Experiments on sticking, restructuring, and fragmentation of preplanetary dust aggregates

We performed laboratory as well as microgravity experiments in which we studied the impact of small fractal aggregates consisting of micrometer-sized dust particles onto solid targets at various velocities. Slow bombardment of the target in the laboratory results in the formation of a fluffy dust layer in which gravity-induced compaction is observed. In order to reduce the gravitational aggregate restructuring and, hence, to investigate the collisional behavior of fluffy dust aggregates, we performed additional experiments in the microgravity environment of a drop tower. We observe that the agglomerates are disrupted as long as the impact velocities are above a few meters per second. For slightly lower collision velocities, both sticking to and removal from the target are detected. At even lower velocities, the impinging dust agglomerates are captured by the target with a sticking probability of unity, and a compact dust layer forms. When the impact energies are no longer sufficiently large to allow for agglomerate restructuring, the internal structures of the impacting aggregates are preserved, and we observe the growth of a very porous dust layer. The results of our experiments are in qualitative, but not in quantitative agreement with theoretical predictions. Full quantitative accordance between computer simulations and experiments can be reached when the recently measured values for the rolling friction force F-roll = 1.2 x 10(-9) N and for the break-up energy E-br = 1.3 x 10(-15) J (valid for 1.9-mu m-diameter SiO2 spheres) are used. Our experimental results suggest that aggregate restructuring in the solar nebula, and hence, the gradual increase of the fractal dimensionality of the dust agglomerates, becomes an important process when the aggregate diameters exceed a few centimeters, Dust aggregates below that size are not expected to be subjected to impact compaction. (C) 2000 Academic Press.