We present results of collision experiments of a dense beam of aggregated 1
.2 mum SiO2 particles entrained in a gas flow with metal targets of differe
nt widths. Depending on the target width (d=25.4, 50.8, and 127 mum) and th
e ambient gas pressure (p=0.5-2.0 mbar), the growth of a dust pile on the t
arget begins at a threshold impact speed (v(imp)=6-12.5 m/s). These thresho
ld velocities for sticking exceed the limit for total disruption of aggrega
tes by more than a factor of 5-10 for the given parameters. We found that a
significant number of fragments (single particles) from the collisions had
a very low coefficient of restitution c(r) at least down to c(r)<0.05 that
is much lower than the value c(r)>0.5 that one. of the single solid micron
-sized particles would have while impinging a rigid target. Due to the drag
of the gas flow these slow fragments are forced back to the target a secon
d time resulting in sticking that eventually leads to the formation of the
dust pile in spite of the high impact velocities. Together, the fragmentati
on, the low coefficients of restitution of a significant number of fragment
s, and the gas flow provide an efficient growth mechanism for bodies that w
ould otherwise lose mass. We consider this an important mechanism for the f
ormation of planetesimals in the solar nebula.