Movie S6.

Meandering patterns formed when a collection of Quincke rollers propel through a random lattice of obstacles (pink color). The vortex structure observed in a pure system is detroyed but local order subsists. Obstacle fraction: Φ_o = 15%. Colloid fraction ρ₀ = 0.08. Chamber diameter: 3mm. Colloid diameter: 4.8 μm. Field amplitude: E₀ = 4.5V/μm. Video recorded at 190 fps, played at 25 fps.