As the user of a modern rheometer you have many different measuring systems available for use. You can rheologically characterize almost all substances. But what are the criteria for choosing the correct measuring system?
To select the best measuring system for your sample, consider the following:
- What is the samples’ consistency? Where is the sample located on the Rheology Road?
- How much sample is available for the measurement?
- How difficult is the cleaning?
- Does the sample sediment or dry out?
What is the samples’ consistency? Where is the sample located on the Rheology Road?
The following figure gives an overview and rough classification of different substances, from low-viscosity liquids to solids. Each column lists the most common test types and a recommended measuring system. The lower the viscosity of a sample, the larger the surface of the measuring bob should be.
Low-viscosity samples are often measured in cylinder measuring systems. At high shear rates the centrifugal forces lead to turbulence in the measuring gap (Taylor vortices). This leads to an apparent increase in the viscosity. Therefore, a critical shear rate limit should not be exceeded.
My recommendation: The more viscous the substance, the more likely it is that a cone-plate or parallel plate measuring system should be used.
Rheology Road (DG: double-gap measuring system, CC: cylinder measuring system, CP: cone-plate measuring system, PP: parallel plate measuring system, SRF: Solid Rectangular Fixture, UXF and SER: Extensional Rheometer Systems)
How much sample is available for the measurement?
Sometimes the choice of measuring system is limited because only a small volume of sample is available and therefore cylinder measuring systems cannot be used.
In this case I recommend using a cone-plate or parallel plate system. With a CP40-0.3 (diameter: 40 mm, angle: 0.3°) cone you only need, for example, 0.09 mL of sample.
How difficult is the cleaning?
If the measuring system is difficult to clean after a measurement, use a parallel plate or cone-plate system instead. Cylinder measuring systems are relatively difficult to clean.
There are disposable systems available for all measuring system types. These can be disposed of along with the sample or cleaned separately. This is required for samples which undergo a curing process.
Does the sample sediment or dry out?
If your sample contains solvent and you use parallel plate, cone-plate or double-gap systems, the sample may dry out around the edge. The resulting measured viscosity is too high.
If possible, use a cylinder measuring system. Even if the sample surface dries out, this does not affect the results when using cylinder measuring systems.
When using cone-plate and parallel plate measuring systems there are several ways of stopping the sample drying out:
- You can coat the sample around the edge with low-viscosity silicon oil.
- You can use a solvent trap which results in a saturated solvent atmosphere.
- If a Peltier-controlled hood is used, you can add an Evaporation Blocker which optimally seals and reduces the sample area.
If your sample tends to sediment, the viscosity values may be too low as only the liquid phase is measured. For such samples I recommend using cylinder measuring systems.
If your sample slides or slips, you should use sandblasted or profiled measuring systems. For very aggressive samples you can request measuring systems made of special materials. If your sample contains particles which are bigger than 1 mm, I recommend using the Ball Measuring System or a vane geometry.
If you use special systems it is important to measure your sample in the same way every time in order to obtain comparable results.
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