(a) Schematic diagram of the experimental establishment. Disk particles on a conveyor belt are driven to flow through an orifice. A high-resolution camera is used to record the number of the particles in the frame. (b) Illustration of the hopper angle. By adjusting the position of the bottom wall (highlighted by white lines), different hopper angles from 0° to 80° can be obtained. d = 11 mm is the diameter of the disk particle used in the experiment.