Static and dynamic ordering of magnetic repelling particles under confinement: disks vs bars

We explored experimentally the self-organization at rest and the compression dynamics of a two-dimensional array of magnetic repelling particles, using two particle geometries, namely, disks and rectangular bars. Despite the non-contact interaction, typical static features of granular materials are observed for both particle shapes: pile formation with an angle of repose and pressure saturation (Janssen-like effect), which can be explained by considering the magnetically-induced torques that generate friction between particles and confining walls. Particle shape effects are mainly observed during compression: while disks rearrange increasing the hexagonal ordering, bars augment their orientational ordering forming larger non-contact force chains; however, in both cases, the resistance to compression rises continuously, in contrast with the fluctuating compression dynamics (stick-slip motion or periodic oscillations) that characterizes granular systems with inter-particle contacts. The continuous response to compression, and the reduction of particle wear due to non-contact interactions, are desirable features in designing magnetic granular dampers.