On the effect of surfactants on drop coalescence at liquid/liquid interfaces

- The surfactant concentration affects interface shape during drop coalescence with interface.
- Velocity fields inside drop during coalescence are reported for the first time.
- Vortices form inside the drop during coalescence whose evolution is affected by surfactant concentration.

In this work the effect of surfactants on the coalescence of a drop with a flat aqueous-organic interface was experimentally investigated. A high speed Particle Image Velocimetry (PIV) system was used to obtain velocity profiles and kinetic energy per unit mass distribution inside the coalescing droplets. Different mass ratios of surfactant to oil below the CMC value, up to ϕ = 5 × 10−4, of a non-ionic surfactant dissolved in the organic phase were studied. It was found that an increase in the surfactant concentration promoted the deformation of the interface before the film that separated the drop from the interface ruptured. A high surfactant concentration also increased the time needed for film rupture. When rupture occurred, two counter-rotating vortices formed inside the droplet on either side of the rupture point, which moved upwards with time. The propagation of the vortices inside the droplet was faster for low surfactant concentrations, while the intensities of the two counter-rotating vortices significantly decreased for increasing surfactant concentration. At the early stages of coalescence after film rupture, the kinetic energy per unit mass was mainly distributed near the bottom part of the droplet, while at later stages it was distributed near the upper part of the droplet.