At the 28th International Forum for Materials Testing, hosted as usual on the ZwickRoell premises in Ulm, Germany, Andy Pye caught up with Malvern Panalytical, a leading provider of scientific instrumentation, including rheometry.
Viscosity measures the resistance of a material to flow and is mathematically defined as shear stress divided by shear rate.
- With Newtonian fluids, viscosity is broadly independent of applied shear rate – water is a common example.
- Non-Newtonian fluids, in contrast, either shear thin – they exhibit lower viscosity at higher shear rates – or (less commonly) shear thicken, meaning that viscosity increases with applied shear rate.
Viscosity is a performance-defining parameter for many industrial products, few of which are Newtonian. Understanding how a material will behave when used, or processed, relies on knowing its properties under the conditions that are routinely applied. Clearly this is much more challenging for non-Newtonian materials than for those that are Newtonian.
Many commonly-used materials and formulations exhibit complex rheological properties, whose viscosity and viscoelasticity can vary depending upon the external conditions applied, such as stress, strain, timescale and temperature.
Often, viscosity is required to be high at low shear rates to prevent sedimentation or slumping, but to thin down at higher shear rates to facilitate application or processing. Think about the last time you visited a restaurant with a bottle of ketchup on the table. If the ketchup resisted pouring, what did you do? You shook the bottle, resulting in the ketchup flowing out more easily. It works because ketchup is a non-Newtonian fluid.
Hence a single viscosity measurement is not sufficient to describe the viscosity of such materials and the viscosity should be measured over a range of shear rates or stresses.
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Viscometer or rheometer?
A viscometer is designed for simple flow measurements on Newtonian materials. A viscometer can also be portable and used for field or remote testing.
Rheometers, though more expensive than viscometers, are more versatile and have a much wider dynamic range of control and measurement parameters. A rheometer allows far greater characterisation of flow, deformation and even the tackiness of a material (for Newtonian and non-Newtonian materials). One manufacturer’s benchtop viscometer costs $2,000-$4,000, while its rheometer range is $4,500 to $25,000.
Typically, a viscometer can measure in the range of about 0.1 to 1000 s-1 while a rheometer extends the measurement range from 0.000001 to 10,000 s-1. A broader measurement range enables relevant data to be obtained by exposing the sample to conditions that are realistic to the conditions applied during product manufacture or use.
However, viscometers and rheometers are complementary: it is not uncommon within a single organisation to find viscometers used for QC testing on products that have been developed using a rheometer. The choice tween the two is increasingly blurred. Most viscometers come with multiple speed range capability, some can measure a non-Newtonian material and show how its viscosity changes as a function of spindle rotational speed.
The software that drives both has progressed substantially in recent years with systems to help the user choose the right test for investigating specific behaviours, guide through measurement, interpretation of the resulting data and creating permanent test records. Some instruments have the embedded capability to analyse data sets using a variety of mathematical models.
Table 1 Common applications of Rheometers (source: Malvern Panalytical)
Types of rheometer
There are two basic types of rheometer: rotational (shear type) and capillary extrusion. Capillary extrusion is normally used for relatively concentrated suspension/pastes while the rotational design is for relatively less concentrated suspensions.
Rotational rheometers are preferred to obtain information on how molecular structure affects processing characteristics. They are relatively easy to use but their measurement inaccuracy is at least +/-10%. Various designs exist: all types have some form of element rotating inside the liquid at a constant rate. One common version has two coaxial cylinders with the fluid to be measured contained between them, while in the cone-and-plate design, the liquid is placed on horizontal plate and a shallow cone placed into it.
Most polymeric materials are processed in their melt state by extrusion or injection moulding and knowledge of rheological properties is essential. High-pressure capillary extrusion is a well-established method for simulating the processing conditions of molten polymers at high temperatures, for which it was specifically developed. It extends the shear rate range attainable in the laboratory beyond that available in a rotational instrument and allows the flow properties to be measured under typical processing conditions.
A sample is forced to extrude through a barrel or die of well-defined dimensions under high pressure. The pressure drop across the barrel or die is measured to give pressure-flow rate data for the fluid, from which viscosity is calculated. Temperature and shear rate can be closely controlled to simulate the processing environment of interest.
Even so, viscous heating and the effect of pressure on viscosity remain concerns with the method. Researchers have investigated this with advanced numerical simulation using finite element methods alongside the viscosity model. Simulations confirm that viscous heating and pressure dependence of viscosity can have a considerable impact on capillary rheometer experiments, but, that they largely compensate for one another.
References
- Capillary Rheometry – A Method to Predict Flow Properties under Processing Conditions
Torsten Remmler, Malvern Panalytical GmbH - More Solutions to Sticky Problems
Brookfield Engineering
https://www.brookfieldengineering.com/-/media/ametekbrookfield/%E2%80%8Ctech%20sheets/more%20solutions%202017.pdf?la=en - Asad Ullah Khan, Nasir Mahmood*, Aqeel Ahemd Bazmi
Department of Chemical Engineering – COMSATS, Institute of Information Technology, Defence Road, Off Raiwind Road, Lahore, Pakistan 54000
Revised: October 31, 2009
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