Surfactant effects in escape from pinch-off during contraction of liquid sheets and 2D drops

In certain spraying, polymer processing, and coating applications, liquids are ejected from nozzles/dies as sheets which have rectangular cross-sections and rounded ends. These ends contract towards each other due to surface tension. Recently, Wee et al. (PRF, 2024) showed that, in the absence of intermolecular forces, slightly viscous liquid sheets (2D drops) escape pinch-off through two distinct mechanisms. At very low viscosities, escape is due to viscous resistance, while at slightly higher, yet small, viscosities, it is attributable to vorticity generated at the highly deformed free surface. However, in many practical applications, e.g. crop spraying, liquid sheets often contain surfactants which are present either by design or as contaminants. Surfactants can profoundly influence the dynamics of sheets. They do so not only by reducing surface tension, but also by giving rise to surface tension gradients, and hence tangential Marangoni stresses, that occur when surfactant concentration is nonuniform at the interface. Here, we investigate how insoluble surfactants affect the escape dynamics by direct numerical simulation of the underlying free-surface flow. Our results offer new physical insights into escape mechanisms that can be directly attributable to surfactants.