Common yield stress and rheological measurement systems are discussed in this article.
Cone and Plate Measuring Systems
While working with a plate set-up on a rheometer, it is often preferable to use a cone measuring system. This is mainly because the shear stress is normally the same over the complete cone surface and the material should yield homogenously across the sample radius.
The measured stress is a function of the applied shear rate in the case of a parallel plate that varies with radius. Hence the sample exposed to the outer radius will yield before that in the central zone and consequently in some tests a plate may give slightly different results.
Roughened cones are available for minimizing the impact of slip, however if the sample contains large particles and/or requires a serrated system due to extensive slippage then a parallel plate may be the only means of proper measurement using a plate set-up. The gap in these tests may also be significant as slip effects are usually more prevalent at small gaps. This is because slip velocity at the plate surface along with the geometry velocity that gets smaller relative to the constant slip velocity as the gap gets smaller.
While working with certain materials, especially paste-like materials, there may be restrictions on the working gap that can be used to obtain accurate yield stress measurements. This is because such materials can show inhomogeneous flow behavior when larger gaps are employed and can cause partial yielding or fracture across the gap.
This is usually visually evident, with close examination revealing two distinct layers moving at different speeds and a characteristic inflection on the shear stress-shear rate curve.
Cylinder and Vane Measuring Systems
During working, a cylinder set on a rheometer, vanes and splined geometries can be used for minimizing slip and to work similarly in the serrated plate systems. The former is often suggested for concentrated dispersions and emulsions, which are prone to slippage, since this maximizes sample-sample contact.
Another advantage of the vane tool is that it can be inserted in the sample with little disturbance to the structure. This can be significant as a number of complex fluids are thixotropic and may take a finite time to recover their structure after loading or in some cases not at all.
It is also possible to use the vane with the product in its original container, dimensions permitting implying there is no need to transfer the sample to a measurement cup, which again prevents structural damage prior to measurement.
Figure 1. Illustration of vane tool in a smooth cup and associated stress equation.
It is important while using the vane tool or a cup and bob system for measuring yield stress that the measurement is made at the bob wall or vane edge as opposed to the midpoint position, which is the standard (ISO3219; DIN53019) for viscosity measurement. This is because stress decreases with radial distance from the bob surface and hence yielding will initially occur at the bob surface.
The rotating vane will circumscribe a path in the sample while using the vane tool and thus can be considered to behave like a cylindrical bob that is made out of sample, as shown in Figure 1. As the sample is in contact with the sample, the minimum slip is encountered at the vane periphery, however, if the cup surface is not profiled then there is the possibility that the sample may slip or yield at the outer wall surface before the sample yields. As there is a decrease in stress with inverse radius from the bob/vane surface, then using a larger measuring gap can minimize such effects. A splined or serrated cup or basket may also be employed.
Yield stress is crucial for characterizing a wide range of complex fluids, and is an important factor for many real-life processes and applications involving such materials.
To obtain robust, relevant and reproducible yield stress data for a specific material, it is required to assess both the test type, and the measurement protocol used to perform the test. It is this background understanding and consistency of approach that will make a difference to obtaining reliable yield stress measurements.
This information has been sourced, reviewed and adapted from materials provided by Malvern Panalytical.
For more information on this source, please visit Malvern Panalytical.