How do force chains form in granular materials?

Force networks are ubiquitous in dense static and slowly deforming granular materials. A particle is in a strong force chain if it experiences forces at or above the mean. Relatively few particles in a system close to jamming are typically in force chains. This raises the questions, what determines whether a particle becomes part of a force chain, and what controls the evolution of force chains once they have formed? We investigate these questions in the context of shear jamming, where force chains emerge from an initially zero-stress state in response to shear. The shear jamming process causes the mean contact number, Z, to rise above its isostatic value. We identify key structures, groups of three contacting particles (trimers) and intersection points of chains (branches) that respond to shear, so as to create a jammed state. We use experimental data to track all trimers and branches to answer the two questions above.