Pattern formation in triboelectrically charged binary packings

Electrostatic self-assembly is an interesting route to aim at creating well-defined microstructures [1]. In this spirit, we study the process of self-assembling for vertically shaken granular materials. Our system consists from 1 to 400 plastic beads of 3mm size made from Teflon and Nylon in 2D and 3D geometries. We find self-organization in four, five and sixfold order which is due to charging of the system via triboelectric effects between the grains. We observe that the binary system solidifies on a time scale of a few minutes. Image processing is used to extract the structural and dynamical properties of the assemblies. The mixture ratio is tuned from 1:5 to 5:1 and the humidity level is varied between 10\% and 90\% leading to various transitions between the morphologies. \\[4pt] [1] B. Grzybowksi et al. , Nat. Materials 2, 241-245 (2003)