Investigating viscous to inertial transitions in granular suspensions with internal imaging*

We discuss experiments that probe the rheology of fluid-saturated granular beds coupled with the micromechanics of their flow using refractive index matching (RIM) techniques. These studies are fundamental to problems of erosion, sedimentation, and biolocomotion in such mediums. We will first discuss a sedimented bed which is sheared in a conical rheometer geometry which allows us to examine the system over prolonged periods under steady driving conditions. The shear experienced as a function of prescribed shear rate can be measured in this system along with the motion of the granular phase and the fluid phase. This allows us to determine the granular and fluid component of shear above and below the bed. We demonstrate that the appropriate control variable for the onset of bed motion with increasing shear is in fact the granular packing of the bed rather than the strength or the duration of the applied shear. Then, we show that the velocity profile of the suspended phase which develops between the clear fluid phase on top and a dense creeping granular bed below can be obtained from the applied strain rate and the Krieger-Dougherty model for the effective viscosity. We shall also discuss the drag and the unsteady flow of the medium resulting from the motion of a large solid intruder through the medium. The differences in the flow and drag encountered versus those observed in a viscous fluid and in dry granular media will be discussed going from the frictional to the inertial regime. We will highlight the difficulties encountered in interpreting the encountered rheology simply in terms of inertial and viscous numbers even in cases where the grains are weakly sedimenting.