Yield Stress is a rheological term that defines how much force is required to cause a liquid or semi-solid material to begin moving. Thick cough syrups, for example, pour slowly out of the bottle; gravity alone appears to provide sufficient force to discharge the fluid. The customer patiently waits until a sufficient quantity comes out and fills the spoon or cup with the recommended dose. Consumers evaluate creams and ointments in part by the ease or difficulty of squeezing a tube to get the product out. Marketing gives input to R&D for the yield stress performance parameter by subjectively describing how the product should flow when used by the customer.
Creep is the steady deformation of a material that is placed under a constant load. When gel caps crack and start to ooze the liquid filler, the force that pushes the material out the crack must exceed the yield stress of the liquid. Gravity alone may be sufficient to cause the seepage. For thicker formulations, there may be other forces in play that assist gravity. Packages of gel caps stacked on top of each other can sometimes experience this problem. The overall weight of the stacked packages can cause those on the bottom to slowly lose their contents over time as product oozes out. This is an example of creep behavior; the consequence can result in partial or total loss of the shipment, depending on the damage caused by the escaping liquid.
Creep is related to yield stress in the sense that the material starts to flow because the yield stress has been exceeded. The onset of creep behavior occurs only when the yield stress threshold has been crossed. So in this sense, the two parameters are clearly related to one another.
How do pharmaceutical manufacturers anticipate the flow problems described above? The science of rheology clearly defines these two parameters and provides R&D with simple tests that can predict flow behaviors related to the above issues. Controlled stress rheometers with cone/plate geometry have the flexibility to measure a broad range of flow parameters, ranging from yield stress to viscosity to creep. They can be used on both easy flowing liquids as well as sticky paste-like ointments, so the advantage of this type instrument is the variety of product types that can be measured and characterized.