The Brownian Motion of Dust Particles in the Solar Nebula: An Experimental Approach to the Problem of Pre-planetary Dust Aggregation

✍ Scribed by Jürgen Blum; Gerhard Wurm; Sascha Kempf; Thomas Henning

• Publisher: Elsevier Science
• Year: 1996
• Tongue: English
• Weight: 321 KB
• Volume: 124
• Category: Article
• ISSN: 0019-1035
• DOI: 10.1006/icar.1996.0221

✦ Synopsis

are subjected to drag-induced frictional forces. These drag forces are in many cases responsible for the occurrence of Laboratory experiments were performed to study the Brownian motion of m-sized dust grains and small aggregates mutual collisions among the dust particles. If the impact under pre-planetary nebula conditions, i.e., in a thin gas atmovelocities in these collisions are reasonably low, dust grains sphere (Epstein drag regime), in the ballistic limit, and under tend to stick to each other due to surface attractions thus microgravity conditions. The results of these experiments, i.e., forming larger compound particles. Weidenschilling and the grain diffusivities, are in quantitative agreement with theo-Cuzzi (1993) have discussed various processes responsible retical predictions for single spherical grains. Deviations from for inter-particle collisions. For the smallest particles with particle sphericity, i.e., in our case aggregates consisting of radii a Շ 10 Ϫ4 m, Brownian motion of the dust grains monodisperse spherical grains, cause only minor deviations is the dominant source of relative velocities and thus of between the Epstein drag formula for spheres and our expericollisions among the particles for both, a laminar and a mental results for equal particle cross section. Thus, we find a turbulent solar nebula.
quantitative agreement of our measurements with the Epstein
In this paper, we will describe our experimental investirelation D ؔ 1/ a between grain diffusivity and geometrical (aerodynamic) cross section. gations on the very first steps of pre-planetesimal dust The results of our investigations can be used for the calculaaggregation, i.e., the Brownian motion of small dust partition of the gas-grain stopping time f ‫؍‬ (m/ a ) (1/ g v m ) cles in the free molecular flow regime. In Section 2, we which, in turn, is an important grain characteristic for the will investigate the theoretical background of solar nebula calculation of pre-planetary dust aggregation. Here, m and a dust interactions for small grains, Section 3 will refer to the are the mass and the aerodynamic, i.e., geometric cross section technique of our experimental investigations, and Section 4 of the grain, g and v m are the mass density of the gas and describes the experimental results. In Section 5 we discuss the mean thermal velocity of the gas molecules, and is a the implications of these results for the understanding of proportionality factor which we determined to be ‫؍‬ 0.68 ؎ pre-planetary dust growth, and Section 6 concludes the 0.10. The gas-grain stopping time describes the strength of general results of our investigations.grain coupling to a given gas motion and its value determines relative velocties and, hence, collision frequencies between dust grains due to sedimentation, drag-induced orbital decay, and 2. BROWNIAN MOTION DRIVEN PRE-PLANETARY gas turbulence in the solar nebula.