Impact of droplets onto surfactant-laden thin liquid films

TL;DR

This study uses advanced numerical simulations to explore how insoluble surfactants influence the impact dynamics of droplets on thin liquid films, revealing their effects on crown evolution, ligament formation, and flow retardation.

Contribution

It provides a detailed numerical analysis of surfactant effects on droplet impact dynamics, including surfactant-induced Marangoni stresses and their influence on breakup mechanisms.

Findings

Surfactants do not alter wave selection via Rayleigh-Plateau instability.

Surfactants delay end-pinching, resulting in longer ligaments before breakup.

Surfactants cause surface rigidification, slowing flow dynamics.

Abstract

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 and 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 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 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, while also promoting the spanwise merging between ligaments. Finally, we show that the addition of surfactants leads to surface rigidification and consequently to the retardation of the flow dynamics.