Jammed solids held together with pins: The effect of pin geometry on structure and mechanical response

Currently, much is known about the theory and broad applicability of the jamming transition. Here, we address unanswered questions on the geometrical role that a scaffolding of fixed particles, or "pins", plays in structure and dynamical response of jammed, soft bi- or polydisperse particles. Our 2d system consists of particles and tiny pins which harmonically repel overlaps, fixed in various geometrical arrangements: square, triangular, or honeycomb lattices, or distributed randomly. While at low pin densities the jamming threshold, φ_j, decreases linearly with pin density, independently of pin geometry; at higher densities it reflects lattice-specific constraints on particle packing, and φ_j may even increase with pin density. The distribution of bond angles may be anisotropic, and contact force distribution reflect the presence of pins. Changes in the linear elastic response can be seen in bulk and shear moduli, their scaling with pressure near jamming, and a Zener ratio indicating that pin geometry might break the mechanical isotropy of the jammed state.