APPLICATION: YIELD STRESS
Go with the flow
Yield Stress and Yield Viscosity (non-equilibrium "flow curve") quantifies the breaking point of the macrostructure with an increasing stress ramp to model properties such as spreadability, pumpability and syringability. It can be a useful tool to screen numerous formulation properties ranging from mass transfer for manufacturing and product delivery to feel on skin (soft, smooth, thick, thin).
Figure 1 shows typical shear rates for common processes. Yield stress measurements are often used to compare the ease (or lack) of dispensing, spreading, and feel during application.
Figure 2 shows the process to produce a non-equilibrium ramp flow curve. With increasing stress, viscosity often increases due to elastic "push-back" until the macromolecular structure "breaks" and flows with resultant viscosity decrease.
Figure 3 shows ketchup Brand 2 having a higher yield stress and yield viscosity that should require more energy input (hitting or shaking container) to flow. It is important; however, to also consider the zero-shear (approximate at rest) and terminal viscosities (highest achievable shear) depending on product requirements.
Figure 4 highlights the potential impact of manufacturing process changes on rheological properties. One product may have better customer appeal, efficacy or performance than the other.