Drag on a sphere while being extruded from a cohesive granular medium

Predicting the drag experienced by intruders in granular materials is important for a variety of systems, such as modeling the anchoring of objects in soils or understanding the locomotion of an earthworm in wet mud. In this study, we experimentally investigate the drag force experienced by a sphere when it is pulled out vertically through a model cohesive granular material. A few sizes of intruders are considered, and are reported for extrusions from a range of initial buried depths. The magnitude of the inter-particle cohesion is varied. For the results observed, a framework based on the relevant stresses is found to be useful, where the confinement stresses on the intruder are compared to the stress applied by the extrusion. When deep enough, cohesion is interpreted through the Mohr-Coulomb yield criterion as a yield stress; however, near the surface the presence of cohesion results in large fluctuations in the measured drag.