Magnetic Resonance Particle Tracking

Granular matter such as gravel, cereals, or pellets, is ubiquitous in nature, daily life, and industry. While sharing some characteristics with gases, liquids, and solids, granular matter exhibits a wealth of phenomena that defy these analogies and are yet to be fully understood. Advancing granular physics requires experimental observation at the level of individual particles and over a wide range of dynamics. Specifically, it calls for the ability to track many particles simultaneously, in three dimensions (3D), and with high spatial and temporal resolution. Here, we introduce magnetic resonance particle tracking (MRPT) [1] and show it to achieve such recording with resolution on the scale of micrometers and milliseconds while preserving particle identity. This is possible by a judicious, fast strategy of exciting and encoding nuclear magnetic resonance, leading to a recursive non-convex optimization problem that can be solved to obtain particle trajectories. MRPT enables the study of a large variety of systems. This includes vibrated granular beds, where we report the direct study of granular glassy dynamics in 3D, investigation of mixing properties in a rotating drum and fast particle dynamics in an air-fluidized bed. These findings illustrate broad utility of MRPT in advancing the exploration, theory, and numerical models of granular matter.

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