Schematic diagram of the grain trajectories. Solid lines in panel (a) illustrate the approximate profile of sheath in vertical section. Equilibrium line L (dashed line) indicates the position of the minima value of confining potential. F₁ represents the horizontal component of the radially confining force when the grain is between the equilibrium line L and the glass wall. Outside this region, it is expressed as F₂. Panel (b) shows four cases of complex trajectories in a horizontal plane. (I), (II), and (IV) show the hypocycloid motions, and (III) shows the epicycloid motion. F_MI represents the inverse Magnus force. R₁ (R₂) is the cyclotron radius when the grain is near (far away from) the glass wall.