Impact of friction and grain shape on the morphology of sheared granular media

DEM simulations in a linear split-bottom shear cell (MercuryDPM)

Key insight

Sheared granular materials behave fundamentally differently depending on particle shape:

  • Spherical particles → no surface deformation
  • Elongated particles → develop a pronounced surface depression

This transition is governed by a competition between:

  • Alignment-driven compaction (dominant at low friction)
  • Friction-driven dilatancy (dominant at high friction)

Using DEM simulations, I show how these microscopic mechanisms directly control shear-band structure and free-surface morphology.

System (visual)

Linear split-bottom shear cell with elongated particles (DEM simulation).
See full simulations →

This work is published in Physical Review Fluids (2026), DOI.

What I did

  • Built DEM simulations of spheres and elongated particles (AR = 1–5) in a linear split-bottom shear cell
  • Computed continuum fields (density, velocity, stress tensor) using coarse-graining
  • Quantified particle alignment inside the shear band using orientation statistics and nematic order parameter S
  • Systematically varied friction (µ = 0.01–0.8) to isolate its effect on alignment, stress, and surface morphology

Methods & tools

  • Simulation: MercuryDPM (multisphere particles, Hertz–Mindlin contact model).
  • Geometry: linear split-bottom shear cell with periodic boundary conditions in the flow direction.
  • Particles: spheres (AR = 1) and stick-shaped grains built from connected spheres (AR = 2–5).
  • Parameter range: friction coefficient µ = 0.01–0.8; aspect ratio AR = 1–5.
  • Post-processing: coarse-graining via MercuryCG, analysis and plotting in Python / MATLAB.

Key results

1. Shape controls surface morphology

  • Spheres (AR = 1) → flat surface
  • Elongated particles (AR > 1) → clear surface depression

This shows that particle geometry alone can fundamentally change macroscopic behavior.

2. Friction weakens alignment

  • Strong alignment at low friction → S ≈ 0.8
  • Reduced alignment at high friction → S ≈ 0.4

Higher friction disrupts particle ordering inside the shear band.

3. Competing mechanisms inside the shear band

Two mechanisms control the system:

  • Alignment → compaction (density increases)

  • Dilatancy → expansion (density decreases)

  • Low friction → alignment dominates → broader depression

  • High friction → dilatancy dominates → localized deformation

4. Key physical insight

Surface morphology is not random — it directly emerges from:

  • particle shape
  • friction
  • shear-induced alignment

This links microscopic particle physics → macroscopic flow behavior.

Takeaway

Particle shape and friction do not just modify granular flow —
they fundamentally determine how shear is localized and how the free surface evolves.

This work shows how microscopic alignment mechanisms translate into macroscopic morphology.

Media


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Publication

H. Rahim, S. Roy, and T. Pöschel
Impact of friction and grain shape on the morphology of sheared granular media
Physical Review Fluids (2026)
DOI · Preprint

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