Modeling growth and anchoring of plant root-inspired structures in dry granular media

Plants initially anchor within media like soil and sand by elongation of primary roots and subsequent growth of lateral protrusions called root hairs. To discover principles of anchoring, we developed a soft robot which achieves tip growth via a pressurized thin-walled tube, initially inverted back inside itself; the model also includes hair-like protrusions which emanate sequentially down the robot during elongation. The robophysical model reveals that incorporation of hairs increases pull-out forces more than twice. To better understand robot dynamics and generate hypotheses for root hair function, we developed an experimentally validated Discrete Element Method simulation of an elongating root-like structure with growing hair protrusions. The simulated root grows into a granular medium of ~1mm spheres. We simulate the root growth by continually adding coupled (via linear bonds) stacks of “rings” constructed from a circular ring of rigidly coupled spheres We observed that depth at which the grower’s anchoring force exceeds the resistive forces at its tip increased with increasing ring diameter Simulations with root hairs revealed that such structures increased the root anchoring forces: growth of a 5mm diameter primary root with hairs having 1 cm vertical spacing covering 13% of the circumferential area achieved 5 times the anchoring force over a hairless root. To understand such dynamics, we monitored particle recruitment by hairs and discovered that hair density played a key role in self-anchoring .