A phenomenological analysis of the coalescence of single and mixed surfactant laden aqueous droplets at oil-aqueous interfaces

Controlling droplet coalescence is essential to optimize the performance of emulsions. In this article, the coalescence behaviour of aqueous surfactant laden droplets containing a nonionic surfactant (EOT), an anionic surfactant (NaDDBS), or their mixture at a mole ratio of n EOT / n NaDDBS = 0.01 , with a flat oil-aqueous interface, is explored. High speed shadowgraphy, planar laser induced fluorescence, and the particle image velocimetry techniques were employed, while the surfactant concentrations varied from below to above their critical micelle concentration. The surfactant laden droplets showed longer rest times than droplets of pure deionized water (an increase of almost 96 % at the highest EOT concentration studied), due to their greater deformation and extended trapped oil films. EOT droplets also exhibited longer rest times than NaDDBS ones, while the mixed surfactant laden droplets had rest times between those of the single surfactant systems. After film rupture, longer merging times were found for the surfactant laden droplets compared to those obtained for the droplets with single surfactants, with an increase of up to 43 % at the highest EOT concentration. Upon film rupture, vortices formed whose shape, evolution, and strength depended on the location of the rupture point and the surfactant concentration. Increasing the EOT concentration reduced vortex maximum intensity by up to ~ 84 % compared to pure aqueous droplets. • Synergistic effects between nonionic and anionic surfactants affect droplet coalescence dynamics. • Compared to concentrations below surfactant Critical Micelle Concentration, coalescing droplets at concentrations above surfactant Critical Micelle Concentration exhibit higher rest and merging times. • Planar laser induced fluorescence and particle image velocimetry reveal how film rupture location, neck growth, as well as velocity fields and vortices within the coalescing droplets, are affected by surfactant concentration.