Rainer Schräpler

The Laplace facility is an advanced microgravity setup launched in May 2026 and will be integrated on the International Space Station (ISS) to explore the initial stages of planet formation. Building upon the insights gained from its predecessor, ICAPS, which was launched aboard the Texus-56 and Texus-58 sounding rockets in 2019 and 2023, Laplace aims to shed light on early dust-agglomeration processes in protoplanetary disks by systematically varying dust-grain sizes, morphologies and materials.Investigating the initial stages of planet formation necessitates the study of many-particle interactions, involving millions of micrometer-sized particles within a low-density gaseous environment (typically a few tens of Pascals pressure), where Knudsen numbers significantly exceed 1. These conditions require extended observation periods, making microgravity environments essential for such experiments. Both ICAPS and Laplace setups are equipped with advanced mechanisms to actively protect the dust cloud from diffusion losses and other external forces, as well as concentrate the particles using an actively-controllable thermophoretic trap, functioning along all three axes.Concurrently, the dust cloud is accessible to multiple observational instruments. The entire cloud is monitored in three dimensions from two perpendicular overview cameras. Additionally, a dedicated, high-speed long-distance microscope provides non-destructive in-situ analysis of the growing dust aggregates in a central volume of 1 mm² cross-section. Repeated measurements of individual particle electric charges are performed using controlled DC or AC fields to determine charge distribution and its evolution over time.This presentation will give a view on the first data acquired by the Laplace facility, show-casing its capabilities and calibration processes.

In a micro-gravity experiment onboard a sounding rocket, we observed the rapid formation of mm-sized fractal dust agglomerates, consisting of micrometer-sized spherical silica monomers, due to an unforeseen (but very favorable) spatial concentration of the dust cloud (Schubert.2024). A convergent gas flow was caused by the interaction between the dust-cloud control mechanism, which generates a homogeneous gas flow using the thermal-creep method, and a thermophoresis-driven flow directed towards a cold spot. This convergent rarefied-gas flow led to a slight compaction of the mm-sized highly porous agglomerates. We found that the resulting packing density - pressure curve with pressures of several 10-7 Pa agrees well with the theoretical predictions of Kataoka. et al. (2013) and Tatsuuma et al. (2023). We also found that already compacted agglomerates cannot be relaxed again by a divergent flow. As far as we know, this is the first direct observation of gas-flow driven compaction of fractal dust agglomerates, a process predicted in the planetesimal-formation model by Kataoka et al. (2013b).In a second step, we applied these results to turbulent protoplanetary disks, where turbulence generates velocity gradients. Describing the disk turbulence with an α model, our calculations show that due to turbulence-induced velocity gradients (e.g., at ,α = 10-4) fractal growth is only possible up to an agglomerate size of mm (at 0.1 AU) to dm (at 100 AU). For larger particles, the turbulent flow forces a correlation between the size and the packing density of the particles, eventually leading to non-fractal (albeit porous) agglomerates.Acknowledgments: This work was part of the ICAPS mission, which was supported by the Deutsches Zentrum für Luft- und Raumfahrt (DLR Space Agency) under Contract nos. 50WM0336, 50WM0636, 50WM0936, 50WM1236, 50WM1536, 50WM1846, and 50WM2146 and the European Space Agency (ESA) through their SciSpacE programme for this project and the TEXUS-56 Sounding Rocket flight. The Belgian Science Policy Office and the ESA PRODEX Programme are kindly acknowledged for their support.

Protoplanetary disks (PPDs), the cradles of planetary systems around young stars, are composed of dust and ice particles immersed in a rarefied gas. These grains possess initial sizes of less than and up to about 1 µm and collide due to Brownian motion, drift motions relative to the gas, and gas turbulence. Provided that the collision velocity remains below a threshold value, grains stick upon first contact and form fractal agglomerates with typical fractal dimensions just below 2. This is the case for collisions caused by Brownian motion, which occur at thermal velocity.The ICAPS experiment (Interactions in Cosmic and Atmospheric Particle Systems) studies these collisions and growth processes under micogravity conditions. To avoid gravity-induced sedimentation and compaction of the agglomerates, the experiment flew on board of the TEXUS-56 and TEXUS-58 sounding rockets. Monomeric, spherical SiO2 dust (1.5 µm in diameter) was injected into a vacuum chamber at ~107 times the number density in PPDs, reducing processes that take ~100 years in nature to a few minutes. A specially designed thermal trap was used to stabilize the dust cloud and center it in the fields of view of two perpendicular overview cameras. Further in-situ observations were made with a high-speed camera attached to a long-distance microscope (LDM, see Fig. 1 for example agglomerates). An electrical field was repeatedly applied to measure particle charges that diminish drastically within the first ~20 seconds of experiment time due to neutralizing collisions (see Fig. 2).Fig. 1: Collage of 190 agglomerates cropped from LDM images (flat-field corrected and inverted).Fig. 2: Temporal evolution of particle charging of the four experimental runs. The mean charge per monomer is shown in gray and the standard deviation is shown in black. Only the more frequent measurements from the second flight of ICAPS (denoted as SR2) reveal the speed at which the charge is neutralized.With the high spatial and temporal resolution of the LDM and high-speed camera, it was possible to analyze the Brownian motion and rotation of a great number of particles, yielding their masses, moments of inertia, as well as translational and rotational friction times. From these data, an ensemble fractal dimension of 1.4 could be derived (see Schubert et al. 2023). Careful cross-calibration of the cameras also allowed for mass-frequency distributions of the whole dust cloud to be obtained from the overview cameras, taking into account the cloud morphology. In one out of the four experimental runs, the cloud volume shrank slowly but significantly due to a subtle, thermal anomaly (see Fig. 3), which, combined with a conserved total mass, lead to the runaway growth (or gelation) of a few oligarch agglomerates. These agglomerates clearly detach from the otherwise self-similar mass-frequency distributions (see Schubert et al. 2024 and Fig. 4).Fig. 3: Shrinking cloud volume across 124 seconds due to a subtle, thermal anomaly in one of the experimental runs (from Schubert et al. 2024).Fig. 4: Mass-frequency distributions of six consecutive experimental sequences (with subtle thermal anomaly), where N is the number of constituent monomers (from Schubert et al. 2024). We modeled the ensemble growth of each of the four experimental runs using a mean-field approximation as well as a Monte-Carlo simulation. The former is based on a version of Smoluchowski's coagulation equation that readsHere, n, K0, N, and κ are the number density, a pre-factor, the mean number of monomers, and an exponent, respectively. The exponent is κ ≈ -0.3 for diffusive agglomeration of fractal aggregates in the free molecular flow limit, whereas it becomes κ ≈ 0.5 in the ballistic regime. A formal fit of the solution of the above equation yields κ = 0.236 with the thermal anomaly and κ = 0.350 without, which suggest that the growth is relatively close to the pure ballistic Brownian limit (see Schubert et al. 2024 and Fig. 5).Fig. 5: Temporal evolution of the frame-wise mean and maximum mass, given as the number of constituent monomers, from the first flight of ICAPS (TEXUS-56), modelled with a mean-field approximation. Data from the first experimental run (thermal anomaly) are denoted in black and those from the second run are shown in gray (from Schubert et al. 2024). [Schubert et al. 2023] Schubert, B., Molinski, N., Borstel, I., Glißmann, T., Balapanov, D., Vedernikov, A., & Blum, J. (2023). Brownian translation and rotation from the ballistic to the diffusive limit and derivation of the physical properties of dust agglomerates. Phys. Rev. E, 107, 034136. DOI: 10.1103/PhysRevE.107.034136[Schubert et al. 2024] B. Schubert, J. Blum, M. Isensee, N. S. Molinski, A. Pöppelwerth, I. von Borstel, R. Schräpler, D. Balapanov, & A. Vedernikov (2024). The formation of large aggregates by Brownian growth and the onset of a runaway process. Europhysics Letters, 145(3), 30001. DOI: 10.1209/0295-5075/ad234

