Drag on a sphere moving through an aging system

We have investigated the drag on a sphere falling through a clay suspension that has a yield stress and exhibits rheological aging. The drag force increases with both speed and the rest time between the preparation of the system and the start of the experiment, but there exists a non-zero minimum speed below which steady motion is not possible. We find that only a very thin layer of material around the sphere is fluidized when it moves, while the rest of the suspension is deformed elastically. This is in marked contrast to what is found for yield-stress fluids that do not age.