  The experiments presented aim to measure the outcome of collisions between
sub-mm sized protoplanetary dust aggregate analogues. We also observed the
clusters formed from these aggregates and their collision behaviour. The
experiments were performed at the drop tower in Bremen. The protoplanetary dust
analogue materials were micrometre-sized monodisperse and polydisperse SiO$_2$
particles prepared into aggregates with sizes between 120~$\mu$m and
250~$\mu$m. One of the dust samples contained aggregates that were previously
compacted through repeated bouncing. During three flights of 9~s of
microgravity each, individual collisions between aggregates and the formation
of clusters of up to a few millimetres in size were observed. In addition, the
collisions of clusters with the experiment cell walls leading to compaction or
fragmentation were recorded. We observed collisions amongst dust aggregates and
collisions between dust clusters and the cell aluminium walls at speeds ranging
from about 0.1 cm/s to 20 cm/s. The velocities at which sticking occurred
ranged from 0.18 to 5.0 cm/s for aggregates composed of monodisperse dust, with
an average value of 2.1 cm/s for reduced masses ranging from 1.2x10-6 to
1.8x10-3 g with an average value of 2.2x10-4 g. From the restructuring and
fragmentation of clusters composed of dust aggregates colliding with the
aluminium cell walls, we derived a collision recipe for dust aggregates
($\sim$100 $\mu$m) following the model of Dominik \& Thielens (1997) developed
for microscopic particles. We measured a critical rolling energy of 1.8x10-13 J
and a critical breaking energy of 3.5x10-13 J for 100 $\mu$m-sized
non-compacted aggregates.
