IROCS: The Influence of Radiation On the Charging of Granular Media.
Experiments with granular grains onboard the ISS indicated, that the process of charging due to collisions changes. Furthermore, generated charge got lost after a short time. One possible explanation for this is that the 100-times higher cosmic radiation on the ISS influences the charging of a grain's surface. Therefore, we participate at the German-Swedish student programme REXUS/BEXUS to test if the electrostatic charge is reduced in about 30 km altitude. An experiment for the stratosphere balloon BEXUS29 is currently set up.

Planetesimals move in protoplanetary disks on Kepler orbits around the central star. They experience head winds of 50 m/s. Depending on the formation model, a planetesimal consists of a loose collection of mm to cm-size dust aggregates. In a low-pressure wind tunnel in parabolic flights the conditions are determined for which these objects are stable against erosion by wind. This project is supported by the DLR space administration with funds provided by the Federal Ministry for Economic Affairs and Energy (50 WM 1760)

LANNISTER: Formation of cm-sized Aggregates by Collisional Charging.
It is still unclear if mm-sized dust particles can continue to grow in mutual collisions during early phases of planet formation. Particles mostly bounce off each other at this size. However, they charge during these collisions and electrostatic attraction can hold particles together. In drop-tower experiments, mm-sized dielectric spheres get charged in collisions and their behavior is observed in a capacitor. Charged attraction leads to the formation of cm-sized aggregates. Due to polarization and dipole moments, the collision properties of charged particles differ significantly from those of uncharged particles.

(Supported by the DLR space management with funds provided by the Federal Ministry for Economic Affairs and Energy, 50WM1762)

Streaming Instabilities in Laboratory Experiments:
Streaming Instabilities are a popular way to overcome collisional barriers in planet formation. In a protoplanetary disk, the gas interacts via drag forces with the particles, but particles also back-react to the gas. It was shown in simulations that this back-reaction leads to particle concentrations and finally to the formation of planetesimals. So far, the so-called streaming instability was only investigated in simulations. We did set out to change that. We study particles (e.g. hollow glass spheres) trapped in a levitation chamber under low pressure. We find that in dense systems particles sediment much faster than in dilute systems. The sedimentation velocity depends on both, particle number density and inter-particle distances or “closeness”.

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The Role of Ferromagnetic Iron in Planetesimal Formation.
Eisen iIron is an important building block of terrestrial planets as it is among the most abundant elements in protoplanetary disks.  As an example, the iron core of Mercury, the innermost planet in the solar system, accounts for around 70% of the overall planet mass. One might ask whether there is a link between the planets’ high iron abundance and the existence of magnetic fields at their place of birth, which are up to several millitesla. We perform collision experiments of levitated iron-rich dust to investigate the influence of magnetic fields on dust aggregation. Due to dipole-dipole forces iron particles tend to build chainlike structures oriented along the field lines (see figure). Furthermore, we study selective iron aggregation in an ensemble of iron and silicates.

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Particle Ejections. Particles can be ejected from a dust bed's surface at low pressures (mbar) by light. Temperature gradients within the dust sample lead to thermal creep forces (photophoresis, thermophoresis, Knudsen compressor effects ...) ejecting the particles into the surroundings. Pressure and intensities to induce this effect are found in proto-planetary disks and bodies within the disk will be eroded, something important for planet formation. But also on Mars this effect might lead to particle ejections and entrainment of dust into the thin Martian atmosphere.

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Photophoresis. Photophoresis is long known dating back to the 19th century. If particles in a gaseous environment are illuminated they move, usually away from the light source. In protoplanetary disks photophoresis transports particles to different locations. The details of the photophoretic force are not known but important e.g. as selection effect. In laboratory experiments and gravity and microgravity we therefore study photophoretic forces in more detail.

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Thermal Properties of Chondrules: Chondrules, mm-size spherical particles, are important consituents of meteorites and therefore asteroids. Photophoresis might have transported these particles and sorted them. In experiments and numerical models we measure and simulate the thermal properties as basic quantity. In experiments we measure the temperature distribution by high resolution thermography for illuminated chondrules. For modeling we start with tomographic data (provided by Jon Friedrich, New York) and build a 3D-model of a chondrules used as basis for further calculation of the heat transfer through an illuminated chondrule. Particle transport, rotation and sorting get accessible then.

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Particle Levitation. We discovered that temperature gradients over dust particles may force therm to levitate at low pressures. Thermal creep forces (Knudsen compressor) lead to an overpressure below the particle. In 1909 Knudsen already found that temperature gradients may lead to creeping of gas along non uniformly heated tubes. The pores within a dust particle aggregate can be interpreted as a collection of micro-tubes. A gas soakage through the particle lets the particle hover. We perform leviation experiments at high (800 K) but also at low (70 K) temperatures to study e.g. collisions or photophoresis.

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Ice Aggregation. Particle levitation can be applied to ice particles as well. This allows the study of ice aggregate dynamics. The objective of this application is to study the collision processes between ice particles at conditions that could be found during planet formation beyond what is called the snowline in protopanetary disks (further out than a few astronomical units from the star). Ice is important here as initial building block for gas giants and Ice is in general widely present in interstellar space. Our work here is related to the European Initial Training Network.

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Whale. Solids in protoplanetary disks do not collide in isolation. They are surrounded by gas and the larger bodies are accompanied by a gas flow. We study the possibility of reaccretion of dust fragments by this gas flow after a collision. Smaller particles are influenced very easy by gas flows. Therefore, on one side, ejected particles might be pressed back towards the parent body by headwind gas flows. On the other side gas flow is also capable of eroding a dust body.

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High Velocity Collisions. It is common belief that planets form through collisions of km-size planetesimals in protoplanetary disks. The standard model for planetesimal formation is also based on collisional growth. We study how far you can go with this theory. Is it possible to stick dusty bodies together at 100 m/s? A crossbow within a vacuum chamber is used to accelerate particles, allowing us to record the dust collisions at high velocities.

This work is also related to a DFG-funded research group.

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