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In protoplanetary disks, zones of dense particle configuration promote planet formation. Solid particles in dense clouds alter their motion through collective effects and back reaction to the gas. The effect of particle-gas feedback with ambient solid-to-gas ratios $\epsilon > 1$ on the stopping time of particles is investigated. In experiments on board the International Space Station we studied the evolution of a dense granular gas while interacting with air. We observed diffusion of clusters released at the onset of an experiment but also the formation of new dynamical clusters. The solid-to-gas mass ratio outside the cluster varied in the range of about $\epsilon_{\rm avg} \sim 2.5 - 60$. We find that the concept of gas drag in a viscous medium still holds, even if the medium is strongly dominated in mass by solids. However, a collective factor has to be used, depending on $\epsilon_{\rm avg} $, i.e. the drag force is reduced by a factor 18 at the highest mass ratios. Therefore, flocks of grains in protoplanetary disks move faster and collide faster than their constituents might suggest.

A planetesimal moves through the gas of its protoplanetary disc where it experiences a head wind. Though the ambient pressure is low, this wind can erode and ultimately destroy the planetesimal if the flow is strong enough. For the first time, we observe wind erosion in ground based and microgravity experiments at pressures relevant in protoplanetary discs, i.e. down to $10^{-1}\, \rm mbar$. We find that the required shear stress for erosion depends on the Knudsen number related to the grains at the surface. The critical shear stress to initiate erosion increases as particles become comparable to or larger than the mean free path of the gas molecules. This makes pebble pile planetesimals more stable at lower pressure. However, it does not save them as the experiments also show that the critical shear stress to initiate erosion is very low for sub-millimetre sized grains.

We developed an experiment to study different aspects of granular matter under microgravity. The 1.5U small experiment was carried out on the International Space Station. About 3500 almost identical spherical glass particles with 856 um diameter were placed in a container of 50*50 mm cross section. Adjusting the height between 5 and 50 mm, the filling factor can be varied. The sample was vibrated under different frequencies and amplitudes. The majority of the data are video images of the particles motion. Here, we first give an overview of the general setup and a first qualitative account of different phenomena observed in about 700 experimental runs. These phenomena include collisional cooling, collective motion via gas-cluster coupling, and the influence of electrostatic forces on particle-particle interactions., Comment: This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in (Review of Scientific Instruments 90:10 2019) and may be found at (https://aip.scitation.org/doi/10.1063/1.5095213)

In the past, laboratory experiments and theoretical calculations showed a mismatch in derived sticking properties of silicates in the context of planetesimal formation. It has been proposed by Kimura et al. (2015) that this mismatch is due to the value of the surface energy assumed, supposedly correlated to the presence or lack of water layers of different thickness on a grain's surface. We present tensile strength measurements of dust aggregates with different water content here. The results are in support of the suggestion by Kimura et al. (2015). Dry samples show increased strengths by a factor of up to 10 over wet samples. A high value of $\gamma = 0.2 J/m^2$ likely applies to the dry low pressure conditions of protoplanetary disks and should be used in the future.

Collisional charging is one potential initial step in generating lightning. In this work, we study the charging of colliding monodisperse, spherical basalt grains depending on ambient pressure. We used grains of 1.0 to 1.2 mm in one set and 2.0 to 2.4 mm in another set. We varied the ambient pressure between 0.03 mbar and 80 mbar. This especially includes Martian pressure being 6 mbar on average. At a few mbar the net charge gathering on colliding grains has a minimum. A smooth incline in charging occurs for larger pressures. Toward lower pressure the charge increases steeply. The pressure dependence is in agreement to a model where the maximum charge is limited by a gas discharge occurring between two charged colliding grains shortly after or before a collision. The capability of building up charge is at a minimum exactly in the range of Martian pressures. The charges on grains are at least a factor 5 smaller than at the highest pressure tested and still smaller compared to ambient pressure on Earth. This implies that on Mars collisional charging and the potential of subsequent generation of lightning or other large scale discharges are strongly reduced compared to Earth. This might result in less frequent and less energetic lightning on Mars.

Dust drifting inward in protoplanetary disks is subject to increasing temperatures. In laboratory experiments, we tempered basaltic dust between 873 K and 1273 K and find that the dust grains change in size and composition. These modifications influence the outcome of self-consistent low speed aggregation experiments showing a transition temperature of 1000\,K. Dust tempered at lower temperatures grows to a maximum aggregate size of $2.02 \pm 0.06$ mm, which is $1.49 \pm 0.08$ times the value for dust tempered at higher temperatures. A similar size ratio of $1.75 \pm 0.16$ results for a different set of collision velocities. This transition temperature is in agreement with orbit temperatures deduced for observed extrasolar planets. Most terrestrial planets are observed at positions equivalent to less than 1000 K. Dust aggregation on the millimeter-scale at elevated temperatures might therefore be a key factor for terrestrial planet formation.