Drag and relaxation in a bentonite clay suspension

We measure the drag force on a sphere moving slowly through a 6% by weight suspension of bentonite clay particles in water. The steady state drag force depends only weakly of the speed of the sphere, indicating that yield stress effects dominate the drag in the range of pulling speeds studied. The drag force increases as a function of time as the suspension gels. The yield stress of the suspension as a function of suspension age is determined from the zero-velocity intercept of our force-pulling speed data. Both the initial transient build-up of the force and its relaxation when the motion stops are described by two exponential time constants, each of which shows an inverse power law dependence on pulling speed. We also determine the yield stress from the force remaining after the relaxation; it is significantly smaller than that determined from the steady-state force data due to the disruption of the structure of the suspension by the motion of the sphere.