Abstract

Granular friction μ is a sensitive and poorly understood function of packing fraction ϕ. Every granular material has a distinct critical volume fraction ϕc that delineates two mechanical deformation modes – compaction for ϕ < ϕc, and dilation for ϕ > ϕc. Here we examine the relation(s) between friction and packing fraction, using quasi-static penetration tests in materials ranging from glass beads to highly heterogeneous lunar regolith simulant. Noncohesive materials collapse onto a master curve that relates changes in friction to the distance from ϕc, confirming that the handoff from compaction to dilation is a phase transition. The mechanical distinction between compaction and dilation regimes is highlighted with the addition of cohesive dust – relevant for lunar regolith – which has little effect for ϕ < ϕc, but drastically enhances strength for ϕ > ϕc. Our simple model improves predictions of granular resistance to intrusion, which can help to explore and manipulate soils on Earth and regolith on other planets. A reanalysis of Apollo era lunar penetration tests demonstrates the promise of our approach.