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arxiv: 2605.30395 · v1 · pith:STJNBR5Cnew · submitted 2026-05-28 · 🌌 astro-ph.IM · astro-ph.EP· cond-mat.soft

Experiments on Settling of Granular and Cohesive Material in Low Gravity

Matthias Keulen , Timo Giese , Kolja Joeris , Jonathan Kollmer This is my paper

Pith reviewed 2026-06-29 00:41 UTC · model grok-4.3

classification 🌌 astro-ph.IM astro-ph.EPcond-mat.soft
keywords granular settlinglow gravitypacking densityregolithcohesive forcesdrop towerbasalt particlesglass beads
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The pith

Granular materials pack less densely in lower gravity, with fine basalt most sensitive.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper conducts settling experiments on three granular samples under reduced gravities from 150 to 1000 mm/s² created inside a drop tower. It measures that all samples reach higher final volumes at lower gravities, which means lower packing densities, and that the effect is strongest for fine basalt particles. A sympathetic reader would care because this shows how inter-particle cohesion, normally weak on Earth, can dominate settling on asteroids and small bodies where gravity is weak. The work supplies measured volume changes to check against models that predict regolith behavior in those environments.

Core claim

When three granular samples settle under artificially reduced gravities, fine basalt (1-200 μm) reaches up to 19.6 % higher volume at 250 mm/s², coarse basalt (2-5 mm) up to 12.2 % higher at 150 mm/s², and glass beads (750-1000 μm) up to 4.25 % higher at 250 mm/s², compared with higher-gravity runs; the authors conclude that volume increase is not determined by particle size alone but also by roughness and uniformity, and that cohesive forces become comparatively important in low gravity.

What carries the argument

A high-precision linear stage inside the drop tower that accelerates the sample containers to produce constant reduced gravity during settling.

If this is right

  • Packing density of granular material decreases as ambient gravity decreases.
  • Fine, rough particles exhibit the largest density drop because cohesive forces matter more relative to weight.
  • Change in settled volume depends on particle size, surface roughness, and size uniformity together.
  • The measured volume changes supply calibration points for simulations of regolith on asteroids and small bodies.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • Models of asteroid surface evolution or spacecraft landing may need to treat packing density as gravity-dependent rather than fixed.
  • The same cohesion-driven loosening could affect how dust and small grains aggregate during planet formation at low relative velocities.
  • Repeating the tests with a wider range of particle shapes or with added electrostatic charging would test how far the observed trends generalize.

Load-bearing premise

The stage motion creates the same settling dynamics that would occur under steady natural low gravity, without added inertial forces, vibrations, or container effects.

What would settle it

Packing-density measurements made on an asteroid surface or in a true constant low-g environment that show no gravity dependence or the opposite trend from the laboratory volume increases.

Figures

Figures reproduced from arXiv: 2605.30395 by Jonathan Kollmer, Kolja Joeris, Matthias Keulen, Timo Giese.

