Reverse segregation and self-organization in inclined chute flows of bidisperse granular mixtures

TL;DR

This study investigates how bidisperse granular mixtures in inclined chute flows exhibit reverse segregation and self-organization, revealing new layering phenomena influenced by particle size ratio and mass fraction, with implications for industrial mixing processes.

Contribution

It provides a detailed analysis of segregation reversal and layer formation in bidisperse granular flows at high diameter ratios using discrete element simulations.

Findings

Segregation transitions from usual to reverse at diameter ratio ~4.

Formation of alternating coarse- and fine-rich layers along the shear gradient.

Layer thickness correlates with coarse particle diameter.

Abstract

In the usual segregation scenario for stable inclined chute flows of bidisperse mixtures of fine and coarse spherical particles, coarse particles rise toward the free surface, forming a coarse-rich region atop the flowing pile. Beyond a threshold coarse-to-fine diameter ratio of approximately 4, conversely, the weight of the coarse particles exceeds the segregation driving forces, causing individual coarse particles to sink within the pile and producing a reversed segregation state. However, an understanding of the collective evolution of the pile structure is still lacking when the particle diameter ratio exceeds 4 {\textit{and}} the coarse particle mass fraction is appreciable. To explore this broadly bidisperse limit, we perform discrete element method simulations considering mean particle diameter ratios of up to 8 and coarse particle mass fractions spanning 0.1 to 0.9. The steady-state flow profiles reveal several intriguing behaviors that depend on the diameter ratio and mass fraction. These include a previously identified transition from usual to reverse segregation and a newfound tendency to self-organize into alternating coarse- and fine-rich particle layers stacked along the shear gradient direction, with layer thickness dictated by the coarse particle diameter. A fuller understanding of segregation at this scale could pave the way for enhanced mixing or demixing techniques at the commercial scale.