Figure 2. Schematic diagram of the experimental setup.

Particles are released at variable discharge rates within an experimental box open at the top and bottom, and filmed at 2000 frames per second while settling. The camera focuses on a small control volume at the center of the stream of settling particles. Small flow width (~0.1 m) with respect to box size minimizes wall effects. Still frames on the right show examples of different particle volume fraction (ϕ_p) within the control volume (see Supplementary Video S2).