Experimental set-up

To study the behaviour of active rotating particles in pure viscous and in dense passive environments, we have designed a hybrid active–passive system composed by ferromagnetic particles (the active units or spinners) and polystyrene particles (the passive units). To measure pairwise interactions between spinners we mix an extremely dilute solution of active ferromagnetic particles with a solution containing passive polystyrene particles. Both particles have a diameter of 5 μm. We first study the case of pure spinners, in the absence of any passive particle. We then study the interactions between spinners in a monolayer of passive particles at a particle area fraction of ϕ_A=0.7±0.1. These solutions, dilute spinners and dense mixtures, are placed between a cover slip and a slide, and placed in our experimental set-up, which is composed by four magnetic coils (Supplementary Fig. 1); this allows us to generate a rotating magnetic field that is parallel to the substrate and rotates around the z axis (see Supplementary Information for more details). The frequency of rotation used in this study was 5 Hz. On actuation of the external rotating magnetic field the magnetic dipole moment of the spinner couples to the applied field, and the particle spins in place around the z axis.