Surfactant-induced Marangoni stresses in drop-interface coalescence*

We study the effect of surfactants on the dynamics of a drop-interface coalescence using fully-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 available experimental data, 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.