🔭 astrophysics

Experiments on Settling of Granular and Cohesive Material in Low Gravity

This paper presents experiments conducted in a zero-gravity drop tower using a linear stage to simulate reduced gravities, demonstrating that granular materials exhibit lower packing densities in weaker gravitational fields with sensitivity dependent on particle size, roughness, and uniformity, thereby highlighting the critical role of cohesive forces in low-gravity environments.

Original authors: Matthias Keulen, Timo Giese, Kolja Joeris, Jonathan Kollmer

Published 2026-06-01

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CC BY 4.0

Original authors: Matthias Keulen, Timo Giese, Kolja Joeris, Jonathan Kollmer

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine you are trying to pack a suitcase. On Earth, if you throw in a bunch of heavy rocks, they settle tightly at the bottom because gravity pulls them down hard. But what happens if you take that same suitcase to a place where gravity is much weaker, like on a small asteroid or the Moon? Do the rocks pack just as tightly, or do they float around more loosely?

This paper is about answering that exact question, but with a twist: the scientists couldn't just wait for a trip to space. Instead, they built a "gravity simulator" inside a drop tower in Germany to see how different types of dirt and sand behave when gravity is dialed down.

Here is the story of their experiment, broken down simply:

The Setup: A Gravity Elevator

The team used a special tower in Bremen, Germany, called a "drop tower." Normally, you drop a capsule to create a few seconds of weightlessness (zero gravity). But this team wanted to test low gravity, not zero gravity.

Think of their experiment like a very precise elevator inside that falling capsule.

The Drop: The capsule falls, creating a zero-gravity environment.
The Push: Inside the falling capsule, a high-speed motor pushes a container of dirt upward with just the right amount of force.
The Result: By pushing up against the fall, they created a "fake" gravity that was weaker than Earth's but stronger than zero. It's like riding an elevator that is falling, but you press the "up" button just enough to make yourself feel lighter, but not weightless.

The Cast of Characters

They tested three different "guests" to see how they settled:

Fine Basalt: Tiny, rough dust particles (like very fine sand or flour), ranging from 1 to 200 micrometers.
Coarse Basalt: Large, rough chunks of rock (2 to 5 millimeters), like small pebbles.
Glass Beads: Smooth, round balls (750 to 1000 micrometers), like tiny marbles.

The Discovery: The "Fluffiness" Factor

When they turned down the gravity, they watched how much space the dirt took up. They found that everything got fluffier (took up more volume) when gravity was weaker.

The Rough Dust (Fine Basalt): This was the most dramatic. When gravity was very low, these tiny, rough particles expanded by nearly 20%. They acted like a crowd of people who are afraid to touch each other; when the "push" of gravity is weak, they stick to each other and form a loose, airy cloud.
The Rough Rocks (Coarse Basalt): These also got fluffier, expanding by about 12%, but not as much as the dust.
The Smooth Marbles (Glass Beads): Surprisingly, these were the least affected. Even though they were similar in size to the fine dust, because they were smooth and round, they only expanded by about 4%. They slid past each other easily, even in low gravity.

The "Stickiness" Metaphor

Why did the rough dust expand so much? The paper explains that in low gravity, the weight of the particles becomes very light. When that happens, tiny invisible forces—like static electricity or the natural "stickiness" of rough surfaces (Van der Waals forces)—become the boss.

Imagine a pile of dry leaves on a windy day. On Earth, the wind might blow them, but they are heavy enough to stay somewhat packed. On a tiny asteroid, those same leaves would be so light that they would cling to each other and float in a big, loose cloud. The rougher the surface, the more they "cling" to their neighbors, creating a fluffier pile.

Why This Matters (According to the Paper)

The scientists wanted to prove that you can't just guess how space dirt will behave by looking at Earth dirt.

Size isn't everything: You might think small particles always act the same, but the rough dust acted very differently from the smooth glass beads of similar size.
Roughness matters: The rough, jagged edges of the basalt made them much more sensitive to low gravity than the smooth glass beads.
Validation: Their real-world data matches up with computer simulations, giving scientists confidence that their models for building habitats or mining on the Moon and asteroids are on the right track.

The Bottom Line

In simple terms: If you are planning to build a base on an asteroid, don't assume the dirt will pack down tight like it does in your backyard. In low gravity, especially with rough, dusty soil, that dirt will stay loose and fluffy, taking up much more space than you expect. The smoother and rounder the material, the less it will change.

