Jürgen Blum

Generating a jet of deagglomerated small particles in vacuum

We describe a dust jet generator, which is a device for the deagglomeration of micron-sized and submicron-sized dust particles. In contrast to other mechanisms, it principally works without any gas and generates a dust jet with a speed up to 100 m/s relative to the surrounding gas. Compressed dust powder is fed onto the front side of a fast rotating cogwheel. Upon contact with the cogwheel, the dust is deagglomerated and the single particles are accelerated at a tangent to the circumference. Design details of the mechanism and characteristics of the dust jet are presented. We will use the dust jet for the investigation of collisions of individual particles with surfaces or other particles. Other possible applications of the dust jet generator are proposed.

The de‐agglomeration and dispersion of small dust particles—Principles and applications

A mechanism for the generation of dust dispersions consisting of single micrometer‐sized particles in a low pressure gas is discussed. The particles are de‐agglomerated and dispersed by injecting a dust sample (powder) together with a small amount of an arbitrary type of gas into a modified turbomolecular pump. The discussed sample release mechanism/dust disperser offers a wide range of possible gas and particle parameters, suitable for different kinds of experiments. We present two applications of our de‐agglomeration mechanism. In the first experiment, homogeneous dust clouds were produced under microgravity conditions, and the Brownian motion of the single dust grains was studied. The second application is a matrix isolation technique for the spectroscopical investigation of single dust particles.

First results from the cosmic dust aggregation experiment CODAG DYNAMICS

Abstract The Cosmic Dust Aggregation Experiment (CODAG), an experimental simulation of the onset of planet formation, was successfully flown on STS-95 (October/November 1998) and on Maser 8 (May 1999). The main objective of the CODAG experiment was a direct observation of the Brownian motion-induced coagulation process of micron-sized dust particles. To overcome rapid sedimentation of the dust grains in the rarefied gas atmosphere, experiments were conducted in a long-duration microgravity environment. In the experiment, we observed that within several minutes the initially deagglomerated dust grains formed fractal dust aggregates due to their thermal motion and subsequent mutual collisions. The results from this experiment are the first experimental proof that the concept of pre-planetary dust coagulation is correct.

New experiments on collisions of solid grains related to the preplanetary dust aggregation

Abstract The formation of planetesimals and cometesimals is due to inelastic collisions between dust grains and due to van der Waals and other attractive surface forces. These low-velocity collisions are believed to result from size-dependent friction with the dilute gas of the solar nebula. Any numerical simulation of the grain growth process must be based on assumptions about the collisional behaviour of these grains, which is a very complex theoretical problem. Therefore, we performed analogous laboratory experiments on individual grain-target collisions of micron-sized particles. We briefly summarize some important results, point out discrepancies to former work, and mention possible applications on solar nebula conditions with respect to the preplanetary dust aggregation, to a possible motion of charged particles due to magnetic forces, and to the formation of meteoritic chondrules due to ‘lightning’.

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.

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.

Coagulation simulations for interstellar dust grains using an N-particle code

Abstract A computer code is presented in which coagulation of interstellar dust grains is realized through the mutual interaction of N individual particles. In contrast to other growth simulations, it is not assumed that a particle always belongs to the cluster it had its first contact with. By solving the equations of motion for each particle individually, stripping of particles as well as fragmentation and compaction of the clusters can occur. Simulations were done using a Hertz-Johnson-type force. Methods for dissipation of energy are discussed.

Drop tower experiments on sticking, restructuring, and fragmentation of preplanetary dust aggregates

For the determination of the aggregation efficiency of preplanetary dust, we performed impact experiments with fractal dust aggregates in the drop tower Bremen. We found that for the lowest impact velocities, the dust aggregates, which consisted of micron-sized, monodisperse SiO2 spheres, hit and stuck with no measurable impact restructuring. For intermediate collision velocities, compact aggregate structures formed, and at the highest impact velocities, aggregates were fragmented. Our experimental results are in quantitative agreement with a numerical dust aggregate collision model (Dominik, C., Tielens, A. G. G. M. 1997, Astrophysical Journal vol. 480, p. 647), when the latest experimental values for the rolling-friction and break-up energies are used. However, the presence of a rarefied gas flow, in which the incoming aggregates were embedded, increased the threshold velocity for sticking. Although the impinging aggregates were disintegrated at high impact velocities, the resulting fragments were dragged back to the target on which they could stick due to a then considerably lower collision velocity. This aerodynamically-supported aggregation process might be responsible for the rapid growth of preplanetary bodies in the size range from ∼0.1 m to ∼10 m. Such a rapid growth is necessary to prevent a loss of most of the solid bodies of these sizes due to gas-drag-induced fast orbital decay.

Experiments on the effects of dust flux exposure on ROSETTA spacecraft materials

Abstract During its comet rendezvous, the ROSETTA/ROLAND spacecraft will be exposed to a particle flux which is caused by the sublimation of volatile cometary material. The dust flux will be characterized by particle velocities of a few 100 ms −1 and particle sizes between 0.5 and 100 μm. The dust flux exposure requires the investigation of possible effects on spacecraft and scientific payload performance. It is of interest whether (1) the particles stick to the spacecraft and thereby form dust layers, (2) the particles cause erosive damage upon impact on sensitive parts, and (3) the impacts electrically charge the spacecraft. Furthermore, it will be possible to better decode dust flux characteristics from spacecraft data (e.g. internal temperatures, spacecraft charging) if the effects on the space probe are more intensively investigated. We present work of an ESA-funded pre-study which showed that an experimental method which consists of generating a dust jet in vacuum and of optically observing individual grain-target collisions is applicable for this purpose, and we point out design features of the future “Cometary Dust Flux Simulation Facility”.

Experiments on dust aggregation and their relevance to space missions

Abstract Dust Aggregation is generally accepted as the physical mechanism to form both planetesimals and come-tesimals. There have been numerous and diverse experimental approaches to the problem of dust aggregation. We point out that this work is relevant to space misssions because of three main reasons: (1) An experimental technique allows to simulate conditions on and around small celestial bodies in the laboratory. (2) Research on the formation process of primitive bodies allows to put constraints on their inner structure, and the study of porous particle layers and fractal aggregates helps to understand regolith properties. (3) Supporting research on pre-planetary dust aggregation helps to develope new applications and instruments for future space missions.