  A two-dimensional granular packing under horizontally circular shaking
exhibits various collective motion modes depending on the strength of the
oscillation and the global packing density. For intermediate packing density
and oscillation amplitude, a high density phase travels along the container's
side wall in clockwise direction, while the oscillation itself is
anti-clockwise. Further increasing packing density towards the hexagonal
packing, the whole packing rotates collectively in clockwise direction. The
core of the packing rotates as a solid and is separated from the boundary by a
fluid-like layer. Both motion modes are associated with the asymmetric motion
of particles close to the side wall.
