Dilatancy-Induced Heap Formation in Dense Cohesive Granular Media

DEM simulations of wet granular shear flow in a split-bottom cell (MercuryDPM)

Key insight

Cohesion fundamentally changes how granular materials respond to shear:

  • Dry systems (no cohesion) → flat free surface
  • Cohesive systems (liquid bridges) → formation of a pronounced surface heap

This behavior emerges from enhanced shear-induced dilatancy inside the shear band, driven by capillary forces.

The competition between:

  • Cohesion (liquid bridges) → stabilizes particle separation and promotes dilation
  • Inertia (shear rate) → suppresses cohesion-driven effects

determines whether heap formation occurs.

What I did

  • Implemented DEM simulations of wet granular media with capillary bridge cohesion
  • Quantified free-surface evolution using a heap area ratio (A/A₀) derived from particle data
  • Computed coarse-grained packing density fields to resolve dilatancy inside the shear band
  • Systematically varied surface tension (γ) and shear rate to isolate competing physical effects

Methods & tools

  • Simulation: MercuryDPM (DEM), Hertz–Mindlin contact model
  • Cohesion model: capillary bridge forces (Willett approximation), controlled via surface tension γ
  • Geometry: linear split-bottom shear cell (LSC), domain size (Lx, Ly, Lz) = (20, 80, 25) dp
  • Particles: polydisperse spheres (uniform size distribution)
  • Cohesion range: γ = 0–0.160 N/m
  • Shear rates: Vₛₕₑₐᵣ = 0.016 – 0.16 m/s
  • Post-processing: coarse-graining via MercuryCG (packing density fields in y–z plane)

Key results

1. Cohesion drives heap formation

  • γ = 0 (dry) → flat surface
  • γ > 0 (cohesive) → clear heap above the shear band

Cohesion introduces a qualitative change in surface morphology.

2. Cohesion enhances dilatancy

  • Increasing γ enlarges the low-density (dilated) region inside the shear band
  • This produces a dome-shaped dilated zone and a corresponding surface heap

3. Heap evolution reaches steady state

  • Heap area ratio A/A₀ increases over time and saturates
  • Higher cohesion → larger steady-state heap

4. Bond number controls the physics

  • Systems with different material parameters collapse onto the same behavior when the equivalent Bond number is matched
  • Heap formation is governed by the ratio of cohesive forces to gravity, not surface tension alone

5. Shear rate suppresses cohesion effects

  • Higher shear rates reduce heap formation
  • Inertial effects dominate over cohesion at high velocities

Takeaway

Cohesion does not simply modify granular flow —
it fundamentally alters how dilatancy develops and how the free surface evolves under shear.

This work shows that heap formation emerges from the interplay between capillary forces, gravity, and shear rate.

Media


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Publication

H. Rahim, T. Pöschel, and S. Roy
Dilatancy-induced heap formation in dense cohesive granular media
Physics of Fluids (2026)
DOI · Preprint

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