From chili sauce to body armour
By Dr. Aaron Goh
The word ‘rheology’ is not a word one would commonly come across. Even electronic spell checkers think it is a misspelling of ‘theology’.
In fact, rheology is a very important scientific discipline. I hope, through this article, to show that rheology is very much a part of our daily lives, though we may not realise it.
What is Rheology?
Rheology is concerned with the ‘deformation’ of a body under the influence of ‘forces’. To an engineer or scientist, ‘deformation’ means the change in the shape of a body, while ‘forces’ may be referred to as the effort to pull or push a body.
In particular, rheology is concerned with how things ‘flow’. For example, when you squeeze a toothpaste tube, the toothpaste has to ‘flow’ out from the tube and onto your toothbrush. When you paint your house, the paint has to ‘flow’ to enable easy painting.
Do all materials ‘flow’ however? Some scientists say that everything flows, especially over a long period of time. An oft-quoted example is that of glass windows of old churches where the windows become thicker at the bottom than at the top. It is believed that the glass gradually flows from the top to the bottom of the windows due to their own weight.
Materials may also flow when there is sufficient force applied to it. I remember when the Exora car was first launched, its brochure contained two terms that caught my attention. The first was ‘hydroforming’ in which highly pressured liquids force a metal against a set of dies to change the shape of the metal. Another was ‘high strength steel’ which has been developed in such that the way it ‘flows’ during a crash, allowing it to absorb significant amounts of impact energy.
Of course, we do not necessarily want all materials to ‘flow’, otherwise the chair that sit on might slowly sag under our weight.
Solids and Liquids
An important concept in rheology is the difference between a solid and a liquid. When a force is applied to an ideal solid and then removed, the solid recovers its initial shape. An example of such a body is a spring.
On the other hand, an ideal liquid changes shape permanently when we apply a force. The simplest example of this is water at room temperature which can fill up a container of any shape that we put it in.
The real bodies that we encounter in our daily lives are often neither ideal solid nor ideal liquid. They tend to show both solid and liquid behaviour depending on the conditions in which we use them.
For example, a piece of steel may be considered a solid under normal usage, but at high forces, we can make it flow to form the shapes of car panels. Paint is easily layered onto walls, but when we stop painting, it remains on the wall and does not drip.
Rheology is concerned with these materials that are neither ideal solids nor ideal liquids. To investigate the rheological properties of these materials, we may pull or compress them.
However, since these materials tend to flow easily, it is more common to shear them. Shearing involves confining the material between two parallel plates that move parallel to each other. An analogy to shearing is placing a deck of cards on a table and pressing the hand gently on the top card and then moving the hand left to right.
Behaviour of Non-Ideal Solids or Liquids
How a material behaves depends on the amount of force we apply to it, and the time that the force is applied. When the force is significantly high, we exceed a yield point after which the material flows. Some materials may break immediately, however, instead of flowing when the force is high.
Similarly, depending on how fast we shear our materials, we may get different types of behaviour. For instance, we may observe what is called ‘shear thinning’, which occurs when the materials flow more easily the faster we shear it. The material is said to have a lower viscosity (resistance to shear) the faster we shear it.
The toothpaste and the paint are good examples of shear thinning materials. When we squeeze the toothpaste out, we are trying to shear the material quickly. The toothpaste then has a low viscosity, which allows it to be squeezed out. When the toothpaste is resting on our toothbrush, however, it has a high viscosity which allows it to retain its shape.
For the paint, it has a low viscosity when we spread it, but once it has been spread on the wall, the paint has a high viscosity, causing it to remain on the wall instead of dripping.
Some materials have an opposite behaviour compared to shear thinning. In these ‘shear thickening’ materials, the faster we shear the material, the higher is its viscosity. The next time you are at the beach, try to move your feet slowly and then quickly on the sand where the sea water is lapping. It will be more difficult to do the latter than the former.
Guests to our Curtin Open Day in 2009 had the experience of ‘walking’ on a liquid. The liquid was a simple starch suspension, which behaved like a solid when walked over quickly, but if the person slowed down or stood still, he or she would sink. These are examples of shear thickening materials.
