Nothing to get stressed about,
just "go with the flow"
Yield Stress and Yield Viscosity (non-equilibrium "flow curve") quantify the "breaking point" of the sample macrostructure when flow initiates during an increasing shear stress ramp. This assay is used to screen and model numerous properties and processes such as mass transfer through pipes for manufacturing (Poiseuille's Law), pumpability, sedimentation potential (Stokes' Law), ease of product delivery (syringability, injectability, spreadability) and sensory.
Figure 1 shows the processes that occur during a shear stress ramp flow curve. With increasing stress, viscosity often increases due to elastic deformation or "push-back" until the sample's macromolecular structure "breaks" and flows with resultant viscosity decrease. The point of flow is defined by the yield stress and yield viscosity. In addition, the shape of the curve can provide insight into the sample. A sharp viscosity decrease suggests a more brittle structure.
Figure 2 highlights the potential problem of slippage at the plate-sample interface that can be resolved with using a roughened or serrated plate. Slippage is similar to applying force to a deck of cards with only the top cards moving. The applied force is not transferring through the sample. Slippage can also occur during other rheological assays for materials having a yield stress and lead to erroneous results and conclusions. Therefore, identifying slippage is important for method development.
Figure 3 shows an application of the shear stress assay. Ketchup Brand 2 (red curve) having a higher yield stress and yield viscosity should require more energy input (hitting or shaking container) to initiate flow. Refer to the example under the Thixotropy tab to compare post-flow rebuilding.