Yield stresses of concentrated suspensions of rigid particles in the volume fraction range of 0.62 to 0.78 via steady torsional flow

The ubiquitous wall slip behavior of viscoplastic fluids renders the characterization of their yield stress values a challenge but also presents an opportunity. Here, a new process for the determination of the yield stresses of viscoplastic fluids is introduced and demonstrated on concentrated suspensions subjected to steady torsional flow, i.e., parallel-disk viscometry based on the understanding of apparent wall slip. Four viscoplastic suspensions (particles with a maximum packing fraction, ϕ_m, of 0.86 mixed with a Newtonian binder at the volume fraction, ϕ, range of 0.62 to 0.78) were used. It is demonstrated that a step change in the slope of the torque versus apparent shear rate (or the rotational speed) occurs at a critical torque that corresponds to the yield stress of the suspension. Below the critical torque the behavior is governed by apparent slip and plug flow while above the critical torque the behavior is governed by continuous deformation and apparent slip. The yield stresses of the four concentrated suspensions were verified by comparisons with those obtained from other methods including from wall slip velocities at various shear stresses.