Extending Granular Resistive Force Theory to Cohesive Powder-scale Media

Intrusions into granular media display complex behaviors due to friction-dominated interactions among particles. Despite such complexities, granular resistive force theory (RFT)—a reduced-order frictional fluid model—quantitatively predicts intrusion forces in dry noncohesive granular media. RFT operates by linear superposition of the elemental forces on intruders. A frictional plasticity continuum PDE model [Askari & Kamrin, Nat. Matt 2016] explains RFT’s success and corresponding force trends obtained from experiments. Based on these insights we hypothesized that linear superposition could be extended to cohesive powders. We used cornstarch powder (20–50 µm diameter) and developed a method to control initial conditions for systematic RFT experiments to measure elemental forces. We observed linear stress-depth relation, which we used to create stress per unit depth heatmaps. These stresses were notably similar to noncohesive media; however, important differences in powder - higher resistance to horizontal intrusions compared to noncohesive media - were observed. We attribute these to the powder’s interparticle cohesive forces. We used our model to assist NASA JPL in identifying geometries that enhance intrusion resistance, focusing on extraterrestrial lander footpads aimed at minimizing sinkage on weak surfaces, posing landing stability challenges. Our results suggest that flat geometries generate larger resistive forces than curved ones across a wide range of landing scenarios.