Dynamic and Static Yield Stress
Static and Dynamic Yield Stress
What’s the difference and which should I use?
The most commonly used method for obtaining a yield stress value is to shear the sample over a range of shear rates, plot the shear stress as a function of shear rate and fit a curve (various models are available) through the data points (see fig 1).
The intersection on the stress axis is then taken as the yield stress, the assumption being that any stress below this is insufficient to cause the sample to flow. Rheologists call this a dynamic yield stress; we are looking at the sample in motion (i.e. under shear) and extrapolating from this how it behaves when not in motion.
However, there’s more than one way to skin a cat! Another approach is to start with the sample in its at-rest state (zero shear) and incrementally increase the stress until we identify at value at which it starts to flow i.e. we record non-zero shear rate (see fig 2)
We call this value a static yield stress - the stress at which we initiate flow - and it is usually considerably higher than its dynamic counterpart for any given product. In reality the sample is undergoing creep flow below this stress but we can assume for many practical purposes that it is static. This test can be performed with a quick (non-equilibrium) stress ramp on a controlled stress rheometer or a constant rate test on a vane-based tester.
So which yield stress should you use?
Well it depends on what you need to know. A good starting point is to match the test type to the flow process of interest: If you are interested in how a fluid stops flowing after shear (such as screen printing, dip coating, enrobing or slumping) then the dynamic yield stress is a key determinant. On the other hand if you are interested in how hard you need to push to get the fluid moving in the first place (spreadability of butters, texture of tubs of cream, mixer and pump start-up etc) then the product’s static yield stress will prove a major factor.
If you have any questions about using static and dynamic yield stress to characterise your products, as always, feel free to drop me a line or give me a call.