Torsten Poppe

Analogous Experiments on the Stickiness of Micron-sized Preplanetary Dust

In the early solar nebula, the formation of planetesimals and cometesimals is believed to be due to inelastic collisions of initially micron-sized grains. The collisions are caused by relative velocities due to size-dependent interactions with the surrounding dilute gas. The grain growth process is determined by the velocity-dependent sticking efficiency upon collisions. Therefore, we performed experiments with eight samples of micron-sized particles consisting of monodisperse silica spheres, of irregularly shaped diamond, enstatite, and silicon carbide grains, and of silicon carbide whiskers. We determined the sticking probability and the energy loss upon bouncing collisions by studying individual grain-target collisions in vacuum. We found a sticking probability higher than predicted by previous theoretical work. Grain size, roughness, and primarily grain shape, i.e., the difference of spherical versus irregular grain shape, is important for the collisional behavior, whereas the material properties are rather unimportant. Our results indicate that the preplanetary dust aggregation is more effective than previously thought.

Experiments on Collisional Grain Charging of Micron-sized Preplanetary Dust

Collisions between micron-sized grains and larger objects with velocities up to several 10 m s-1 are believed to be an important physical process in the solar nebula with respect to the preplanetary dust aggregation. Former collision experiments demonstrated that grain-target collisions of micron-sized particles were marked by obvious electrostatic effects. Among those were the observation of particles which, after mechanical rebound, returned to the target and finally stuck, and of particle deposition on targets influenced by the presence of conducting materials. Therefore, it is clear that the dust aggregation process cannot adequately be described without investigating collisional grain charging experimentally. We present experiments on the collisional grain charging of micron-sized grains impacting target surfaces which, in contrast to former work, consist both of nonconducting material and the experiments involving smaller particles than before. Collisional grain charging is stronger than previously discussed with respect to preplanetary grains and should be considered concerning the preplanetary dust aggregation, the formation of lightning in the solar nebula, and a coupling of charged grains to magnetic fields.

Magnetic aggregation: II. Laboratory and microgravity experiments

Abstract In a previous publication (Dominik and Nubold, 2002, Icarus 157, 173–186), we presented analytical expressions and theoretical considerations concerning preplanetary dust aggregation with magnetized grains in the solar nebula. The present work is dedicated to the experimental study of magnetic aggregation in a ground-based laboratory as well as under microgravity conditions on parabolic flights. We conducted aggregation experiments with dust analogues in order to study the temporal evolution and the structural outcome of grain growth processes dominated by or comprising exclusively magnetic grains. Within aggregation times ranging from a couple of seconds to a few minutes only, formation of huge chain-like and/or web-like dust aggregates was observed. After aggregate retrieval we were able to study the sizes and structures of the aggregates in great detail. We established the fractal dimension of the aggregates as Dfs=1.20±0.05 for single chains and Dfc=1.50±0.21 for inter-connected web-like structures. This is considerably lower than for non-magnetic grain growth. The dynamic exponent z for the mass increase with time according to tz was found to be z=2.7 from in-situ video images of the microgravity aggregation runs. The results are compared with the theoretical considerations presented earlier as well as with previous experimental work on the same and on related topics, respectively.

The cosmic dust aggregation experiment CODAG

For the simulation of the first stage of preplanetary dust aggregation, we developed the cosmic dust aggregation experiment (CODAG). With CODAG, we intend to study the aggregational behaviour of a cloud of micron-sized dust particles due to Brownian motion of the grains. For a realistic simulation of the processes in the young solar system, the dust grains have to be dispersed in a rarefied gas so that mutual collisions are ballistic. Fast sedimentation of the grains in the Earths gravitational field leads to unrealistic collision velocities and to a rapid loss of particles to the container walls. Therefore, CODAG was designed to work in a microgravity environment. In this paper, we present an overview of the experimental design of CODAG which was recently flown in a Get Away Special container during the STS-95 mission.

Experiments concerning the influence of grain magnetization on preplanetary dust aggregation

Abstract Coagulation of small dust grains is considered the basic mechanism forming planetesimals and cometesimals in a protoplanetary disk environment. Up to now, mainly mechanical and electrostatic interactions were treated. However, there is evidence that magnetic interactions between permanently magnetized dust grains could have played an important role in this aggregation scenario. Long-range magnetic forces are expected to accelerate the grain growth process, to align the constituents of the resulting aggregates, and to form chain-like structures of low fractal dimension D | . We performed coagulation experiments in a dilute gaseous environment using micron-sized magnetized dust samples. We observed rapid growth of chain-like aggregates with very low fractal dimension D | = 1.2. Implications for grain growth in the protoplanetary disk and future perspectives of the experimental work are discussed.

The CODAG sounding rocket experiment to study aggregation of thermally diffusing dust particles

Abstract The initial step of planetesimal formation in the solar nebula is due to a low velocity, Brownian motion — induced aggregation process of the pre-planetary dust particles. In order to reveal the physics of such agglomeration processes, e.g. sticking efficiency, temporal evolution of cluster mass distribution, and morphology of the (fractal) dust aggregates, we developed the simulation experiment CODAG-SRE for ESAs MASER 8 sounding rocket flight (launch April/May 1999) which is described in this article.

Optical particle and particle motion analysis with PATRICIA

We present a portable instrument for a comprehensive particle and particle motion analysis for a large variety of applications. The instrument presently exists as a prototype. It combines two optical techniques which were originally developed for astrophysically-motivated laboratory research. One is based on bright-field microscopy to image micrometre-sized particles and to give structural information. The other is based on stroboscopic trajectory imaging to yield the motion of particles. The latter is applicable for particle sizes down to less than 100 nm. The direction of a particle and its speed can be determined. Due to a long focal distance (80 mm), the instrument does not interfere with the particles and their environment but can be used for in situ and real-time measurements.