Impact of droplets on surfactant-laden thin liquid films*

The occurrence of drop impacts on liquid surfaces has been observed in a broad range of natural phenomena and industrial applications. Most of those streams may be contaminated with surfactants (i.e., surface-active agents), whose concentration variations lead to surface tension gradients, which in turn, results in the formation of Marangoni stresses. We study the effect of insoluble surfactants on the impact of droplets on surfactant-laden thin liquid films via a fully three-dimensional direct numerical simulations, and taking into account surfactant-ind­uced Marangoni stresses due to gradients in interfacial surfactant concentration. We focus on the "crown-splash regime", and we observe that the crown dynamics go through various stages: from the growth of linear modes (through a Rayleigh-Plateau instability) to the development of nonlinearities leading to primary and secondary breakup events (through droplet shedding modulated by an end-pinching mechanism). We show that the addition of surfactants does not affect the wave selection via the Rayleigh-Plateau instability. However, the presence of surfactants affects the late stages of the dynamics as soon as the ligaments are driven from the rim. Surfactant-induced Marangoni stresses retard the end-pinching mechanisms to result in longer ligaments prior to their capillary singularity, and promote the spanwise merging between ligaments. Additionally, we have studied the limiting ca­se of three-dimensional rim-driven retraction dynamics of thin water sheets in air.