(color online) (a) Cumulative rotation angle β of a rigid circle as a function of time t for various board lengths l. The inset figure is a schematic illustration of the rotation angle at time t₀, and the straight black lines are linear fitting curves. (b) The corresponding relationship between average angular velocity of a rigid circle and board length l. Here the angular velocity ω of chiral active particles is 0.1, the area fraction ρ is 0.1, the number of boards n_B is 4, and the tilt angle γ of boards is π/2. (c) Resultant moment M_Z of rigid circle suffered from the collision of active particles on various collision positions. The inset figures show the division of collision positions and the definition of M_Z.