Measurement of the translational and rotational Brownian motion of individual particles in a rarefied gas.

We measured the free Brownian motion of individual spherical and the Brownian rotation of individual nonspherical micrometer-sized particles in rarefied gas. Measurements were done with high spatial and temporal resolution under microgravity conditions in the Bremen drop tower so that the transition from diffusive to ballistic motion could be resolved. We find that the translational and rotational diffusion can be described by the relation given by Uhlenbeck and Ornstein [Phys. Rev. 36, 823 (1930)]. Measurements of rotational Brownian motion can be used for the determination of the moments of inertia of small particles.

Growth and form of planetary seedlings: results from a sounding rocket microgravity aggregation experiment.

In a second microgravity experiment on the formation of dust agglomerates by Brownian motion-induced collisions we find that the agglomerates have fractal dimensions as low as 1.4. Because of much better data, we are now able to derive the diffusion constant of the agglomerates as a function of mass, to show that a power law with an exponent of 1.7 describes the temporal evolution of the mean agglomerate mass very well and to prove that the collision cross section is proportional to the geometrical cross section. In addition to that we derived the universal mass-distribution function of the agglomerates.