  The collective behavior of vibrated hexagonal disks confined in a monolayer
is investigated experimentally. Due to the broken circular symmetry, hexagons
prefer to rotate upon sufficiently strong driving. Due to the formation of
liquid bridges, short-ranged cohesive interactions are introduced upon wetting.
Consequently, a nonequilibrium stationary state with the rotating disks
self-organized in a hexagonal structure arises. The bond length of the
hexagonal structure is slightly smaller than the circumdiameter of a hexagon,
indicating geometric frustration. This investigation provides an example where
the collective behavior of granular matter is tuned by the shape of individual
particles.
