Droplet impact on surfactant-laden thin liquid films: Vortex ring dynamics

TL;DR

This study experimentally explores how surfactants influence vortex ring behavior and mixing during droplet impacts on thin liquid films, revealing stabilization effects and modified wave dynamics.

Contribution

It provides new insights into the stabilizing role of surfactants on vortex rings and establishes an empirical regime map for instability thresholds.

Findings

Surfactants stabilize vortex rings and suppress azimuthal instabilities.

Marangoni stresses alter early capillary-wave dynamics.

A regime map links film parameters to vortex stability.

Abstract

Droplet impact on surfactant-laden thin liquid films is investigated experimentally with emphasis on vortex ring dynamics. Bottom- and side-view imaging reveal that increasing surfactant concentration progressively stabilize vortex rings, suppress azimuthal instabilities and promote concentric mixing patterns. A regime map is established in terms of film thickness, Reynolds number, and surface-tension ratio, yielding an empirical instability threshold. Shadowgraphy observations suggest that Marangoni stresses modify early capillary-wave dynamics, potentially altering vortex ring formation and delaying instability onset. These findings clarify the link between interfacial stresses, vortex ring dynamics, and mixing patterns in thin-film droplet impact.