Yield stress is one of the most important rheological properties of practical use in the prediction of the behaviour of dispersions. Several different experimental methods are available on modern rheometers such as the NETZSCH Kinexus which will be discussed in this webinar. There will be consideration of the advantages of those more direct methods with controlled stress measurements compared to those where strain is controlled with time.
Methods to be discussed include:
- Flow curve (viscosity vs. shear rate) extrapolation back to the intercept with the y-axis, to determine the stress at which viscosity is infinite.
- Incremental (constant) stress stepping in creep mode determines the yield stress by producing a series of creep curves, which are plots of compliance versus time for a given applied stress.
- Ramping stress with time and measuring the change in strain/shear (strain) rate or “instantaneous” viscosity.
- Ramping strain with time and determining the stress maximum.
- Measuring the Linear Viscoelastic Region (LVER) in standard oscillation (SAOS), and the reasons why the crossover of G’ and G’’ should never be used, but instead measure the peak in elastic stress.
The conclusion will include consideration of the differing yield stresses in a sample and how these can affect the measured result. These being wall yield stress and a bulk (or interior) yield stress, with relatively smooth (measurement) surfaces leading to the wall yield stress being lower than the bulk, resulting in premature slippage.
Presenter: Dr. Adrian Hill, Host: Dr. Shona Marsh
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