Collective dynamics and active turbulence in swarms of synchronized self-assembled spinners

Active magnetic colloids have proven to be an excellent model system to explore emergent out-of-equilibrium dynamics and structures. We demonstrate that ferromagnetic microparticles, suspended at an air/water interface and energized by an external rotating magnetic field, form dynamic ensembles of synchronized self-assembled spinners. The balance between the magnetic and viscous torques determines the size of an individual self-assembled spinner, which can be controlled by the frequency and strength of the applied magnetic field. Each spinner generates local hydrodynamic flows such that the collective interactions of the multiple spinners allow the formation of dynamic crystal lattices. We investigate active diffusion of passive cargo particles in such spinner ensembles and analyze the structure of the underlying self-induced surface flows. We show that induced flows exhibit properties of an active turbulence. The energy spectra of the active turbulence in such synchronized spinner ensembles reveal reverse energy cascade with the exponent significantly different from the classical 2D turbulence.