Rheology – How to Select the Appropriate Measuring Geometry

Fig. 1. Selection of upper bob geometries. From left to right: smooth, splined, spiralled, vane, paddle

Rheometers can measure the viscosity and viscoelasticity of a material by applying a range of shear deformations. In simple terms, the viscosity of a material is its resistance to flow and viscoelasticity can explain whether a material behaves more like a liquid (‘viscous’) or solid (‘elastic’). This information can help scientists in R&D, for example, determine whether an intravenous drug can be injected or an oral dose can be swallowed, and even if it is likely to be a stable dispersion over time to prevent overdosing. It is also used in QC environments to assess whether a material passes or fails important performance criteria.

Kinexus Series

The Kinexus series of rheometers are class-leading rotational rheometers. These rheometers possess a custom air bearing making them incredibly sensitive to small material differences. Their torque sensitivity capabilities are even better than the equivalent of dropping an eyelash onto the instrument! What does that mean in practice? It allows you to easily measure materials under ‘at rest’ conditions. Therefore, we can determine whether products are going to be stable after having sat in the bottle on the shelf, i.e., their shelf life.

Geometry Choice

The measuring geometry selection is deliberately extensive. This is to ensure you have an appropriate measuring tool for both the type of test you wish to perform and the nature of your sample. The categories of standard geometries are: plate systems (parallel plates, cone & plates) and cylinder systems (cup & bobs).

Parallel Plates

These simple sets of flat upper and lower plates come in various materials, diameters and surface finishes, and are incredibly versatile.

  • Size – ranging from 4 mm to 60 mm in diameter as standard. This wide range of sizes is available to accommodate for different viscosities. The smaller geometries (<25 mm) are suited to highly viscous(> 10 Pa·s) samples and the larger geometries (>50 mm) are for low viscosity (<0.1 Pa·s) materials.
  • Surface finish – can be smooth, roughened (sandblasted) or serrated. Different surface finishes are available to accommodate those stubborn samples! Emulsions and slurries, for example, may be prone to slippage. This manifests itself as a lowering/drop in viscosity during a shear rate measurement. If you see a sudden drop in your viscosity and suspect slip, swap to using a roughened surface finish (see figure 2) for those samples. In order to encourage the material to flow, provide an extra grip using a modified surface interface.
  • Measuring gap – can be changed with parallel plates. This flexible feature means that gaps can be tailored to match the samples' viscosity (i.e., smaller gaps for lower viscosity samples) and to achieve different shear rates. Smaller gaps subject samples to higher shear rates (for the same angular velocity), whilst larger gaps will only achieve lower shear rates. As a compromise for the modifiable gap with these measuring systems, an average shear rate is applied to the sample and therefore, results are not absolute (as with cones and plates). In addition, as a general rule of thumb, if particles are present, select a measuring gap of at least 10 times larger than the largest particles. This is to prevent particles from jamming during the measurement, which will cause artefacts in the results.
  • Materials – the standard geometries on offer are made of stainless steel (SS316L) which is perfect for most laboratory environments as they are compatible with a wide range of sample types and can easily be cleaned with solvents. However, in some circumstances when working with acidic samples, a polymeric geometry may be more suitable. For example, PEEK and acrylic geometries (see figure 3) can be selected. The added advantage is that they are lighter and hence useful for high-frequency oscillation measurements on low-viscosity samples. In addition, titanium, aluminum and hastelloy steel geometries are also available.
Fig. 2. Lower pedestal plate to match a 20-mm upper geometry. Roughened surface finish.
Fig. 3. Alternative material upper plate geometries: PEEK and acrylic

Cones and Plates

Cone-and-plate combinations consist of a flat lower plate with an upper cone-shaped geometry and come in a variety of materials and surface finishes, e.g., roughened to prevent sample slippage. The tip of the cone is truncated and any measurements with these geometries are performed at a set gap (automatically controlled by the software). This is to allow for absolute viscosity measurements, so that wherever the sample is on the surface of this cone, it will be subjected to the same shear rate – a significant advantage over parallel plate geometries.

