| Summary: | We study the effect of insoluble surfactants on the impact of surfactant-free droplets
on surfactant-laden thin liquid films via a fully three-dimensional direct numerical
simulations approach that employs a hybrid interface-tracking/level-set method, and by
taking into account surfactant-induced Marangoni stresses due to gradients in interfacial
surfactant concentration. Our numerical predictions for the temporal evolution of the
surfactant-free crown are validated against the experimental work by Che & Matar
(2017). We focus on the ‘crown-splash regime’, and we observe that the crown dynamics
evolves through various stages: from the the growth of linear modes (through a RayleighPlateau 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 plays a key role in
the late stages of the dynamics as soon as the ligaments are driven out from the rim.
Surfactant-induced Marangoni stresses delay the end-pinching mechanisms to result in
longer ligaments prior to their capillary singularity. Our results indicate that Marangoni
stresses bridge the gap between adjacent protrusions promoting its collision and the
merging of ligaments. Finally, we demonstrate that the addition of surfactants leads to
surface rigidification and consequently to the retardation of the flow dynamics.
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