Force propagation in isostatic granular packs.

Area of Science:

• Physics
• Materials Science
• Complex Systems

Background:

• Granular materials near the jamming transition are often isostatic, implying a null stress law.
• Previous studies suggested stress rays propagate obliquely from point forces in such systems.
• Anisotropic packing histories in prior experiments may have influenced these observations.

Purpose of the Study:

• To investigate stress propagation in frictionless granular packs at the jamming transition.
• To test the null stress law in isotropic granular packs.
• To understand the influence of boundary conditions and packing history on stress distribution.

Main Methods:

• Creation of initially isotropic frictionless granular packs.
• Introduction of controlled boundary conditions to break symmetry.
• Analysis of pack response to point forces using continuum stress models and geometric considerations.

Main Results:

• Isotropic packs exhibited highly sensitive responses to point forces, diverging from continuum models.
• Stabilizing packs with an additional boundary regularized their response.
• The stabilized packs' behavior resembled hyperstatic systems, lacking the expected stress rays.
• Isostaticity does not necessitate adherence to the null stress condition.

Conclusions:

• The null stress law may not be a universal requirement for isostatic granular packs.
• Stress propagation patterns might be governed by geometric factors like contact angles.
• Boundary conditions significantly influence stress distribution in granular systems near jamming.