Role of friction on the formation of confined granular structures.

Metastable fluidized granular systems can spontaneously defluidize, forming glass- or crystal-like structures. We performed experiments with polymer spheres of different friction and roughness fluidized in a vertical water pipe flow. Velocity fluctuations were higher for high friction materials. Monodisperse particles form a crystal-like shell on the cylinder wall for a range of flow rates and number of particles. For polytetrafluoroethylene (PTFE) spheres with a friction coefficient near 0.1, structural organization was assessed through nearest-neighbor angle analysis, featuring hexagonal packing with defects. At such low friction, defects decreased, while contact chains became longer and more aligned. These findings highlight the role of surface properties in the emergence of ordered or disordered structures, offering new insights into the mechanisms governing glass- and crystal-like arrangements in fluidized particle systems.