We investigate the effective friction encountered by an intruder moving through a sedimented medium which consists of transparent granular hydrogels immersed in water, and the resulting motion of the medium. We show that the effective friction µ e on a spherical intruder is captured by the inertial number I given by the ratio of the time scale over which the intruder moves and the inertial time scale of the granular medium set by the overburden pressure. Further, µ e is described by the function µ e (I) = µ s + αI β , where µ s is the static friction, and α and β are material dependent constants which are independent of intruder depth and size. By measuring the mean flow of the granular component around the intruder, we find significant slip between the intruder and the granular medium. The motion of the medium is strongly confined near the intruder compared with a viscous Newtonian fluid and is of the order of the intruder size. The return flow of the medium occurs closer to the intruder as its depth is increased. Further, we study the reversible and irreversible displacement of the medium by not only following the medium as the intruder moves down but also while returning the intruder back up to its original depth. We find that the flow remains largely reversible in the quasi-static regime, as well as when µ e increases rapidly over the range of I probed. * akudrolli@clarku.edu 1 arXiv:1807.07833v1 [physics.flu-dyn] 20 Jul 2018Recently, it was demonstrated [21] that a sphere dragged through granular hydrogels immersed in water can be described by an effective friction which scales with inertial number I [22], and increases non-linearly from a non-zero static value. The form was found to be similar to that derived from the Herschel and Bulkley model [23], which is used to describe non-Newtonian fluids and muds [24]. Building on that study, we probe the dynamics of an intruder settling through granular hydrogels immersed in water as a model of wet granular medium or mud consisting of soft granular medium immersed in water. This is a much

We investigate the development of mobility inversion and fingering when a granular suspension is injected radially between horizontal parallel plates of a cell filled with a miscible fluid. While the suspension spreads uniformly when the suspension and the displaced fluid densities are exactly matched, even a small density difference is found to result in a dense granular front which develops fingers with angular spacing that increase with granular volume fraction and decrease with injection rate. We show that the time scale over which the instability develops is given by the volume fraction dependent settling time scale of the grains in the cell. We then show that the mobility inversion and the non-equilibrium Korteweg surface tension due to granular volume fraction gradients determine the number of fingers at the onset of the instability in these miscible suspensions.