Intruder spacing induces particle jamming during intrusion into granular media

Robotic feet often encounter granular substrates that can display complex behavior ranging from solid to fluid in a single step. The effects of foot morphology on the interaction between feet and granular substrates remain poorly understood. Inspired by the presence of toes on animal feet, the goal of this study was to characterize the effect of spacing between two intruders (“toes”) on granular material behavior. To this end, we measured and numerically simulated the force response to intrusion of two parallel square rods into dry poppy seeds while varying the rod gap spacing. Our results show that a peak in total force occurs at a gap of ~3 particle diameters, which was 20 % greater than the force at large separation (>11 particle diameters), beyond which the total produced force plateaued. We propose that this peak in force is the result of particle jamming between the intruders. To quantify the degree of particle jamming, interparticle forces were calculated from simulations. As expected, the total number of strong forces—identified as force chains—indicated greater particle jamming at gap spacings close to those corresponding to the peak force. These findings indicate that intruder separation and particle jamming can significantly affect force response to granular intrusion.