  We study the effect of surfactants on the dynamics of a drop-interface
coalescence using full three-dimensional direct numerical simulations. We
employ a hybrid interface-tracking/level-set method, which takes into account
Marangoni stresses that arise from surface tension gradients, interfacial and
bulk diffusion, and sorption kinetic effects. We validate our predictions
against the experimental data of Blanchette and Bigioni [Nat. Phys.
2(4):254-257 (2006)] and perform a parametric study that demonstrates the
delicate interplay between the flow fields and those associated with the
surfactant bulk and interfacial concentrations. The results of this work
unravel the crucial role of the Marangoni stresses in the flow physics of
coalescence with particular attention paid to their influence on neck reopening
dynamics in terms of stagnation-point inhibition, and near-neck vorticity
generation. We demonstrate that surfactant-laden cases feature a rigidifying
effect on the interface compared to the surfactant-free case, a mechanism that
underpins the observed surfactant-induced phenomena.
