Experiments on Settling of Granular and Cohesive Material in Low Gravity

The regolith of rocky bodies, such as planets or asteroids, generally settles under gravity conditions different from those of the earth. The behavior of granular material is not easily scalable for different gravities. To predict these highly complex systems where cohesive inter particle forces can be comparable to gravitational forces, we need simulations and experiments. We did experiments on settling of three different granular samples in varying reduced gravities and examined their packing densities. For this we used a high precision linear motor as a stage to artificially induce milligravity inside the zero g environment provided by the ZARM drop tower and observe the settling of opur samples. The three samples were fine basalt with particle diameters of 1–200μm, coarse basalt with 1–3mm diameters and glass beads with 825–1000 μm. The artificial gravities were 150, 250, 500, 750 and 1000 mm/s² and therefore ranged from large asteroid gravity to almost moon gravity. We unsurprisingly saw the granular samples with higher volume in lower gravities and therefore lower packing densities, however we also saw the fine basalt be the most sensitive to changes in gravity, up to +19.6% in volume for 250 mm/s² , followed by the coarse basalt particles, up to 12.2% in volume for 150 mm/s² and the glass beads packing density being the least sensitive to changes in gravity, up to 4.25% in volume for 250mm/s². With these experiments we show change in volume is not solely dependent of particle size but also roughness and uniformity, we provide real life experimental data to validate theoretical works and highlight the role of cohesive forces in low gravity environments.