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Grain-target collision experiments and astrophysical implications T. Poppe and Th. Henning a aAstrophysical Institute and University Observatory, Schillergafichen 3, 07745 Jena, Germany
The collisional behaviour of micron-sized grains in the solar nebula is regarded to be important for the preplanetary dust aggregation. Collision experiments with analogous materials and velocities between about 0.1 and 100 m s_1 demonstrated that both the sticking efficiency and the collisional grain charging is higher than previously assumed. We summarize the results and we discuss some implications in the frame of models of preplanetary dust aggregation. Based on the experimental results, we also point out the possible role of magnetic forces on charged particles and make conclusions concerning the possible chondrule formation by nebular lightning.
It is generally accepted that the first step of the planet formation process is the dust aggregation of initially submicron-sized and/or micron-sized grains in the dilute gas of the early solar nebula. Among the small grains, the tiny gravitational interaction played no significant role, but the grains formed planetesimals due to other types of interactions involving surface forces, electrostatic forces, and aerodynamic forces. Any modelling of the dust aggragtion process must be based on assumptions about their collisional behaviour. Therefore, we conducted laboratory experiments on dust grain collisions whose results are useful to draw conclusions on the dust aggregation process and on other physical conditions in the forming solar system.
We directly observed individual grain-target collisions in vacuum by imaging particle trajectories close to the target. A detailed description of the experimental work can be found in  and . Here, we will give a short summary.
We determined the sticking probability as a function of impact velocity, the energy loss in bouncing collisions and the collisional grain charging of micron-sized (0.14... 1.4 (am diameter) grains consisting of materials (Si02, MgSiOa, diamond, SiC) which are either astrophysically relevant themselves or have properties typical for certain relevant materials. We found that spheres impacting flat surfaces have a capture velocity, which is of the order of 1 m s_1 for particles of 1 micron diameter and which is both higher and less well defined for smaller particles. The capture velocity is determined by an effective dissipation of kinetic energy in the bulk material, whereas the deformation of surface roughnesses and the surface energy are less important for the analogous materials we used. Irregularly-shaped grains stick better due to their shape which supports multiple contacts and, thereby, they more effectively dissipate the impact energy. This leads either to a many-times higher capture velocity or, even more often, to a constant or slightly decreasing sticking probability in the investigated velocity interval. A fraction of several 10 percent of the submicron-sized grains stuck even at velocities exceeding 50 m s_1. The influence of shape, particle size, surface roughness, and primarily the difference between spherical and irregular grain shape, turned out to be important, whereas an influence of material was not found among the grains used. Generally, in a bouncing collision, most of the kinetic energy is lost.
Silica spheres of 1.2 pm diameter impacting silica targets acquire several 100 elementary charges in collisions with impact velocities of up to 50 m s-1. The charge density at the point and time of contact is of the order of 10_4C m-2, and there is a maximum overall charge density of 10_5C m-2 on silica targets and silica spheres which cannot be increased by further collisions. Temporarily higher charge densities at the point of contact decrease as charges spread across the surface. The charging in a collision with a certain impact velocity varies over 1 order of magnitude. The translational impact energies investigated ranged over 3 orders of magnitude from Ekin = 10—15J to 10_12J. Within this interval, the number n of net acquired elementary charges was found to be between 0 and a few hundred and scaled roughly linearly with impact energy. The sign of particle charging was mostly, but not exclusively, negative. There is no evidence that even highly precharged particles consisting of insulating material ever discharged upon a bouncing contact with a target carrying charge of the opposite sign. Collision-induced charging often resulted in attracting the particles which mechanically rebounded. Then the particles often returned to the target and, after literally 'jumping' on the target, eventually stuck.
The strong collisional grain charging must not be regarded as an exceptional effect of the materials used. It occured with all dust sample materials. Moreover, further experiments showed that collisional grain charging is not an effect restricted to insulators only: We observed 'jumping' SiOj spheres also on a metal-coated silica plate  and on aluminium targets (not published elsewhere) which were marked by the same altitude of a few hundred microns. This experimental basis demonstrates (1) that the charging in collisions between insulating and metallic materials is of the same order of magnitude as between two insulating materials and (2) that collisional grain charging between targets and particles made of the same material is not fundamentally different from the charging between targets and particles made of different materials.
2.3. Comparison of results to assumptions on grain collisions in the literature
The theoretically calculated capture velocity of micron-sized spherical grains  was too low by more than one order of magnitude. Furthermore, the even higher sticking efficiency of irregular grains was not treated and the lack of a capture velocity for many irregular grains was not discussed. Also, collisional grain charging is by one to two orders of magnitude stronger than previously assumed in astrophysical literature (e.g.  and ). Only recently, Desch and Cuzzi  pointed out that collisional charging could be much stronger. However, they assume a grain charging in excess of our measurements and they address a charging mechanism which did not cause the charging we observed. Based on , Desch and Cuzzi  assumed that, due to a different contact potential, electrons migrate during collision. This necessarily requires (1) the contact of different materials to allow charging, (2) an influence of the material combination on the charging strength, and (3) a uniform charge sign in a given particle-target combination which all is not in agreement with our results.
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