Digging through 2D densely-packed coarse granular media as a critical phenomenon

We create a mechanical digger able to move within 2D densely-packed granular media, in a manner intrinsically different from the existing biomimic diggers. The characteristics of our design include that the average grain size is about one tenth as large as the digger, and the area packing density ϕ is about 90% of the jamming density ϕc. Unlike conventional autonomous diggers that move by fluidizing the surrounding granular media with a much finer grain size, our digger swiftly bypasses this mobility mechanism as coarse grains with interparticle friction are hard to be fluidized. To cope with high interparticle friction, the digger has a circular shape and singly captures granular particles near the front entrance of the recess formed by the center unit running across its body and then ejects the captured particles backwards. We validate this moving strategy in both experiments using a manual digger and the corresponding numerical simulations, and demonstrate that the moving efficiency can be enhanced by the judgement of the human operator but the effectiveness is not significantly influenced by the shape of granular particles. In addition, localized vibration is needed to degrade friction between interlocked irregular-shaped particles. Our numerical results show that the distribution of both moving distances and time intervals between consecutive ejections follow a power-law Further, we experimentally verify this finding by demonstrating that similar power-laws are observed from an automated digger which regularly randomizes its digging direction. Finding these spatio-temporal power laws indicates that digging within a densely-packed granular environment could be a critical phenomenon.