An Introduction to Rheology and its Practical Uses
Liquids and other substances can flow and behave in different ways depending on certain conditions. They can change texture and thickness (viscosity) after being stirred or shaken, when they’re poured out of a container, or when they’re applied to a surface. The science of how materials behave, flow and deform when certain forces are applied is called rheology.
What is viscosity?
To understand what rheology is, we first need to understand viscosity. Viscosity relates to the resistance fluids show to various stresses. When we think of fluids, they might be ‘thick’ (like honey) or ‘thin’ (like water). This perceived thickness or thinness can be measured as viscosity.
The viscosity of a given fluid isn’t fixed. Ambient temperature can play a role in the thickness or thinness of a fluid, and some fluids are composite or have different reactions to different industrial and commercial processes. The viscosity of a calibrating oil, for example, changes by approximately 7% when there is a temperature increase of just 1°C.
Fluids have stresses within themselves that govern how they behave. A lot of materials tend to be solid or gel-like while they’re at rest, but as soon as some force is applied to them, they transform to behave more like a liquid. Think of how toothpaste stays in the tube even when the lid is off, but you can easily squeeze some toothpaste onto your toothbrush when needed. This process is called ‘high yield stress’. A dollop of yoghurt sitting solidly on a spoon with nice, creamy appearance is said to have high yield stress, while a thinner and more watery equivalent has a ‘low yield stress’.
However, the measurement of fluid behaviour is more than just their thickness or thinness. Some fluids are composite or have different reactions to different industrial and commercial processes. The study of how fluids and soft solids flow is called rheology.
What is rheology?
Rheology and its effects accompany us everywhere we go. Whenever a material flows or is being deformed, rheology is involved. Water, honey, oil, toothpaste, bitumen, steel – each and every substance has rheological properties.
Rheology is primarily concerned with fluids and soft solids, but it’s more than just the study of a liquid’s viscosity. Rheology can be applied to materials with complex structure, so the reporting of the flow of these materials cannot be reported with a single number. The viscosity of these materials change depending on certain stresses.
Rotational rheometers, using a range of geometrics (including vane tools, cone and plate and concentric cylinders), are particularly useful in measuring fluids and soft solids that are composed of more than one material. This is because they are able to sample effectively without slipping or disturbing the sample.
Using rheology for practical solutions
Since materials can react differently to different stresses, it’s important to test liquid flow in a way that’s representative of the process it might undertake in an industrial or commercial process. Knowledge of rheological behaviour is essential in numerous processing operations that involve slurries or pastes, including beneficiation (wet mixing and milling, atomization and filtration); shape forming (extrusion, injection and roll forming) and coating/deposition (dipping, enameling and spraying).
Take yoghurt, for example. Many yoghurts are comprised of solid and liquid components (the yoghurt and the fruit). In the manufacturing process, it’s important that the composition of the materials remains at a certain mixture (i.e you don’t want all the fruit sinking to the bottom or floating to the top). To properly test the Rheology of this substance, the testing method should be similar to the manufacturing process.
Temperature, which is an extremely important parameter, can also be tested and measured directly in the measuring cell very close to the sample.
How can scientific instruments help?
To measure the rheological properties of a material, rheometers are used. Rheometers help those involved in industries such as sciences, geophysics, human biology, pharmaceuticals and food science measure how substances respond to particular forces or stressers.
Rheometers can help solve problems such as why a pharmaceutical suspension isn’t stable, or why a particular finish doesn’t have the right shine to it. Different rheometers are tailored for different users, meaning that even if rheology is not your field of expertise, you can still use a rheometer to measure the rheological properties of a material.
Malvern Panalytical provides two main rheometry techniques (analytical instruments used to measure flow properties of materials). They are:
- Rotational Rheometers – Sometimes called ‘shear rheometers’ they measure viscosity, thixotropy, shear stress, and shear strain and are used to determine how a liquid flows.
- Capillary Rheometers – Measure viscosity of polymer melts as a function of temperature and rate of deformation.
Rotational Rheology is ideal for testing the structural and compositional changes of a sample. Rotational Rheology is a convenient means of testing:
- Materials that are heterogeneous
- Materials that have more than one component
Rotational rheometers are arguably the most versatile rheological tools available and can be configured for a number of different rheological methods, to probe the structure and performance of suspensions. Test types range from the generation of simple viscosity flow curves (plots of viscosity against shear) over many decades of torque, through yield stress measurement and on to precise sequences that simulate the chewing of food. They enable close matching of the test method to the specific process or in-use environment of the product. Innovative software is increasingly helpful in allowing even novice rheologists to generate and interpret relevant data.
Rotational rheometers are used for a broad range of sample types from pastes and gels to the most weakly structured liquids. Applied shear can be precisely controlled into the very low shear stress region making these instruments suitable for stability studies and the measurement of yield stress.
Samples are loaded between two plates, or other similar geometry such as cone and plate or alternatively a cup and bob or vane system. Applying a torque to the top plate exerts a rotational shear stress on the material and the resulting strain or strain rate (shear rate) is measured. Rotational rheometers and viscometers share the same operating principle, but the former have far greater functionality. This is most evident in the accuracy and range over which shear stress can be applied, their facility for oscillatory testing and the level of control over the normal force applied during rotational testing.
Originating in the polymer industry, capillary rheometry is useful for measuring the viscosity profiles of suspensions and slurries containing relatively large particles, at high particle loadings. Industrial examples include polymer melts, ceramic slurries, foodstuffs, inks and coatings. Capillary rheometers can apply very high force, which enables the exploration of behavior at far higher shear rates than is possible with rotational rheometry. High shear rate performance is pertinent in many industrial processes, such as extrusion and spraying. For certain applications the sample size required for capillary rheometry, around a liter for the generation of a flow curve, is a limitation.
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.
Malvern Kinexus Ultra+
The Malvern Kinexus Ultra+ is a high performance rotational rheometer suitable for benchtop use. It is suitable for testing the shear viscosity and viscoelasticity of sample materials and other advanced applications in the study of flow properties.
With a wide torque range and unmatched axial control capabilities, the Kinexus Ultra+ combines innovative design with expert guided software to achieve reliable and repeatable results. Unique sequence-driven rSpace software enables fully customisable test design for users to set-up and investigate tailored rheological test protocols. Find out more about this top of the range Rheology product here and contact ATA Scientific to find out how we help you stay on the forefront of scientific application. Talk to us today.