Selecting the correct accessories for rheology and viscosity testing.
In order to obtain relevant and accurate rheology and viscosity testing results the correct selection of rheometer and viscometer accessories is essential. Here is a quick run-down of the measuring systems available for defined shear rate and shear stress measurements, with the benefits of each approach
Cone and Plate
The cone/plate measuring system is often considered the perfect measuring system for a rheometer or viscometer. The system comprises a lower temperature-controlled plate on which a small amount of sample (typically no more than 2 ml) is placed and an upper rotating or oscillating cone of a very shallow angle (typically 1 or 2 degrees) that is brought down to make contact with the sample. Although often truncated slightly to eliminate contact friction, the point where the tip of the cone would be is in contact with the lower plate.
On rotation a uniform shear rate is generated across the entire sample.
The sample is small so temperature equilibration is rapid.
In most cases the cone/plate system is open so the sample is visible throughout a measurement.
The small, flat measuring surfaces are easy to clean.
Plate-Plate or Parallel Plate
The parallel plate measuring system is very similar to a cone plate except the upper cone is replaced by a flat plate. The gap set between the plates to hold the sample typically ranges from around 0.3mm to 1mm and the rheometer software takes this gap into account for calculations. Although the parallel plate system does not generate a uniform shear rate (shear rate is at a maximum at the edge and drops to zero at the centre) it has some key advantages.
Up to a certain point the gap can be widened to accommodate particles in a sample. A typical rule-of-thumb here is to set a gap of 10 times the maximum particle size.
A large gap can be set to reduce the shear applied to a sample during loading. This is particularly useful where a delicately-structured sample is to be measured for zero-shear viscosity or viscoelasticity measurements.
When performing oscillation testing on stiff gels or pastes it is desirable to avoid any axial stresses present in the sample prior to taking a measurement. A large gap enables such samples to be loaded without generating such stresses.
The single greatest benefit from using a parallel plate is that the surfaces can be roughened, by sandblasting or crosshatching – to eliminate the wall-slip or wall-depletion effects that are guaranteed to ruin low-shear viscosity testing. See this article for more information on wall-slip.
The concentric cylinder or cup-and-bob system comprises an outer cylinder (the cup) into which the sample is poured and the inner cylinder (the bob) introduced. The sample then occupies the narrow gap between the two. On rotation of the bob shear is generated in the sample. End effects are usually accommodated for in shear stress calculations.
Concentric cylinders are particularly useful for viscosity measurements on samples that would be likely to exhibit edge drying in a cone/plate or plate/plate approach.
Concentric cylinders can allow high shear rates to be applied to samples without fear of expellation or edge failure typical in cone/plate or plate/plate set-ups. However unwanted secondary flows can form in low viscosity fluids resulting in the appearance of apparent shear-thickening as shear rate is increased.
A significant disadvantage of concentric cylinder measuring systems is the high degree of shear a sample is subjected to during loading into the cylinder renders the approach less desirable for structure testing on colloidal gels.