The compaction of highly porous dust agglomerates by a gas-velocity gradient and its application to dust growth in turbulent protoplanetary disks

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 model, our calculations show that due to turbulence-induced velocity gradients (e.g., at , = 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 fr 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.