The fluid dynamics of collective vortex structures of plant-animal worms

Circular milling, a stunning manifestation of collective motion, is found across the natural world, from fish shoals to army ants. It has been observed recently that the plant-animal worm Symsagittifera roscoffensis exhibits circular milling behaviour, both in shallow pools at the beach and in Petri dishes in the laboratory. Here we investigate this phenomenon experimentally and theoretically, from a fluid dynamical viewpoint, focusing on the effect that an established circular mill has on the surrounding fluid. Unlike systems such as confined bacterial suspensions and collections of molecular motors and filaments that exhibit spontaneous circulatory behaviour, and which are modelled as force dipoles, the front-back symmetry of individual worms precludes a stresslet contribution. Instead, singularities such as source dipoles and Stokes quadrupoles are expected to dominate. We analyse a series of models to understand the contributions of these singularities to the azimuthal flow fields generated by a mill, in light of the particular boundary conditions that hold for flow in a Petri dish. A model that treats a circular mill as a rigid rotating disc that generates a Stokes flow is shown to capture basic experimental results well, and gives insights into the emergence and stability of multiple mill systems.

Automated summary

Circular milling behavior is observed in a plant-animal worm, and is investigated experimentally and theoretically from a fluid dynamical viewpoint. Singularities such as source dipoles and Stokes quadrupoles are expected to dominate the flow fields generated by a mill, unlike other systems modeled as force dipoles. A model treating a circular mill as a rigid rotating disc that generates a Stokes flow captures basic experimental results and provides insights into the emergence and stability of multiple mill systems.