In this work the coalescence of an aqueous drop with a flat aqueous-organic interface was investigated in a thin
gap Hele-Shaw cell. Different concentrations of a nonionic surfactant (Span 80) dissolved in the organic phase
were studied. We present experimental results on the velocity field inside a coalescing droplet in the presence of
surfactants. The evolution of the neck between the drop and the interface was studied with high-speed imaging.
It was found that the time evolution of the neck at the initial stages of coalescence follows a linear trend, which
suggests that the local surfactant concentration at the neck region for this stage of coalescence can be considered
quasiconstant in time. This neck expansion can be described by the linear law developed for pure systems when the
surfactant concentration at the neck is assumed higher than in the bulk solution. In addition, velocity and vorticity
fields were computed inside the coalescing droplet and the bulk homophase using a high-speed shadowgraphy
technique. The significant wall effects in the Hele-Shaw cell in the transverse axis cause the two vertical velocity
components towards the singularity rupture point, from the drop and from the bulk homophase, to be of the
same order of magnitude. This movement together with the neck expansion creates two pairs of counteracting
vortices in the drop and in the bulk phase. The neck velocity is the average of the advection velocities of the two
counteracting vortex pairs on each side of the neck. The presence of the surfactant slows down the dynamics of
the coalescence, affects the propagation direction of the pair of vortices in the bulk phase, and reduces their size
faster compared to the system without surfactant.