Tribocharged granular assembly of polystyrene beads confined between plates using acoustic vibrations†

In this work, we investigate the transition from an amorphous state to crystal ordering of 3 mm-sized polystyrene beads in a two-dimensional system between two plates, one of them fixed and the other is mobile and can move across the beads. The system is vibrated vertically, and we have investigated the effect of the frequency, the number of particles, and the different materials covering the bottom and top plate. We found that crystallisation is promoted by the motion of the upper plate, which pushes particles to the centre of the plate's configuration and by the onset of tribocharged-induced electrostatic interactions. The motion of the upper plate creates an effective pressure over the particles. Thus, the upper plate motion drives the ordered particle configuration, and electric charges stabilise it. By changing the bottom plate surface and experimenting with steel balls, we found that strong charging can demote crystal formation, and no charging inhibits crystal formation.