Rigid spheres moving through soft solids

We present the results of an experimental investigation into buoyant rigid spheres rising through highly concentrated collections of hydrated hydrogel particles. The volume fraction of particles is such that the mechanical properties of the material are intermediate between a very viscous fluid and a soft solid. Despite the established time dependent, non-Newtonian character of hydrogels, we find that when the surface of the material is free, an immersed buoyant sphere rises with a constant speed. The effects of the motion are observed to be highly localized around the sphere. When the stress exerted on the material is changed by varying the mass of the sphere, its terminal velocity is found to depend exponentially on its buoyancy. Qualitatively distinct behavior is found when a solid lid is placed on the surface of the material. In this case, a seemingly thixotropic, sublinear time-dependent motion is found. It is observed that linear motion of the sphere is accompanied by flow at the surface of the material whereas fluid movement is suppressed when a lid is present. We use these observations to provide a hypothesis which links the exponential stress dependence of the rheology of the material to the effects of the boundary conditions on the kinematics of the intruder.