Alignment-induced depression and shear thinning in granular matter of nonspherical particles

Rheology of elongated particles in a linear split-bottom shear cell (DEM, MercuryDPM)

Key insight

Particle shape simultaneously controls surface morphology and bulk rheology in granular shear flow:

  • Spheres (AR = 1) → dilatancy inside the shear band → flat surface
  • Elongated particles (AR > 1) → alignment-driven compaction → surface depression

The same alignment mechanism also governs rheology:

  • Increased alignment → higher packing density
  • Modified contact network → shear-thinning behavior

This shows that microstructure (alignment) directly links morphology and flow resistance.

System (visual)

Linear split-bottom shear cell with elongated particles (DEM simulation).
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This work is published in Physical Review Fluids (2024).
DOI

What I did

  • Simulated granular shear flow for aspect ratios AR = 1–5
  • Quantified particle alignment and its localization inside the shear band
  • Computed packing density fields to distinguish dilatancy vs compaction regimes
  • Performed rheological analysis using stress tensor, normal stress differences, and inertial number scaling

Methods & tools

  • Simulation: MercuryDPM, Hertz–Mindlin viscoelastic contact model
  • Geometry: linear split-bottom shear cell (LSC), periodic in flow direction
  • Driving: two L-shaped walls moving at ±Vx/2 (Vx = 0.038 m/s)
  • Particles: multisphere elongated grains (overlapping and non-overlapping), AR = 1–5
  • Parameter range: AR = 1–5 (including fractional AR for overlapping particles)
  • Post-processing: coarse-grained continuum fields Q(y, z), averaged in x and steady state

Key results

1. Alignment localizes inside the shear band

  • Strong alignment develops within the shear band
  • For AR = 5: orientation decreases from 0.27π → 0.15π (~44% reduction)

2. Shape controls dilatancy vs compaction

  • Spheres (AR = 1) → dilatancy → lower density in shear band → flat surface
  • Elongated particles (AR = 5) → alignment-driven compaction → higher density → surface depression

3. Shape increases macroscopic friction

  • Macroscopic friction increases with aspect ratio
  • Saturation occurs at larger AR
  • Overlapping particles show higher friction due to enhanced contact interactions

4. Shear-thinning emerges across all aspect ratios

  • Effective viscosity decreases with inertial number → shear-thinning behavior
  • At fixed inertial number: viscosity increases with AR
  • Rescaling by macroscopic friction collapses data into two regimes:
    • slope ≈ −1 → solid-like behavior
    • slope ≈ −2 → fluid-like behavior

Takeaway

Particle alignment is the key mechanism linking structure, surface morphology, and rheology in granular flow.

This work shows how microstructural evolution controls both compaction and shear-thinning behavior in dense granular systems.

Media


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Publication

H. Rahim, V. Angelidakis, T. Pöschel, and S. Roy
Alignment-induced depression and shear thinning in granular matter of nonspherical particles
Physical Review Fluids (2024)
DOI · Preprint

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