  In laboratory experiments, we studied collisions of ensembles of compact
(filling factor 0.33) millimeter dust aggregates composed of micrometer quartz
grains. We used cylindrical aggregates, triangular aggregates, square
aggregates, and rectangular aggregates. Ensembles of equal size aggregates as
well as ensembles with embedded larger aggregates were studied. The typical
collision velocities are 10-20 mm $\rm s^{-1}$. High spatial and temporal
resolution imaging unambiguously shows that individual collisions lead to
sticking with a high probability of 20 percent. This leads to connected
clusters of aggregates. The contact areas between two aggregates increase with
collision velocity. However, this cluster growth is only temporary, as
subsequent collisions of aggregates and clusters eventually lead to the
detachment of all aggregates from a cluster. The contacts are very fragile as
aggregates cannot be compressed further or fragment under our experimental
conditions to enhance the contact stability. Therefore, the evolution of the
ensemble always leads back to a distribution of individual aggregates of
initial size. This supports and extends earlier experiments showing that a
bouncing barrier in planetesimal formation would be robust against shape and
size variations.
