Unraveling 3D Granular Mixing Dynamics in Rotating Drums via Magnetic Resonance Particle Tracking

Rotating drums are canonical systems for studying granular flow and mixing, where particle collisions within a flowing layer drive particle dispersion. Yet, the mechanisms governing these three-dimensional dynamics remain mostly elusive due to the opacity of granular media. Conventional tomographic methods such as MRI offer limited spatiotemporal resolution and cannot resolve individual particle trajectories. Here, we employ Magnetic Resonance Particle Tracking (MRPT), a recently developed technique capable of tracking multiple particles with precision on the scale of milliseconds and per-mils of particle diameter [1]. Using MRPT, we probe granular dynamics across flow regimes (avalanching, rolling, and cataracting) in a rotating drum. The resulting trajectories enable spatially resolved measurements of local occupation probability, mean velocity, and granular temperature, revealing heterogeneous regions of particle dynamics. Additionally, particle-scale tracking uncovers partial layering within the solid body and permits statistical analysis of inter-layer transitions. These findings shed light on the emergence of structure in dense granular flows and advance models of granular mixing.

[1] M. Suter, J. P. Metzger, A. Port, C. R. Müller, and K. P. Pruessmann, (2025), arXiv:2503.22425.