Figure 1
Figure 1. Figure 1: Set-up as used inside the Bremen drop tower cap￾sule and the residing microgravity. With the high precision linear motor accelerating the experiment according to figure 2 and with the sample tubes in view of the camera. We conducted our experiments at the drop tower facil￾ities of the ZARM (Zentrum f¨ur angewandte Raumfahrt￾technologie und Mikrogravitation) in Bremen. Here we used the GraviTower Pro Bremen… view at source ↗
Figure 2
Figure 2. Figure 2: Example trajectory of the stage in zero g for a run in 750 mm/s2 , including initializing, shake up phase (red background), partial gravity phase (green background) and the way back to the home position (blue background) to re￾duce the distance the stage can travel in the deceleration of the drop tower capsule. Please note that this distance trav￾eled by the stage is completely independent from the travel￾… view at source ↗
Figure 4
Figure 4. Figure 4: shows the size distributions of the fine basalt and the glass beads samples determined using a Master￾sizer 3000+. Included are insets of microscope images of the samples for roughness comparison. The coarse basalt particles were too big to be analyzed using the Mastersizer. Here we used a series of ten pic￾tures of the sample material on a white backgroun and analyzed them using the image processing progr… view at source ↗
Figure 5
Figure 5. Figure 5: Sample container (a) schematic as shown on the Hositrad webshop, with total length L and outer diameter D (Hositrad Holland B.V. 2026) and (b) as used in the ex￾periments, here filled with the 825–1000 µm glass beads. 2.3. Image Processing The videos obtained through these experiments were analyzed regarding the filling height of the granular sam￾ples to conclude the change in volume and therefore the pack… view at source ↗
Figure 7
Figure 7. Figure 7: Normalized brightness of pixel rows of figure 6 (a) and (b) in comparison. Note that the y-axis is the actual pixel row converted to height in mm ferent samples and gravities using the corresponding densities ρ of basalt and soda lime glass and the weight m of the samples, with: ϕi,mg = mi · ρi Vmg (2) [PITH_FULL_IMAGE:figures/full_fig_p004_7.png] view at source ↗
Figure 9
Figure 9. Figure 9: Packing density for different gravities. Symbols with error bars mark the averages for 150, 250, 500, 750 and 1000 mm/s2 and the error bars are the standard deviations of these averages. The horizontal lines show values in 1 g. with all three sets of data in one picture thus broadening the range. What it shows is the packing densities approaching the Earth’s gravity values and the switch between fine and c… view at source ↗
Figure 8
Figure 8. Figure 8: Change in volume in percent for different gravities. Symbols with error bars mark the averages for 150, 250, 500, 750 and 1000 mm/s2 and the error bars are the standard deviations of these averages. We see an increase in volume for lower gravities as cohesive forces become more and more important com￾pared to gravity, in other words for higher granular Bond numbers (Castellanos 2005). It is also apparent t… view at source ↗
Figure 10
Figure 10. Figure 10: Packing density ϕ against d · p g/gEarth and fitted to ϕ = ϕ∞ · n 1 − C · exp h −D −1 ϕ · d p g/gEarthio cor￾responding to the works of Elekes & Parteli (2021). Our fits are shown in the same color as individual measurements. The results of Elekes & Parteli (2021) for simulated monodisperse glass particles are shown in dashed and purple. Additionally packing densities for Earth conditions are included as … view at source ↗
read the original abstract

The regolith of rocky bodies, such as planets or asteroids, generally settles under gravity conditions different from those of 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. We used a high precision linear stage to artificially induce reduced gravities inside the zero $g$ environment provided by the ZARM drop tower and observe the settling of our samples. The three samples were fine basalt with particle diameters of $1\text{-}200\,\mu$m, coarse basalt with $2\text{-}5\,$mm and glass beads with $750\text{-}1000\,\mu$m. The artificial gravities were $150,\,250,\,500,\,750$ and $1000\,$mm/s$^2$ and therefore ranged from large asteroid gravity to almost moon gravity. We saw the granular samples have higher volumes in lower gravities and therefore lower packing densities, we also saw the fine basalt be the most sensitive to changes in gravity, up to $+19.6\,\%$ in volume for $250\,$mm/s$^2$, followed by the coarse basalt particles, up to $+12.2\,\%$ for $150\,$mm/s$^2$ and the glass beads packing density being the least sensitive to changes in gravity, up to $+4.25\,\%$ for $250\,$mm/s$^2$. 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.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

3 major / 2 minor

Summary. The manuscript reports drop-tower experiments on the settling of three granular samples (fine basalt 1-200 μm, coarse basalt 2-5 mm, glass beads 750-1000 μm) under artificially reduced gravities of 150, 250, 500, 750 and 1000 mm/s² produced by a high-precision linear stage inside the ZARM zero-g facility. The central observational claim is that settled volumes increase (packing densities decrease) at lower g, with material-dependent sensitivities: fine basalt up to +19.6 % at 250 mm/s², coarse basalt up to +12.2 % at 150 mm/s², and glass beads up to +4.25 % at 250 mm/s². The authors conclude that volume change depends on particle size, roughness and uniformity and that cohesive forces play an increasing role at low g.