Technical Summary: Experiments on Settling of Granular and Cohesive Material in Low Gravity

Problem Statement
The mechanical behavior of regolith on rocky bodies (planets, asteroids) differs significantly from Earth due to varying gravitational conditions. Predicting the packing density, angle of repose, and cohesion of granular matter in these environments is critical for future In-Situ Resource Utilization (ISRU), lunar outpost construction, and asteroid interaction missions. A primary challenge in this field is that granular behavior is not easily scalable across different gravities. In low-gravity regimes, such as those found on small asteroids, cohesive inter-particle forces (e.g., van der Waals, electrostatic) can become comparable to gravitational forces, complicating theoretical predictions. While theoretical simulations exist, there is a lack of high-quality experimental data in reduced gravity regimes to validate these models.

Methodology
The authors conducted experiments at the ZARM drop tower facility in Bremen, Germany, utilizing the GraviTower Pro Bremen (GTB) to provide a microgravity environment with residual accelerations of $10^{-6} g$ . To simulate specific partial gravity conditions, the team employed a high-precision linear motor (Newport IMS-LM300) inside the drop tower capsule. This setup allowed for the artificial generation of gravity levels ranging from 150 to 1000 mm/s² (approximately 0.015g to 0.1g), covering regimes from large asteroid gravity to below lunar gravity.

Three distinct granular samples were tested:

Fine Basalt: Particle diameters of 1–200 µm.
Coarse Basalt: Particle diameters of 2–5 mm.
Glass Beads: Spherical particles with diameters of 750–1000 µm.

The samples were dried at 120°C for 24 hours to minimize moisture and surface water layers. They were placed in cylindrical vacuum tubes (inner diameter 16.3 mm) maintained at pressures between 0.21 and 0.78 mbar to minimize gas drag. The experimental sequence involved an initialization phase, a vigorous shake-up to ensure consistent packing, and a settling phase under the specific artificial gravity trajectory. High-speed video analysis was used to determine the filling height, from which volume changes ( $\Delta V$ ) and packing densities ( $\phi$ ) were calculated relative to Earth's gravity ( $1g$ ).

Key Results
The experiments revealed that granular samples occupy larger volumes and exhibit lower packing densities in reduced gravity compared to Earth conditions. The magnitude of this change is highly dependent on particle characteristics:

Sensitivity to Gravity: The fine basalt sample was the most sensitive, showing a volume increase of up to +19.6% at 250 mm/s². The coarse basalt showed a maximum volume increase of +12.2% at 150 mm/s². The spherical glass beads were the least sensitive, with a volume increase of only +4.25% at 250 mm/s².
Role of Roughness and Uniformity: The results indicate that volume change is not solely a function of particle size. The rough, irregular basalt particles exhibited higher sensitivity to gravity changes than the uniform, spherical glass beads, suggesting that surface roughness and particle uniformity significantly influence packing behavior in low gravity.
Packing Density Trends: As gravity decreased, packing densities approached values lower than those observed on Earth. The coarse basalt demonstrated lower packing densities than the fine basalt at gravities below 750 mm/s², further highlighting non-linear scalability.

Modeling and Validation
The experimental data were fitted to a theoretical model proposed by Elekes & Parteli (2021), which describes the asymptotic growth of packing density as a function of particle diameter ( $d$ ) and the square root of gravity ( $\sqrt{g}$ ). The authors utilized the dimensionless parameter $d \cdot \sqrt{g/g_{Earth}}$ to standardize the data across different particle sizes. While the experimental data generally followed the theoretical trend, the data points did not collapse onto a single curve as seen in simulations of monodisperse particles. The authors attribute this deviation to the significant size distributions of their samples (particularly the coarse basalt) and the presence of boundary effects in the sample containers.

Significance and Claims
The paper claims to provide the first real-life experimental data for granular settling in gravity regimes between 0.015g and 0.1g, a range previously uninvestigated experimentally. The study demonstrates that:

Cohesive forces play a dominant role in low-gravity packing, causing significant volume expansion in rough and fine-grained materials.
Particle properties matter: Roughness and uniformity are critical factors in determining how granular matter responds to reduced gravity, not just particle size.
Validation of Theory: The data serves to validate and refine theoretical works and simulations (such as those by Elekes & Parteli) regarding packing density and cohesion in low-gravity environments.

The authors conclude that these findings are essential for understanding the mechanical behavior of regolith for future space missions, particularly regarding the handling and transport of materials on asteroids and the Moon. They note that while their setup minimized noise and Coriolis forces, factors such as tribocharging and boundary effects remain areas for further investigation.