Statistical measurement of the average particle velocity inside the NanoRocks experiment cells during a shaking cycle. (a) Definition of the quantities used to link the particle moving surface to the average particle speed: between the times t and t + dt, the particle center displacement is dx. The particle is spherical with a radius R and the height of the red circle portion is defined as h. The area highlighted by the processing described in Sect. 3.1.1 is striped on this schematic. (b) Average particle speed inside the NanoRocks tray 3 during one shaking cycle, measured as described in the text.