Surfactant-driven escape from endpinching in nearly-inviscid filaments

Routinely encountered in spraying and drop-on-demand applications, highly stretched liquid filaments are known to exhibit complex and counterintuitive dynamics as they recoil. Instead of simply retracting to a sphere, low-viscosity filaments of aspect ratio exceeding a critical value undergo localized pinch-off near their two ends, a phenomenon called endpinching. However, surfactant-free, Newtonian filaments of intermediate viscosity have been shown to escape endpinching even when in advanced stages of pinch-off. It has been hypothesized that this unexpected behavior can be attributed to a viscous mechanism, and known to accompany the formation of a vortex ring inside the filament. Here we show that strikingly similar 'escape' events can also be induced in nearly-inviscid filaments when the interface is covered with surfactant. Simulations of the full 3D-axisymmetric equations reveal the complex sequence of processes that lead to the final escape and conclusively identify the dominant role of Marangoni stress. Further, we observe how the escape processes occurring in two starkly distinct physical systems, viscous surfactant-free filaments and inviscid surfactant-laden ones, are manifestations of the same physical phenomenon brought about by two distinct mechanisms.