Self-propulsion, interactions, and flocking of active vortlets in three dimensions

Vorticity - a measure of the local rate of rotation of a fluid element - is the elemental building block of incompressible flow. In Newtonian fluids, such as water, powering bulk flows requires the continuous injection of vorticity from boundaries to counteract the diffusive effects of viscosity. Here we experimentally power a flow from within by suspending approximately cylindrical particles and driving them to rotate at Reynolds numbers in the intermediate range (Re = 5 − 200). We find that a single spinner generates a localized three dimensional region of vorticity around it which we term a 'vortlet'. This active element drives a number of novel behaviors. Slight asymmetries in the particle shape deform the vortlet and cause the particle to self-propel. Interactions between spinners are similarly rich, generating bound dynamical states with periodicity. When a large number of spinners interact, the combination of propulsion and adhesion culminates in spontaneous collective motion akin to flocking. These cohesive flocks propel, split and merge as a collective, with their speed governed by the self-propulsion of their constituents. As the number of spinners in a flock increases we find that their collective behaviors become more pronounced. Our findings demonstrate how flow rotated from within at intermediate Reynolds numbers gives rise to a new form of active matter and provides a long-awaited controlled physical system for the quantitative study of three-dimensional flocking in non-sentient systems, as well as a platform for the study of three-dimensional active inertial chiral fluids.