Effect of rotation on penetration: Toward a seed awn-inspired self-burrowing probe

In nature, seeds of some flowering plants such as Erodium and Pelargonium can bury themselves in the ground effectively and efficiently. This self-burial strategy has several key features: it is achieved by the hygroscopic coiling and uncoiling movement of the awn; the helical structure and movement induce net vertical penetration force; and the awns leverage anisotropic bristles to enhance the anchorage. In this study, another feature, namely the effect of rotation on penetration resistance, is studied using the discrete element method (DEM). A sudo-cylindrical (with a solid polygon cross section rather than a circular or helical one) was used to isolate the rotational effect. A series of rotational penetration tests were conducted with a set of calibrated microscale parameters from a previous study. It was observed that the rotational movement could reduce the penetration resistance. The reduction becomes significant at higher rotational speeds, but the corresponding power first decreases and then increases with the increase of rotational velocity, indicating the existence of an optimal condition. Further analysis of particle-probe contact forces unveils the underlying mechanism of the rotational effect on penetration. Specifically, rotation of the cone and shaft causes decrease in contact number and the vertical components of the particle-penetrator contact forces; in addition, the reduction of the contact normal force is more significant than that of the contact shear force.