Transients and Shear Thinning due to Material Acceleration during Rapid Forced Intrusions into Granular Materials

The dynamics of forced intrusions are central to biological or robotic agents that excavate or locomote atop granular materials. We examine velocity-dependent effects of rapid constant-speed intrusions (up to 75 cm/s) into granular media. Using a force-instrument robotic arm, we vertically plunged rigid intruders into a bed of loosely packed poppy seeds at a constant speed, v, from 1 to 75 cm/s. For slow intrusions, force increased monotonically with intrusion depth. As v increased, a transient force developed over a characteristic time; excess work scaled with v². Particle imaging (with the intruder against a clear side-wall) indicates that the transient forces correlate with the formation and rapid acceleration of a jammed structure beneath the intruder [Aguilar & Goldman. 2015. Nat Phys]. Once the transient decayed, force increased with depth across tested speeds. Surprisingly, in the steady state regime, force per unit depth decreased as v increased (18% from 1-75 cm/s) analogous to shear thinning in particulate suspensions. Imaging during the steady state shows grains above the intruder in a state of partial free-fall. A model incorporating free-fall suggests shear thinning results from reduced lithostatic pressure for rapid constant-speed intrusions.