Granular Materials

1. Granular Materials R. Behringer Duke University Durham, NC, USA

2. Outline • Overview • What’s a granular material? • Numbers, sizes and scales • Granular phases • Features of granular phases • Why study granular materials? • Special Phenomena • Open challenges—what we don’t know

3. Issues/ideas for granular gases • Kinetic theory • Hydrodynamics • Clustering and collapse • Simulations • Experiments

4. Issues/ideas for dense granular systems • Friction and dilatancy • Force chains • Janssen model • Constant flow from a hopper • Forces under sandpiles • Texture

5. Models for static force transmission • Lattice models: Q-model, 3-leg, elastic • Continuum limits of LM’s • Classical continuum models • Summary of predictions

6. Experimental tests of force transmission • Order/disorder • Friction • Vector nature of force transmission • Textured systems • So where do we stand?

7. Force fluctuations in dense systems • Force chains • Fragility • Anisotropy

8. Transitions • Jamming • Percolation • Relation to other phenomena—e.g. glasses • Clustering (see gases) • Fluidization • Subharmonic Instabilities (shaken systems) • Stick-slip

9. “Classical” systems • Shaking (convection, waves…) • Avalanches • Rotating flows • Hoppers and bunkers • Shearing • Mixing and segregation

10. Special techniques • Discrete element models (DEM or MD) • Lattice models • Special experimental techniques • NMR • Photoelasticity • “Carbon paper”

11. What is a granular material? • Large number of individual solid particles • Classical interactions between particles • Inter-particle forces only during contact • Interaction forces are dissipative • Friction, restitutional losses from collisions • Interaction forces are dissipative • A-thermal—kBT << Etypical ~ mgd • Other effects from surrounding fluid, charging may occur

12. Numbers, Sizes and Scales • Sizes: 1m < d < 100m– powders -100m < d , 0.5cm—grains d > 0.5 cm—pebbles, rocks, boulders… • Size range of phenomena—packed powers (pills– mm to mm • A box of cereal—mm to 10 cm • Grains in a silo—mm to 10’s of m • Sahara desert—mm to many km • Rings of Saturn, intergalactic dust clouds—up to 1020m

13. Granular Phases and Statistical Properties • Qualitative similarity of fluid, gas and solid states for granular and molecular systems • Difficult question: how do granular phase changes occur? • Open question: what are the statistical properties of granular systems? • Caveat: No true thermodynamic temperature—far from equilibrium • Various possible granular ‘temperatures’ proposed

14. Distinguishing properties of phases • Solids resist shear • Fluids are viscous, i.e. shear stresses scale with the velocity gradients • Gases are also viscous, have lower densities than fluids, and have Maxwell- Boltzmann-like distributions for velocities

15. Properties of granular gases • Characterized by pair-wise grain collisions • Kinetic theory works reasonably well • Velocity distributions are modified M-B • Gases can only persist with continuous energy input • Subject to clustering instability • Models (may) show granular collapse

16. Granular Clustering –(Luding and Herrmann)

17. Properties of granular solids • Persistent contacts (contrast to collisional picture for gases) • Dense slow flows or static configurations • Force chains carry most of the force • Force chains lead to strong spatio-temporal fluctuations • Interlocking of grains leads to jamming, yield stress, dilation on shearing

18. Example of Force Chains from a Couette Experiment

19. Solids, continued • Dilation under shear (Reynolds) • Grains interact via friction (Coulomb) Note frictional indeterminacy history dependence • Persistent contacts may limit sampling of phase space • Conventionally modeled as continuum • Strong fluctuations raise questions of appropriate continuum limit

20. Granular ‘phase’ transistions • Clustering in gases • Elastic to plastic (semi- ‘fluid’) in dense systems—jamming • Jamming and fragility • Note: gravity typically compacts flows—many states not easily accessible on earth

21. Do granular materials flow like water? • Example: sand flowing from a hopper: • Mass flow, M, independent of fill height • M ~ Da a ~ 2.5 to 3.0 • Why—force chains, jamming…

22. Visualization in 2D by photoelasticity (more later)

23. Note: method of pouring matters for the final heap (History dependence)

24. Mass flow rate vs. hopper opening diameter

25. Simple argument to predict flow rate • M = rV D2 • V ~ (gD)1/2 • M ~ D5/2.

26. Why study granular materials? • Fundamental statistical and dynamical challenges • Related to broader class of systems • e.g. foams, colloids, glasses • Important applications: • Coal and grain handling • Chemical processing • Pharmaceuticals • Xerography • Mixing • Avalanche phenomena • Earthquakes and mudslides

27. Some technical ‘problems’

28. Close to home—about a mile from the Duke University Campus

29. Interesting phenomena • Pattern formation • In shaken systems • Hopper flows • Mixing/segregation • Clustering—granular gases • Avalanches • Rotating flows • Granular convection • Jamming/unjamming

30. Applications • Significant contribution to economy (~1$ trillion per year (?) – in US) • Granular industrial facilities operate below design—large financial losses result • Large losses due to avalanches and mudslides

31. Friction: Granular and otherwise • Two parallel/intertwined concepts: • ‘Ordinary’ friction • Granular friction • Both referenced to Coulomb’s original work • Mohr-Coulomb friction.

32. C. A. Coulomb, Acad. Roy. Sci. Mem. Phys. Divers Savants7, 343 (1773)

33. Ordinary Solid Friction

34. e. g. block on plane

35. Indeterminacy of frictional contacts

36. Hertz-Mindlin contact forces

37. Reynolds Dilatancy

38. Example of Reynolds dilation in before and after images from a shear experiment

39. Microscopic origin of stresses, Fabric, Anisotropy • Fabric tensor • Microscopic origin of stress tensor • Shape effects–

40. Fabric and fragility (e.g. Cates et al. Chaos9, 511 (1999))

41. Other effects leading to anisotropy

42. Aligned force chains/contacts lead to texture and anisotropy

43. Example—simple shear creates texture

44. Force chains, Spatio-temporal fluctuations • What happens when dense materials deform? • Strong spatio-temporal fluctuations • Examples: hopper, 2d shear, sound. • Length scale/correlation questions

45. Fluctuations during hopper flow

46. Spectrum of stress time series

47. Sound measurements (Liu and Nagel, PRL 68, 2301 (1992)

48. 2D Shear Experiment—stress chains break and reform

49. Example of stress chains: Couette shear (Bob Hartley)

50. Closeup of sheared material (Bob Hartley)