There are fewer applications for shear thickening effects than there are for shear thinning effects. One notable application for shear thickening liquids is in body armour, where the liquid makes the armour soft enough to wear but when a bullet strikes the armour at high speeds, the shear thickening effect turns the armour into a hard, impenetrable solid.
Another fun application is to use a shear thickening liquid to cushion an egg when dropped so that it does not break. There are videos on Youtube which show that the eggs do not break even when dropped from a height of 100 feet! So, next time, when you enter an egg-drop competition, bring some starch, water and a plastic bag rather than a parachute.
Shear thinning and shear thickening effects are due to interactions within the material itself. For example, one of the reasons for shear thinning is that molecules may get entangled with each other, so that when the material is at rest, the entanglement gives the material a high viscosity.
However, when the material is sheared quickly, the molecules may realign to the shear direction, and this alignment leads to a lower degree of entanglement between the molecules and hence a lower viscosity.
On the other hand, in thick suspensions which have many particles stacked closely together, the shear thickening effect is because the particles cannot move easily relative to one another at short times.
Rheology in Our Everyday Lives
One of the important materials in which rheology has a large influence on is our foods. We are basically thinking rheology whenever we ask whether a certain type of margarine can be spread easily on our bread, or whether a sauce can be easily poured out of the bottle or jar, or whether the noodles are springy and firm rather than soft and pasty. Even the growth of bubbles in dough during baking is influenced by the rheological properties of the dough.
The shear thinning behaviour is used to good effect in many food applications. For example, a chilli sauce that has some seeds in it needs to be thick enough to suspend the seeds. On the other hand, it needs to be thin enough to be able to be poured from a bottle.
It is also important to monitor the rheological properties of foods at intermediate processing stages. During processing, the temperature of a food may be raised to cook it, or lowered if other frozen ingredients are added to it.
In the case of chilli sauce, insufficient control of the conditions of the manufacturing processes may lead to unfavourable results. Pipes may be jammed up because of very high viscosities at low temperatures. The seeds may sediment to the bottom of the tank at high temperatures when the viscosity is reduced. If the seeds are not mixed well, the intermediate sedimentation will lead to some bottles containing more seeds than the rest.
Rheology is important not just because of its role in texture and processing, but also because it allows us to have a quantitative measure of the properties of our foods. Rheological tests are used to determine ingredient functionality in product development. For example, if we change from one gelatin type to another, rheological tests allow us to determine how each gelatin sample may affect the texture of our food.
Rheology also allows us to study the microstructure of ingredients. For example, different types of pectin are used in normal jams compared to low-sugar jams. Different pectins have different molecular configurations which give rise to different rheological properties, which consequently require different means of preparing the jam.
Conventionally speaking, rheology is associated with materials that are more liquid than solid. Thus, rheology is more commonly identified with engineers who work in the area of processing and soft solids than by engineers who work on the strength of materials.
There are no specific degree programmes on rheology itself, but the study of rheology may be embedded in a mechanical, chemical or civil engineering, or material science, degree programme. Rheology may also be studied as part of other degrees in specific fields such as food science.
At Curtin Sarawak, the study of rheology is embedded in our engineering programmes and hands-on exposure to the field is obtained through performing undergraduate or postgraduate research in relevant fields. We have a rheometer that is used for measuring the rheological properties of liquid-like materials. In addition, we also have universal tensile machines to perform tests on solid-like materials.
Associate Professor Dr. Aaron Goh is an academic staff of Curtin Sarawak’s School of Engineering and Science and the newly-appointed inaugural Director of the Curtin Sarawak Research Institute (CSRI) at the Curtin Sarawak campus. He is an expert in the mechanical strength of materials, in addition to having a keen passion for food. As the Director of the CSRI, Dr. Goh will provide leadership in developing and implementing the strategic plan and research activities of the CSRI, as well as in raising the research profile of Curtin Sarawak. He can be contacted at +60 85 443939 or by e-mail to firstname.lastname@example.org.