Movie S1.

Flocking motion emerges in an ensemble of half a million colloidal rollers. The circular chamber has a diameter of 6mm and includes no obstacle. When the DC electric field is turned on, the rollers start propelling along random directions. After a short transient, they self-organize into a polar fluid. The polar fluid flow self-organizes to form a steady macroscopic vortex. Colloid area fraction ρ₀ = 0.25. Colloid diameter: 4.8 μm. Field amplitude: E₀ = 4.5V/μm. Video recorded at 190 fps, played at 50 fps.