Generation and stability of toroidal droplets in a viscous liquid

We use a simple method to generate toroidal droplets and study how they transform into spherical droplets. The method relies on the viscous forces exerted by a rotating continuous phase over a liquid which is extruded from an injection needle; the resultant jet is forced to close into a torus due to the imposed rotation. Once formed, the torus transforms into single or multiple spheres. Interestingly, we find there are two routes for this process depending on the aspect ratio of the torus. For thin tori, classical hydrodynamic instabilities induce its breakup into a precise number of droplets. By contrast, for sufficiently fat tori, unstable modes are unable to grow, and the torus evolves through a different route; it shrinks towards its center to coalesce onto itself, to finally form a single spherical droplet.