Extending Granular Resistive Force Theory to Cohesive Powder-scale Media

TL;DR

This paper extends resistive force theory to cohesive powders, enabling accurate prediction of intrusion forces on complex shapes, which is useful for designing robots and landers in powdery environments.

Contribution

The authors adapt resistive force theory to cohesive dry powders through experiments and modeling, expanding its applicability beyond non-cohesive granular media.

Findings

Cohesive powders exhibit higher resistance to intrusion than non-cohesive media.

Flat surfaces generate maximum stress across various angles.

Curved surfaces provide maximum resistance during vertical intrusion.

Abstract

Intrusions into granular media are common in natural and engineered settings (e.g. during animal locomotion and planetary landings). While intrusion of complex shapes in dry non-cohesive granular materials is well studied, less is known about intrusion in cohesive powders. Granular resistive force theory (RFT) -- a reduced-order frictional fluid model -- quantitatively predicts intrusion forces in dry, non-cohesive granular media by assuming a linear superposition of angularly dependent elemental stresses acting on arbitrarily shaped intruders. Here we extend RFT's applicability to cohesive dry powders, enabling quantitative modeling of forces on complex shapes during intrusion. To do so, we first conduct intrusion experiments into dry cornstarch powder to create stress functions. These stresses are similar to non-cohesive media; however, we observe relatively higher resistance to horizontal intrusions in cohesive powder compared to non-cohesive media. We use the model to identify geometries that enhance resistance to intrusion in such materials, aiming to minimize sinkage. Our calculations, supported by experimental verification, suggest that a flat surface generates the largest stress across various intrusion angles while a curved surface exhibits the largest resistance for vertical intrusion. Our model can thus facilitate optimizing design and movement strategies for robotic platforms (e.g. extraterrestrial landers) operating in such environments.