Avalanching on airless metallic bodies with remnant magnetic field

Doctoral Thesis

NASA’s OSIRIS-REx mission collected Asteroid Bennu’s regolith using the TAGSAM - Touch-and-Go Sample Acquisition Mechanism. The arm was supposed to touch and collect samples, but unexpectedly, it kept penetrating the surface. The spacecraft fired thrusters to disengage with the asteroid, or it would have crashed on the surface. The near-zero cohesion between the asteroid regolith caused this unexpected behavior. The whole ordeal pointed to our limited understanding of regolith dynamics in micro-gravity environments.

OSIRIS-REx's TAGSAM operation. Notice how the arm sinks in when it touches the surface of the asteroid.

While we have been studying regolith processes for traditional (stony) asteroids, NASA is endeavouring to explore a new world. Psyche is a metallic asteroid in the main belt between Mars and Jupiter. It is likely a planetesimal - a building block of planets left over from the early solar system. Researchers think that it is a striped core, just like one inside our Earth. Therefore, we expect Psyche to have a remnant magnetic field. Magnetic field could lead to a new cohesion between regolith particles - Magnetic cohesion. I am working to model this magnetic force and understand its effects on bulk regolith properties.

While working through the literature on existing magnetic force models, I realized that most magnetic cohesion studies considered less accurate models based on either Fixed Dipole or Mutual Dipole. (Keaveny & Maxey, 2008) developed a more accurate model, Inclusion model, that includes multipoles along with dipoles. First, I used this model to develop a new empirical formula to calculate the force between two paramagnetic particles placed in a uniform magnetic field. We can use this empirical formula to define a more accurate magnetic bond number: $B_{m a g} = \frac{F_{m a g}}{F_{g r a v}} .$ The bond number helps us compare different forces against surface gravity to understand which force could overpower gravity; making it a significant to research and study. This work resulted in my first journal publication in the Planetary Science Journal: (Sikka et al., 2023).

Since then, I have implemented and validated the Inclusion Model in an open-source Discrete Element Model (DEM) software: LIGGGHTS. We validated the model against the experimental results of (Sunday et al., 2024), making this the first experimentally validated magnetic force model for DEM. The pre-print of our article for this model can be found on (Sikka & Hartzell, 2026). Results for Psyche to follow soon. Meanwhile here are some pretty videos :D

2.5 mT: Granular Flow

15.0 mT: Correlated Regime

25.0 mT: Plastic Regime

Mass Wasting of Steel Balls in different applied magnetic fields.

References

2026

An Experimentally Validated Magnetic Force Model for Discrete Element Modeling of Paramagnetic Granular Media

Anmol Sikka, and Christine M. Hartzell

Jan 2026

Abs

Magnetic interactions between metallic granular particles can lead to magnetic cohesion, influencing the flow characteristics of granular media. This magnetic cohesion has been studied in the context of Magneto-Rheological Fluids (MRF) for their unique flow properties and use in multiple industries. In Planetary Science, magnetic cohesion can influence the behavior of regolith on metallic asteroids with remnant magnetic fields. The upcoming NASA Psyche mission will study the metallic asteroid 16 Psyche, which is expected to have a surface magnetic field. Modeling and simulating the effect of magnetic cohesion on granular media is crucial for accurately simulating the behavior of magnetic granular materials in both terrestrial and planetary applications. We introduce an improved magnetic force model in LIGGGHTS, an open-source discrete element modeling software, to calculate magnetic forces between paramagnetic grains. The model is based on the Mutual Dipole Method and the Inclusion Model, extensions of the Fixed Dipole Method. We validate the model using 1-D unit tests and compare the results from avalanche simulations of paramagnetic regolith with experiments. This work contributes to understanding the role of magnetic cohesion in small body surface processes and provides a tool for future studies of magnetic granular materials in DEM.

