Trajectories of travelling (a)–(e) and stationary (f) grains. Dashed line in each image indicates schematically the glass ring. Panels (a) and (b) are located outside the glass ring. Panels (c)–(f) are located inside the glass ring. (a): epicycloid motion; (b): cuspate epicycloid motion; (c): hypocycloid motion; (d): cuspate hypocycloid motion; (e): circular motion; (f): stationary grain. Exposure times of panels (a)–(f) are: 0.39, 0.26, 0.53, 0.16, 0.32, 1 s, respectively.