Creeping through a rigidity transition in packings of soft spheres

The ability of soft granular packings to have both liquid and solid-like behavior has widespread fundamental and applied significance. We probe this transition in packings of soft spheres with spherical and cylindrical intruder tests. We find power law creep sinking behavior, with diffusive dynamics. The creep amplitude is exponentially dependent on both applied stress and the packing density of spheres. At the highest densities, a transition to constant sinking speeds is observed. Results are independent of intruder shape. Our experiments provide benchmark results for existing frameworks of particle packing mechanics and point towards relevance of particle contact level physics in understanding the role of fluctuations in soft structures.