Significance. If the acceleration field is shown to be free of significant transients or vibrations and the volume measurements are repeatable, the data would supply useful empirical benchmarks for discrete-element models of regolith on asteroids and other small bodies where cohesion becomes comparable to gravity. The material-specific trends and the use of a drop-tower linear-stage method constitute a concrete experimental contribution to planetary-science instrumentation literature.

major comments (3)
  1. [Methods/Results] Methods/Results: The reported volume increases (e.g., +19.6 % for fine basalt) are given without error bars, standard deviations, number of repetitions, or any description of the volume-measurement procedure (optical, mass-based, or otherwise), preventing assessment of whether the material-dependent differences exceed experimental uncertainty.
  2. [Methods] Methods: No quantitative characterization is supplied of the acceleration time-history delivered by the linear stage (uniformity during the settling interval, start-up transients, residual vibration spectrum, or container-wall normal forces), which is required to substantiate that the observed packing densities arise from the intended constant low-g body force rather than apparatus-specific inertial or vibrational effects.
  3. [Discussion] Discussion: The attribution of sensitivity differences to 'roughness and uniformity' in addition to size rests on the three samples differing simultaneously in multiple properties; without additional control experiments or particle-property quantification, this causal claim is not yet load-bearing.
minor comments (2)
  1. [Abstract] Abstract: 'dependent of particle size' should read 'dependent on particle size'.
  2. [Methods] Notation: Gravitational accelerations are consistently given in mm/s², but the manuscript should state whether these values are the commanded stage accelerations or measured values.

Simulated Author's Rebuttal

3 responses · 0 unresolved

We thank the referee for the constructive comments on our manuscript. We address each major comment point by point below, indicating the revisions we will make.

read point-by-point responses

  1. Referee: [Methods/Results] The reported volume increases (e.g., +19.6 % for fine basalt) are given without error bars, standard deviations, number of repetitions, or any description of the volume-measurement procedure (optical, mass-based, or otherwise), preventing assessment of whether the material-dependent differences exceed experimental uncertainty.

    Authors: We agree that these statistical and procedural details are essential. The volumes were determined optically from the height of the settled material in a transparent cylindrical container using calibrated image analysis. Five independent repetitions were performed for each sample-gravity combination. In the revised manuscript we will add a dedicated methods subsection describing the optical procedure, report the repetition count, and include error bars derived from the standard deviation across repetitions. revision: yes

  2. Referee: [Methods] No quantitative characterization is supplied of the acceleration time-history delivered by the linear stage (uniformity during the settling interval, start-up transients, residual vibration spectrum, or container-wall normal forces), which is required to substantiate that the observed packing densities arise from the intended constant low-g body force rather than apparatus-specific inertial or vibrational effects.

    Authors: We acknowledge that quantitative verification of the acceleration field is required. Accelerometer data were acquired during the runs. The revised methods section will include time-series plots and quantitative metrics (uniformity within ±5 % after ramp-up, peak transient acceleration, and RMS vibration amplitude) demonstrating that the settling phase occurred under the commanded constant low-g with negligible residual motion. revision: yes

  3. Referee: [Discussion] The attribution of sensitivity differences to 'roughness and uniformity' in addition to size rests on the three samples differing simultaneously in multiple properties; without additional control experiments or particle-property quantification, this causal claim is not yet load-bearing.

    Authors: The referee correctly notes that the three samples vary in several properties simultaneously. We will revise the discussion to state that the observed material-dependent volume changes are consistent with the combined influence of particle size, surface roughness, and size uniformity, supported by the provided particle-size distributions and qualitative surface descriptions. We will add quantitative particle characterization (SEM-derived roughness estimates and full size-distribution histograms) while explicitly acknowledging that isolating individual factors would require additional controlled experiments not performed here. revision: partial

Circularity Check

0 steps flagged

Purely experimental report; no derivations, equations, or fitted predictions present.

full rationale

The manuscript reports direct experimental measurements of packing volumes for three granular samples under controlled low accelerations produced by a linear stage in a drop tower. All reported results (volume increases of +4.25% to +19.6% at specific accelerations) are presented as observed outcomes without any claimed theoretical derivation, model fitting, or predictive equations. No self-citations of prior uniqueness theorems, ansatzes, or renamings of known results appear. The work contains no load-bearing steps that reduce to their own inputs by construction, satisfying the default expectation of no circularity for an observational study.

Axiom & Free-Parameter Ledger

0 free parameters · 0 axioms · 0 invented entities

Experimental study; central claims rest on validity of the drop-tower setup and volume measurements rather than mathematical axioms or free parameters. No model equations or postulated entities appear in the abstract.

pith-pipeline@v0.9.1-grok · 5872 in / 1085 out tokens · 34970 ms · 2026-06-29T00:41:27.097208+00:00 · methodology

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