Yield stress is a commercially useful rheological phenomenon evident in many every day products. Toothpaste and tomato ketchup are excellent examples of materials that exhibit a yield stress and good demonstrators of what it means in practical terms. Both these substances are stationary and solid-like at rest. However, apply a stress that exceeds a critical value – typically by giving the packaging a squeeze or the bottle a sound tap – and out they flow. There’s a lovely rhyme from the American humourist Ogden Nash (1902-1971) that alludes to the yield stress and shear thinning properties of ketchup…..or catsup…….or tomato sauce (depending on where you come from!), and that is:
The Catsup Bottle
First a little
Then a lottle
Many complex fluids exhibit this kind of behaviour, and, because it can help in developing products with certain preferential performance attributes, it’s a topic of interest to a number of industries, in particular for consumer products such as Foods, Personal Care and Cosmetics.
My colleague at Malvern, Dr John Duffy, is currently working on a white paper on the topic of yield stress but in this blog I simply wanted to take an introductory look at why it occurs.
Why do fluids yield?
The term yield stress is highly descriptive of what essentially happens at the microstructural scale in fluids that exhibit this type of behaviour. Such fluids can be thought of as having a ‘structural skeleton’ that extends throughout the entire volume of the system. This structure has some elasticity so when low levels of stress are applied it simply stretches or tensions as a result. Release the stress and the structure returns to its original state.
At some point though, if applied stress is increased, the elasticity in the system is stretched to breaking point and the structure starts to yield or fail. The material can then move more easily and starts to flow. If the material is then shear thinning……that flow can turn into the ketchup avalanche before you know it!
The importance of timescale
Despite material yielding clearly being apparent in a range of daily activities, the concept of a true yield stress is the topic of much debate among rheological experts. Many materials that show solid-like behaviour at short timescale deformations, will in fact show liquid-like behaviour if left for long enough time, and creep or flow. This is why measured values are often referred to as an ‘apparent yield stress’.
What’s important practically is whether the material exhibits a yield stress under the conditions that will be applied during use. This is why it’s important to carefully consider test conditions if you want to make meaningful measurements.
That’s a topic for next time, but until then if you have any questions about yield stress just ask.