  Experimental measurements of contact forces are limited to spheres and disks
in three and two dimensions, making the evaluation of the shape effect and
universality of force distributions and the comparison between experiments and
theories extremely difficult. Here we present precise measurements of vector
contact forces in photoelastic ellipses and disks subject to isotropic
compression and pure shear. We find the local, instead of the global, stress
ratio, control the width of the force distributions for forces larger than the
mean, regardless of the particle shape and preparation protocols. By taking
advantage of the anisotropic particle shape, we can determine the anisotropic
growth of contacts in ellipses subject to isotropic compression, revealing the
role of non-affine particle motions in homogenizing force distributions. Our
results uncover the role of local anisotropy in the statistical framework of
granular materials and open a new regime of exploring the role of particle
shape on the mechanical and dynamical properties of granular materials in
depth.