  • Cone angles – the upper geometry angle can vary from typically 0.5° to 4°. The selection allows you to select your cone choice to achieve different shear rates. The smaller the cone angle, the higher the achievable shear rate. However, the presence of particles (and size) still needs to be considered. Cone and plates have a fixed (nominal) measuring gap; for a 1° cone, the gap is 30 microns; 70 microns for 2° cones and 150 microns for 4°. Particles still need to be at least 10 times smaller than these gaps to prevent them from jamming at the apex of the geometry. This can be a particular limitation for the use of cones with particulate dispersions considering the small truncation gap, and plate geometries are more suitable for highly filled samples as the measuring gap can be changed to accommodate for this. If no particles (or very small particles) are present, then no worries!

Cups and Bobs

Cup-and-bob geometries are simply a lower cup to accommodate the sample and an upper bob to measure it. Just like the other measuring systems, there are options for surface finishes and different materials. They are useful for lower viscosity samples because there is extra surface area which makes them more sensitive. The relatively large gap between the upper bob and the wall of the lower cup is advantageous if samples possess larger particles because they will not jam. However, for low- viscosity materials being measured with any larger gap, one needs to be cautious of the onset of Taylor (non-shear) flow affecting the results. This can be detected by a false increase in viscosity at higher shear rates. Cups can be selected with fill-up marks for ease of sample loading and with removable bottoms to allow for easier cleaning between measurements, although this is not as straightforward as cleaning a lower flat plate, so consideration should be given to how easy your samples are to clean.

Fig. 4. Double gap upper bob and lower bob
  • Surface finish – for those slippery samples one can also use a roughened (sandblasted) or splined (~1 mm square pyramid “teeth”) cup and bob. If there are particles present in the sample and sedimentation occurs, a spiralled bob may help slow/prevent the dispersion from settling during the 
    measurement. If the dispersion is very unstable, then using a paddle will be more effective (see figure 1).
  • Vane tools – are useful for measuring samples with very delicate structures such as foams or soft solids with a yield stress like yoghurt. The shape of the vane (see figure 1) lends itself to slicing into the sample without disturbing/destroying too much of the structure prior to measurement (in comparison to a solid bob). 
  • Double gap – for extremely low viscosity samples, these geometries are a good option. As can be seen (figure 4), the upper bob is hollow, providing an extra measuring surface area and consequently, improved sensitivity. The use of these geomteries is recommended for more volatile samples at elevated temperatures due to the relatively large volume requirements (for relatively volatile samples at elevated temperature, the double gap must be used with a solvent trap).

Questions to Ask Yourself

There is no hard-and-fast rule for selecting a geometry as this article highlights a number of factors which could come into play. But when considering a new sample and geometry selection, ask yourself:

What is the general viscosity of my sample?

  • If you have a water-like low viscosity, select a large diameter cone/plate or plate/plate geometry (>50 mm).
  • If you have a free-flowing liquid (e.g., shower gel), a medium-sized geometry will work well (40 mm.)
  • If you have a very stiff, thick sample (treacle), a small geometry should be selected (<40 mm).
  • If you have a very low-viscosity or volatile sample, consider using a cup and bob or double gap. For evaporating samples, a solvent trap should be used.

Do I have particles in my samples?

  • If the answer is yes, what size? The measuring gap should be at least 10 times larger than the largest particle size which can be changed for parallel plates.
  • Cup and bob systems should also be considered, especially for settling samples where circulating grooved bobs are advantageous.

What is the composition of my sample?

  • Is my sample prone to slippage? Emulsions or concentrated dispersions can slip on the smooth geometries. Consider using a roughened or serrated surface finish (for plates) and roughened or splined (for bobs).
  • Does my sample have a delicate structure? A vane tool can be used on samples such as foams or soft solids for yield stress measurements.
  • Is my sample aggressive? Acidic samples can be measured with polymeric PEEK materials instead.

Start with these simple questions and review your results. The Kinexus is very forgiving and provides extra information to give users confidence that they have selected the correct geometry. Its clever feature of being able to easily swap to a different geometry and automatic recognition will make testing new samples fun and effortless!