IngentaConnect Experiments on Preplanetary Dust Aggregation

Abstract:

For the study of low-velocity dust interactions in the early solar nebula, we have performed two sets of experiments. In the first type of experiments, we studied the grain-mass evolution of a dust cloud embedded in a rarefied turbulent gas environment in which the initially monodisperse spherical SiO 2 grains (1.9 mum diameter) rapidly aggregate. The analysis of the resulting aggregate structures revealed that the clusters were formed by ballistic cluster-cluster aggregation without restructuring and follow a mass-size relation of the form m ∝ aDfg , where Df ~ 1.91 is the fractal dimension and ag is the radius of gyration of an aggregate with the mass m. A comparison with model calculations shows that the mean collisional velocity vc falls into the interval 0.07 m s-1 ≲ vc ≲ 0.5 m s-1. By extraction of the fractal aggregates from the turbomolecular pump and injection into a levitation tube in the second set of experiments, we were able to observe individual collisions between the aggregates. These types of simulations were performed in the laboratory so that the dominant source of the collisions was relative sedimentation. We investigated 28 collisions between aggregates with monomer numbers between i = 1 and i ~ 100 in the collision velocity range 0.001 m s-1 ≲ vc ≲ 0.01 m s-1. Our observations show a sticking efficiency of betac = 1 for the above-mentioned aggregate masses and collision velocities with no signs of grain restructuring.

As we have attached importance to the similarity between our laboratory experiments and the situation in the solar nebula, e.g., grain size and composition, collision velocities, and friction regime, the results of our investigations are directly applicable to the solar nebula modeling and may be used for time-scale estimations of the aggregate growth in the early Solar System. Our experiments suggest that an ensemble of dust grains which is collisionally self-interacting caused by gas drag effects, such as sedimentation, radial drift, or gas turbulence, will adopt a bell-shaped mass distribution. This evolution results in fractal aggregates with fractal dimensions below or close to Df = 2. Copyright 1998 Academic Press.

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