form during partial coalescence. The surfactants resulted in a thin liquid cylinder, where the Rayleigh-Plateau Instability occurred. Due to the instability, the cylinder broke into two or three droplets. Numerical simulations were also conducted to help analyze the mechanism driving the generation of multiple drops.