2024

PhysRevE

Avalanching Behavior of Magnetic Granular Mixtures

Cecily Sunday, Charles T. Pett, Adam Ben Youssef, Daisy Achiriloaie, and 3 more authors

Physical Review E, Oct 2024

Abs

We conducted avalanching experiments with an external magnetic field and granular samples of different grain sizes (3.18 mm, 6.35 mm, and 8.73 mm) and different materials (low-carbon steel, alloy steel, stainless steel, and brass). The magnetic field was varied to control the magnetic Bond number (the ratio between the magnetic and the gravitational forces in the system). For each test, we compared the angle of repose and the surface roughness of the material in its postavalanche state. The samples containing only steel beads transitioned through three flow regimes as the magnetic field increased. Initially, the grains flowed freely. Above a threshold magnetic field, the material began to move in clumps, and above a second threshold, it solidified completely. The steel-brass mixtures with low magnetic susceptibilities only transitioned through the first two states. We find that the angle of repose and surface roughness increase linearly with magnetic cohesion in the first regime, but that the trends in the second regime depend on the composition and magnetic susceptibility of the mixture. When the angle of repose and surface roughness are expressed in terms of the magnetic Bond number, the homogeneous samples that vary in grain size and magnetic susceptibility collapse onto a single curve, but the mixtures (i.e., the samples that contain more than one type of material) do not.

2023

Development of an Empirical Model of the Force between Paramagnetic Particles in Uniform Magnetic Field on M-type Asteroids

Anmol Sikka, Ian DesJardin, Thomas Leps, and Christine Hartzell

Jul 2023

Abs Bib

M-type asteroids may have remanent magnetic fields. Regolith particles on M-type asteroids are likely to have metallic components, causing them to be paramagnetic and respond to an external magnetic field. Paramagnetic particles placed in an external magnetic field are influenced by induced magnetic moments of neighboring particles. Therefore, the magnetic force between regolith particles on an M-type asteroid can change the net cohesion of the regolith. Previous works have shown the influence of cohesive forces in the evolution of rubble-pile asteroids. This work characterizes the magnetic force between regolith particles on M-type asteroids. We implement existing models of the magnetic force between paramagnetic particles placed in an external magnetic field and then present an empirical model of this force for two magnetic field orientations as an essential step toward a general semiempirical model that the wider planetary science community can more easily use to investigate the significance of this force.
@article{sikka_development_2023, title = {Development of an Empirical Model of the Force between Paramagnetic Particles in Uniform Magnetic Field on M-type Asteroids}, volume = {4}, issn = {2632-3338}, doi = {10.3847/PSJ/ace323}, pages = {129}, number = {7}, journaltitle = {The Planetary Science Journal}, shortjournal = {Planet. Sci. J.}, author = {Sikka, Anmol and {DesJardin}, Ian and Leps, Thomas and Hartzell, Christine}, year = {2023}, month = jul, date = {2023-07-26}, langid = {english}, }

2008

JCP

Modeling the Magnetic Interactions between Paramagnetic Beads in Magnetorheological Fluids

Eric E. Keaveny, and Martin R. Maxey

Journal of Computational Physics, Nov 2008

Abs

In this study, we develop and compare new and existing methods for computing the magnetic interactions between paramagnetic particles in magnetorheological (MR) fluids. The commonly employed point-dipole methods are outlined and the inter-particle magnetic forces, given by these representations, are compared with exact values. An alternative finite-dipole model, where the magnetization of a particle is represented as a distribution of current density, is described and the associated computational effort is shown to scale as O(N). As the dipole moments and forces given by this model depend on the length scale of the current distribution, a sensitivity analysis is performed to reveal a proper choice of this length scale. While the dipole models give a good estimation of the far-field interactions, as two particles come into contact, higher order multipoles are needed to properly resolve their interaction. We present the exact two-body calculation and describe a procedure to include the higher multipoles arising in a pairwise interaction into a dipole model. This inclusion procedure can be integrated with any dipole or higher-multipole calculation. Results from relevant three-body problems are compared to exact solutions to provide information as to how well the inclusion procedure performs in simulations of self-assembly and estimating the yield strength of structures.