Transients and Shear Thinning due to Material Acceleration during Rapid Forced Intrusions into Granular Materials
Publication Type:
Conference
Authors:
Co-Authors:
Jennifer Rieser, Jeffrey Aguilar, Andras Karsai, and Daniel Goldman
Year Published:
2018
Abstract:
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.
Conference Name:
APS March Meeting 2018
Conference Location:
Los Angeles, CA
Other Numbers:
Refereed Designation:
Date Published:
3/5/2018