Effect of bi-modality on sphere penetration into granular media

We investigate the penetration of spheres impacting onto granular media, which are compositions of two discrete size ranges, thus creating a bi-modal material. We systematically vary the volume fraction of the two materials and measured the penetration depth over a wide range of impact speeds (0 to 5 m/s), as well as different sphere densities. We see maximum lubricating effects when we add 55{\%} of the small 31-micrometer glass beads to 178-micrometer glass beads. Here the expected penetration depth increases by 60{\%} when compared to a simple model, which is based on interpolating between the values obtained with the two pure monodisperse grains. Less effect is observed when 178-micrometer beads are mixed with a large 425-micrometer beads. We also study the lubrication effect of fine beads on larger rough Ottawa sand, seeing significant effects.