Experimental study on the reduction of penetration resistance by rotation

Numerous organisms live with or within the soil, and some have evolved efficient strategies to move within soil. Seeds of the Erodium cicutarium and Pelargonium can bury themselves into the ground for future germination with a continuous rotational motion. Worm lizards tunnels by oscillating their heads along the long axis of the trunk. It is believed that the penetration resistance of a penetrator could be reduced by incorporating the rotational movement during the penetration process. A holistic experimental campaign was conducted to reveal the correlations between rotational penetration resistance and its influence factors. The results indicate that the reduction of the penetration resistance increases significantly at first and then gradually with the increase of the relative slip velocity (the ratio between the rotational velocity and the vertical velocity), the roundness of the penetrator, and the interfacial friction angle between the penetrator and the granular media. However, both the relative density and particle shape of the granular media do not exhibit significant effects on the reduction of the penetration resistance. The robotic application of the findings was demonstrated by a soft burrowing robot with a rotating cone.