Growing and anchoring of plant root-inspired soft structures in granular media

Plant roots anchor by growing structures into soft substrates via tip extension. The forces exerted by a plant root during growth and pullout differ from those of a rigid body due to the difference in the amount of displaced material. Investigating growing structures remains underexplored due to the challenge of creating a tip-extending device like a plant root. To discover principles of interaction of elongating bodies within granular media, we use a soft robot 12 mm in diameter that elongates from its tip through eversion of a pressurized tube. We systematically studied how substrate properties affect intrusion and anchoring forces, using low-friction media like glass beads, more frictional materials like sand, and fine particles like powder. We measured and compared peak intrusion and extrusion forces of intruded vs elongated objects of comparable dimensions. The difference in insertion and pullout forces between intruders and tip-extenders was minimal in low-friction media. In fine sand and powder, the tip-extender's insertion force was 5 times less than the intruder's, while its pullout force was about 5 times greater. We hypothesize that the force differences of intrusion vs growth arise from the smaller material reorganization during elongation compared to intrusion, while higher inter-particle friction plays a key factor. Understanding the mechanics of plant-inspired anchor and granular matter interactions is crucial for agricultural applications and the design of burrowing devices.