We present quantitative measurements and mat hematical analysis of the granular drag reduction by rotation, as motivated by the digging of Erodium and Pelargonium seeds. The seeds create a motion to dig into soil before germination using their moisture-responsive awns, which are originally helical shaped but reversibly deform to a linear configuration in a humid environment. We show that the rotation greatly lowers the resistance of soil against penetration because grain rearrangements near the intruder change the force chain network. We find a general correlation for the drag reduction by relative slip, leading to a mathematical model for the granular drag of a rotating intruder. In addition to shedding light on the mechanics of a rotating body in granular media, this work can guide us to design robots working in granular media with enhanced maneuverability.
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Letter| April 17 2017
Reduction of granular drag inspired by self-burrowing rotary seeds
Sung Mok Choi;
Wonjong Jung, Sung Mok Choi, Wonjung Kim, Ho-Young Kim; Reduction of granular drag inspired by self-burrowing rotary seeds. Physics of Fluids 1 April 2017; 29 (4): 041702. https://doi.org/10.1063/1.4979998
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