A slotted plate device to measure yield stress
Description
A slotted-plate device was constructed with a balance and a linear-motion platform to directly measure static yield stresses of suspensions by moving the plate in the suspension in a similar mode as is done in the well-known Wilhelmy-plate technique for measuring surface tension. The yield stress is obtained by determining the point deviating from linearity on the force versus time curve. Wall effects associated with the original plate yield-stress instrument were minimized by opening a series of slots on the plates. The suspension filling the slots is assumed to move with the plate. Shearing with the bulk suspension occurs at the slot edges Yield-stress experiments were conducted on both high-concentration and low-concentration aqueous suspensions. The new setup avoids the disadvantages of the vane instrument, possible secondary flow between the blades as well as a non-uniform stress distribution along a virtual cylindrical surface. Yield stress values of TiO2 suspensions were compared with the values obtained via a variety of other methods, including indirect extrapolation from steady-shear data, vane creep testing, and vane stress-ramp measurements using an SR-5000 rheometer. Very small yield stress (up to ∼0.0001 Pa) measurements of low-concentration bentonite suspensions could be determined only with the slotted-plate device. Relaxation tests on high-concentration suspensions indicated that these suspensions may not be purely elastic below yield stress The pre-yielding stress evolution was investigated using a finite element method. A stress-strain instead of stress-shear rate material model was used. Both solid as well as slotted plates were simulated. At the yielding point, the stress was uniformly distributed along the solid plate surface or along the slot edge, supporting the assumptions put forward towards the yield stress determinations Finally, yield stress values of silicon nitride suspensions were measured at a variety of concentrations, temperatures and pH values using the slotted plate device. Mixtures of silicon nitride, alumina and yttria were also investigated to determine the relationship between the yield stress and the component fractions