Plant root inspired protrusions enhance intruder anchoring force in dry granular media

Diverse systems that interact with soft substrates can take advantage of complex structures to modify local stresses and flows to improve intrusion resistance [Agarwal et al., Soft Matter 2021]. Less is known about how extrusion resistance is modified by complex structures. This is particularly relevant in plants as studies have shown that networks of hairs lining roots can increase anchoring force and enhance soil stability [Bengough et al.,J Exp Bot 2016]. To discover the principles for biological and engineering anchoring resistance, we study forces experienced by rigid cylinders (8.3mm diameter) incorporating rigid protrusions (3mm by 5mm) as it is pulled from a model noncohesive soft substrate, dry granular media (1mm diameter particles). In laboratory experiments and validated Discrete Element Method simulations we measured peak pullout force as a function of distance between hairs. We observed that roots exhibited limited anchoring force for both too sparse and too dense hairs and peaked by approximately 3x in force at intermediate spacing of 1-2cm between hairs. Our results are in accord with tests of a novel burrowing soft robot, generating hair-like structures during intrusion. Investigating the plant-inspired complex interactions with GM could inform future robot designs capable of effective robotic burrowing, and anchoring.