Recoil of a liquid filament: escape from pinch-off through creation of a vortex ring

A liquid filament recoils because of its surface tension. It may recoil to one sphere, the geometrical shape with lowest surface, or otherwise segment to several pieces which individually will recoil to spheres. This experiment is classical and its exploration is fundamental to the understanding of how liquid volumes relax. In this paper, we uncover a mechanism involving the creation of a vortex ring which plays a central role in escaping segmentation. The retracting blob is connected to the untouched filament by a neck. The radius of the neck decreases in time such that we may expect pinch-off. There is a flow through the neck because of the retraction. This flow may detach into a jet downstream of the neck when fluid viscosity exceeds a threshold. This sudden detachment creates a vortex ring which strongly modifies the flow pressure: fluid is expelled back into the neck which in turn reopens.