The field of planetary system formation relies extensively on our understanding of the aerodynamic interaction between gas and dust in protoplanetary disks. Of particular importance are the mechanisms triggering fluid instabilities and clumping of dust particles into aggregates, and their subsequent inclusion into planetesimals. We introduce the timed Epstein multi-pressure vessel at low accelerations, which is an experimental apparatus for the study of particle dynamics and rarefied gas under micro-gravity conditions. This facility contains three experiments dedicated to studying aerodynamic processes: (i) the development of pressure gradients due to collective particle–gas interaction, (ii) the drag coefficients of dust aggregates with variable particle–gas velocity, and (iii) the effect of dust on the profile of a shear flow and resultant onset of turbulence. The approach is innovative with respect to previous experiments because we access an untouched parameter space in terms of dust particle packing fraction, and Knudsen, Stokes, and Reynolds numbers. The mechanisms investigated are also relevant for our understanding of the emission of dust from active surfaces, such as cometary nuclei, and new experimental data will help interpreting previous datasets (Rosetta) and prepare future spacecraft observations (Comet Interceptor). We report on the performance of the experiments, which has been tested over the course of multiple flight campaigns. The project is now ready to benefit from additional flight campaigns, to cover a wide parameter space. The outcome will be a comprehensive framework to test models and numerical recipes for studying collective dust particle aerodynamics under space-like conditions.

The paper Schräpler, Landeck & Blum ( 2022) has typos in equations (3), (4), (6) and (7). However, the corresponding figs 1 and 2 are correct. Additionally, a sentence on the interpretation of fig. 13 in section 6.2 is wrong. The corresponding fig. 13 is correct.

In dead zones of protoplanetary discs, it is assumed that micrometre-sized particles grow Brownian, sediment to the mid-plane and drift radially inward. When collisional compaction sets in, the aggregates collect slower and therefore dynamically smaller particles. This sedimentation and growth phase of highly porous ice and dust aggregates is simulated with laboratory experiments in which we obtained mm- to cm-sized ice aggregates with a porosity of 90 per cent as well as cm-sized dust agglomerates with a porosity of 85 per cent. We modelled the growth process during sedimentation in an analytical calculation to compute the agglomerate sizes when they reach the mid-plane of the disc. In the mid-plane, the dust particles form a thin dense layer and gain relative velocities by, e.g. the streaming instability or the onset of shear turbulence. To investigate these collisions, we performed additional laboratory drop tower experiments with the high-porosity aggregates formed in the sedimentary-growth experiments and determined their mechanical parameters, including their sticking threshold velocity, which is important for their further collisional evolution on their way to form planetesimals. Finally, we developed a method to calculate the packing-density-dependent fundamental properties of our dust and ice agglomerates, the Young’s modulus, the Poisson ratio, the shear viscosity, and the bulk viscosity from compression measurements. With these parameters, it was possible to derive the coefficient of restitution which fits our measurements. In order to physically describe these outcomes, we applied a collision model. With this model, predictions about general dust-aggregate collisions are possible.

In a protoplanetary disk, dust aggregates in the μm to mm size range possess mean collision velocities of 10–60 m s⁻¹ with respect to dm- to m-sized bodies. We performed laboratory collision experiments to explore this parameter regime and found a size- and velocity-dependent threshold between erosion and growth. By using a local Monte Carlo coagulation calculation and along with a simple semi-analytical timescale approach, we show that erosion considerably limits particle growth in protoplanetary disks and leads to a steady-state dust-size distribution from μm- to dm-sized particles.