60th Annual Report on Research 2015

We have developed experiments to examine the dynamics of an intruder settling in a visco-plastic material as a first step in understanding the settling dynamics of particles in a clay suspension. Examples of such systems can be found in the processing of mature fine tailings produced in extraction of bitumen from oil sands, and settling of fines through a bentonite clay used in drilling. Few studies have focused on fundamental studies exist of sedimentation in such system, which is further complicated by the fact that the rheology of such materials are not well known or characterized. Our experiments use hydrogels immersed in water as the visco-plastic material. The hydrogels sediment weakly in water and act as the matrix through which the intruder settles. The size of the hydrogels can be varied but in all cases can be considered as athermal. Exploiting the relative transparency of the hydrogel medium, we visualize the motion of the intruder as well as the fluidization of the medium as the intruder passes through the medium to understand the macroscopic properties. The experiments were designed so that the density of the intruder could be changed without changing the intruder size and surface properties.

We find that the intruder transitions from moving continuously to showing stick-slip motion before coming to rest at a depth that depends on the density difference and the overburden pressure. Thus, this material shows a yield stress just as a clay suspension. By extracting the position of the intruder as a function of time using images obtained with a high frame rate camera, we obtain the instantaneous velocity and acceleration of the intruder. Then, by using force balance, we extract the effective resistive force acting on the intruder by the suspension as a function of settling rate. Using this extracted dynamics, we show that the intruder dynamics is consistent with the motion of a particle through a Hershel-Buckley kind of visco-plastic material with a yield stress and effective viscosity which scales as a power-law of the shear rate. However, a significant difference is that the yield stress and the viscosity are found to depend on the overburden pressure.