INTRODUCTION

At the onset of the planet formation, single micrometer-sized dust particles collide at low velocities and immediately stick due to the van der Waals force (e.g. Weidenschilling & Cuzzi 1993, Blum 2004 and refs. therein). By this process, they form fractal dust agglomerates, grow in mass and are eventually restructured and compacted in further collisions (Dominik & Tielens 1997, Blum & Wurm 2000). At 1 AU this leads to the formation of centimeter- to decimeter-sized porous dust aggregates (Blum 2004), which may then collide with such high collision velocities that the collision does no longer result in mass gain of the larger aggregate. Contrariwise, the smaller body will probably fragment and even the larger body will lose mass.

One possibility to explain the further growth phase is collisional tribo-charging with subsequent reaccretion (Fig. 1; Blum 2004): if the fragments of the smaller agglomerate (projectile) charge uniquely and oppositely to the larger aggregate (target; Fig. 1(b)), the target may establish a large electric field, following a number of collisions (Fig. 1(c)). This electric field may be strong enough to force charged fragments to reaccrete (Fig. 1(d)). Collisional charge separation was found for impacts of micrometer-sized particles with solid and porous targets (Poppe & Schräpler 2005; Schräpler, pers. comm.). This work will focus on the outcome of fragmentation (see first results) and on the measurement of charge separation in these events.

EXPERIMENTAL SETUP

The experiments are carried out in a vacuum chamber at an air pressure of 5·10^-4 mbar. Millimeter-sized, porous projectiles of 1.5 µm SiO₂ spheres are accelerated to velocities of up to 4 m/s and hit a solid target. For velocities exceeding ~1 m/s, the projectiles fragment upon impact. A high-speed camera with back-light illumination (Fig. 2) observes this event in order to measure impact velocity and size of the projectile and the trajectories and sizes of the fragments. A capacitor with a strong electric field will soon be installed to measure the charging of the fragments from the deflection in the electric field.

FIRST RESULTS

First experiments with mm-sized projectiles of 3 - 17 mg mass and collision velocities of 1.2 - 4.5 m/s were performed, in wich fragmentation always occured. The image series taken by the high-speed camera (Fig. 3) is binarized to measure the size of each fragment and to follow its trajectory. These trajectories are cleaned from gravity acceleration and are linearly fitted (Fig. 3, right bottom).

First analysis of the fitting parameters show that the angles of the escaping fragments are preferentially flat (within 20° parallel to the target), which can already be estimated from the image sequence in Fig. 2 (at 2.3 and 6.6 ms). The velocities of the fragments are always slower than the initial collision velocity and most fragments velocities are within 20% of the collision velocity. The mass-distribution of the fragments follows a power-law which was already found in similar aggregate-aggregate collisions by Blum & Münch (1993).

Easy calculations of possible reaccretion of the escaping fragments by electrostatic forces were performed. They showed that reaccretion can occur for the measured sizes, velocities and angles if charging is assumed according the measurements of Poppe & Schräpler (2005) by scaling the surface charge.

OUTLOOK

The experiments on fragmenting dust aggregates will be continued to gain a reliable statistics of angular, velocity and size measurements. Finding correlations between those would be important to include the measured data into numerical models. Concerning the fragment sizes, we aim for the determination of the slopes of the fragment size distributions at different collision velocities to compare them with model predictions of Blum & Münch (1993). For this purpose, a new accelerator is being bulit which will allow velocities of 10 m/s without destroying the fragile projectile before the collision.

Furthermore, the process driving the charge separation will probably differ from simple collisional tribocharging measured by Poppe & Schräpler (2005). For example, friction inside the aggregate might play an important role and the scaling of charge on larger fragments is widely unknown. Thus, charge measurements by deflection of the fragments in an electric field (see experimental setup, Fig. 2) will soon be performed to calculate the efficiency of electrostatic reaccretion in solar nebula conditions and to evaluate the importance of this process.

LITERATURE

• Blum, J. & Münch, M. (1993). Experimentatal Investigations on Aggregate-Aggregate Collisions in the Early Solar Nebula. Icarus, 106, pages 151-167.
• Blum, J. (2004). Grain Growth and Coagulation. In Witt, A. N., Clayton, G. C., and Draine, B. T., editors, ASP Conf. Ser. 309: Astrophysics of Dust, pages 369-391.
• Blum, J. & Schräpler, R. (2004). Structure and Mechanical Properties of High-Porosity Macroscopic Agglomerates formed by Random Ballistic Deposition. Physical Review Letters, 93(11):115503.
• Blum, J. & Wurm, G. (2000). Experiments on Sticking, Restructuring, and Fragmentation of Preplanetary Dust Aggregates. Icarus, 143, pages 138-146.
• Dominik, C. & Tielens, A. G. G. M. (1997). The Physics of Dust Coagulation and the Structure of Dust Aggregates in Space. The Astrophysical Journal, 480:647-673.
• Poppe, T. & Schräpler, R. (2005). Further Experiments on Collisional Tribocharging of Cosmic Grains. Astronomy & Astrophysics, pages 1-9.1
• Weidenschilling, S. J. & Cuzzi, J. N. (1993). Formation of Planetesimals in the Solar Nebula. In Levy, E. H. and Lunine, J. I., editors, Protostars and Planets III, pages1031-1060.

PUBLICATIONS

• Güttler, C., Blum, J., Zsom, A., Ormel, C.W., Dullemond, C.P. (2009). The outcome of protoplanetary dust growth: pebbles, boulders, or planetesimals? I. Mapping the zoo of laboratory collision experiments, Astronomy & Astrophysics, submitted
• Güttler, C., Krause, M., Geretshauser, R., Speith, R., Blum, J. (2009). The Physics of Protoplanetesimal Dust Agglomerates. IV. Towards a Dynamical Collision Model, The Astrophysical Journal, 701, in press (ADS link)
• Güttler, C., Krause, M., Geretshauser, R., Speith, R., Blum, J. (2009). Towards a Dynamical Collision Model of Highly Porous Dust Aggregates, Powders & Grains 2009 Conference Proceedings, in press (ADS link)
• Weidling, R., Güttler, C., Brauer, F., Blum, J. (2009). The Physics of Protoplanetesimal Dust Agglomerates. III. Compaction in Multiple Collisions, The Astrophysical Journal, 696:2036-2043 (ADS link)

This project is funded by DFG with in the
research group FOR 759.