YIELD STRESS
Determine rheological break point (yield stress) and flow curve.
Helpful to model processes and applications.
A shear stress ramp measures the yield stress and yield viscosity (non-equilibrium "flow curve") to quantify a sample's rheological "breaking point" when flow is initiated. This assay is used to screen and model numerous properties and processes such as transfer through pipes and tubes, pumpability, sedimentation potential (Stokes' Law), ease of product delivery (syringability, injectability, spreadability) and sensory.
It is important to note that the results for this assay are very dependent on the rate of stress increase. An analogy is stretching a rubber band that will break if very quickly pulled and will tend to stretch more if slowly pulled.
Yield stress can also be determined with a stepwise series of creep tests and also in the oscillatory mode with an Amplitude Sweep assay; however, values may not necessarily match those obtained with rotational assays.
Checkout the 20 second video showing plate and sample movement during a shear stress ramp assay.
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 rapid viscosity decrease. The point of flow initiation 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 roughened or serrated plates. 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 that may generate erroneous results and conclusions. Therefore, identifying the potential for slippage to occur is important for method development.
Figure 3 shows an application of the shear stress ramp 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 compared to Brand 2 (blue curve). Refer to the example under the Thixotropy tab to compare post shear-thinning rebuilding curves for these 2 ketchup samples.
