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Another method for yield point determination is the vane technique. It is a very quick and easy method for analysing the yield point of paste-like samples. A vane spindle with four thin blades arranged at equal angles is used for the test. A constant low speed is preset on the rotational viscometer. The maximum yield stress, which can be detected during the measurement, is the yield point value. To illustrate the yield point in a graph, the torque is plotted against the time (Figure 5). The diagram has three typical regions [4]:
• The shear stress increases due to deformation as an elastic response.
• A shear stress peak is achieved due to a collapse of the microstructure of the material. This point is called the yield point.
• A stress decay due to the structural breakdown can be visualised.
The Herschel-Bulkley model gives the product’s yield stress as well as the product’s fl ow index and consistency index (Figure 9). 93.38 N/m² need to be applied on the tube to make the gel fl ow. In order to prevent liquids like gels from fl owing out of the tube when no force is applied, the yield point has to be analysed. A fl ow index of 0.4155 (<1) means that the gel has a shear-thinning fl ow behaviour. Gels have to show shear-thinning fl ow behaviour because the viscosity has to decrease when a force is applied, for example, when squeezing it out of a tube or applying it to the skin.
Figure 5. Yield point determination with the vane technique (red line = yield point).
Example applications with various rotational viscometer confi gurations
Concentric cylinder systems: The viscosity of cough syrup was analysed using a spring-type viscometer (R-model for regular viscosity samples) with a concentric cylinder measuring system (Figure 6).
Concentric cylinder systems are absolute measuring systems. Due to the defi ned spindle geometry, it is possible to calculate shear rate values. The sample volume needed for the measurement is relatively low (approx. 2 mL to 20 mL). In contrast, for a typical rotational viscosity test with standard spindles, a sample volume of 500 mL is required.
Figure 9. Flow curve diagram and yield point determination of gel using the Herschel-Bulkley mathematical regression model.
Vane spindles: The yield point of porridge was analysed using a spring-type viscometer (R-model for regular viscosity samples) with a vane spindle (Figure 10).
Figure 6. Concentric cylinder spindles.
In contrast to, for example, water and juices, cough syrup has a relatively high viscosity in order to coat the surface of the throat (Figure 7). To evaluate the fl ow behaviour of the sample, it is useful to calculate the shear-thinning index. If the value is >1, the sample is shear-thinning. If the value is <1, the sample is shear-thickening. The measured cough syrup has a shear-thinning index of 1, which means that the sample shows a so-called Newtonian behaviour. This means that the sample’s viscosity does not change, even if a higher speed is applied, for instance, during swallowing.
Vane spindles are required for measuring the yield point of paste-like samples (also with particles). These spindles reduce slippage and minimise structural changes of the sample during immersion of the spindle. The spindles are also suitable for viscosity measurements at a certain speed. Vanes are relative measuring systems but are the only solution for measuring paste-like samples with particles. They are also cheap.
The measured porridge shows a yield stress of 252 N/m2
Figure 10. Vane spindles. (Figure 11). This parameter
can be important for production, as it defi nes how much pump power is needed. Furthermore, it is known that the mouthfeel of food products is related to the yield stress. Creaminess correlates with a higher yield stress value.
Figure 11. Yield stress measurement of porridge with vane spindle Figure 7. Viscosity measurement of cough syrup.
Cone-plate measuring systems: The quality of a gel for muscle and joint pain was controlled by rotational viscosity testing using a spring-type viscometer (R-model for regular/medium viscosity samples) with a cone-plate measuring system (Figure 8).
The cone-plate system offers the chance to reduce the required sample volume for a viscosity measurement down to 0.5 mL to 2 mL. Moreover, cone-plate measuring systems provide absolute viscosity determinations for sophisticated and reliable measurements.
Figure 8. Cone-plate spindles used with a rotational viscometer from Anton Paar GmbH
T-bar spindles with motorised stand adapter: The viscosity of mayonnaise was measured using a spring-type viscometer (R-model for regular viscosity samples) with a motorised stand adapter and T-bar spindle (Figure 12).
A motorised stand adapter with T-bar spindles is a cheap and reliable measurement confi guration for pasty, non-fl owing samples like mayonnaise (without particles). The motorised stand adapter replaces the hand-wheel of the viscometer.
Figure 12. Rotational viscometer with motorised stand adapter and T-bar spindle (Anton Paar GmbH).
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