Radial and axial segregation in horizontal rotating cylinders studied by Magnetic Resonance Imaging (MRI)

TL;DR

This study uses advanced MRI techniques to visualize and analyze the 3D structures and real-time dynamics of radial and axial segregation in rotating cylinders filled with millet and poppy seeds, revealing new mechanisms of granular segregation.

Contribution

First-time application of high-resolution 3D and real-time 2D MRI to study granular segregation, providing detailed insights into underlying mechanisms and dynamics.

Findings

Radial segregation involves millet and poppy core formation.

Axial segregation features traveling and merging poppy-rich bands.

Core diffusion is a significant mechanism in segregation processes.

Abstract

The dynamics of granular materials, mostly radial and axial segregation in horizontal rotating cylinders filled with millet and poppy seeds, is studied by Magnetic Resonance Imaging (MRI). For the first time, full 3D structures and real-time 2D MRI movies showing the progress of segregation over many hours are reported. Data are acquired with sufficiently high temporal and in-plane spatial resolutions (74 ms and 0.94 mm \times 0.94 mm, respectively), giving new insights into the underlying mechanisms. The millet and poppy mixture composition is calibrated based on the signal intensity and is quantified throughout segregation. As for radial segregation, millet and poppy mixture core formation is observed in cylinders of 75% and 82% filling level. The size of the core is calculated and the avalanche layer thickness is therefore determined. 2D MRI movies showing real-time radial segregation suggests that initial radial segregation is due to the stopping and percolation of poppy seeds. Axial segregation is characterized by the formation, traveling and merging of poppy-rich bands. In all cases studied, the formation of poppy-rich bands is observed, after which individual bands start to travel at \sim 3 \mu m s^{-1} until they are within \sim 3 cm of a stationary band. Adjacent bands then merge into a single, enlarged poppy band as millet seeds move out of the merging region. In both radial and axial segregation, core diffusion is shown to be another mechanism pathway for segregation besides the free surface.