How do fixed pins affect elasticity and rheology of jammed, sheared particles?

It has recently been shown that fixing degrees of freedom by adding pinned particles tunes static and dynamic properties of a two-dimensional, soft granular system near jamming. These properties include the jamming threshold, linear elasticity, presence of bond order, and force network topology [1].



We now investigate elasticity and plasticity via molecular dynamics simulations, with rough walls to establish constant pressure and shear rate. Macroscopically: The presence of pins decreases the linear elasticity and increases the plasticity. Mesoscopically: Compressive oscillations are damped and transverse velocity is attenuated as per a simple model with Hookian springs [2]. Long-lasting fluctuations from affinity are suppressed by pins, and new circulation patterns are established around them. A vertical pressure gradient is established for the highest pin densities studied. Microscopically: Non-affine deformations of clusters of particles, quantified using D²_min, are more pronounced around pins. The total number of locations of high D²_min increase with pin density, and the behavior of D²_min is correlated with the local shear.

[1] P. Wentworth-Nice et al., Soft Matter 16, 5305 (2020); A.L. Zhang et al, Phys. Rev. E 106, 034902 (2022).

[2] B. Tighe, personal communication (2022).