Locomotion in Granular Media: A Geomechanical Perspective on Symmetry Breaking

Biological organisms exploit cyclic body shape changes to locomote underground. These body shape changes are either symmetrical or asymmetrical in time. Examples include the cyclic extension/contraction adopted by the Atlantic razor clam to burrow upward from sands, and the helical drilling used by scarab beetle larva and some seed awns for submerging in soil. Overall, locomotion requires organisms to overcome resistances to advance forward and form anchorage to prevent slip backward. This study investigates the correlation between underground locomotion and the symmetricity of cyclic body shape changes from a geomechanics perspective. We design a modular robot that features an extensible cylindrical body and a rotatable helictical tip. The robot can move in granular media horizontally and automatically through cyclic extension and contraction, facilitated by tip rotation. Nonetheless, negligible movement occurs as the robot extends and contracts without the tip. We then evaluate the robot's burrowing performance, and monitor the particle displacement fields around the robot to facilitate understanding of the underlying burrowing mechanism. Insights into the general principle of locomotion in granular media are discussed by comparing to swimming in low Reynolds number fluids.