Rotational Penetration in Sand: Experiments, Modeling, and Implications

Soil-dwelling organisms have evolved diverse strategies for efficient subterranean movement. For example, seeds of Erodium cicutarium and Pelargonium species use continuous rotational motion for self-burial, while the angled worm lizard ( Agamodon anguliceps ) burrows by oscillating its head around the trunk axis. These rotational movements can reduce penetration resistance and provide inspiration for self-burrowing robotic systems. This study presents systematic experiments and a theory-informed model to investigate how rotational penetration force and energy consumption are affected by velocity ratio, penetrator geometry, surface condition, material type, density, penetration depth, and top surcharge. Results show that the force ratio decreases rapidly at low tangential-to-axial velocity ratio, u , and then approaches a limiting value, consistent with a hyperbolic-like reduction trend. Penetrator geometry strongly affects both the magnitude and rate of reduction; tapered and less-rounded geometries generally mobilize force reduction more effectively than flat-ended or smoother geometries. Relative density and material type have secondary but coupled effects, especially for tapered penetrators. Depthwise force ratios are nonuniform and nonmonotonic, while top surcharge reduces the normalized effectiveness of rotation, particularly in the deeper penetration phase. Energy analysis shows that although rotation reduces force-related penetration work, torque work increases with u . The modified cost–benefit ratio therefore peaks at low velocity ratios and decreases at larger u . For self-burrowing robot design, low-to-moderate velocity ratios are recommended to balance axial force reduction and energy efficiency.