At Curtin Sarawak, the study of rheology is embedded in its engineering programmes
THE word “rheology’ is not a common one. Even electronic spell checkers think it is a misspelling of “theology”. However, according to Assoc Prof Dr Aaron Goh of Curtin Sarawak’s School of Engineering and Science, and the newly-appointed inaugural director of the Curtin Sarawak Research Institute (CSRI), rheology is an important scientific discipline.
Goh said 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 force is 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,” he said.
Curtin Students in Lab
“Do all materials ‘flow’ however? Some scientists say 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.”
Goh said materials may also flow when there is sufficient force applied to them.
“An important concept in rheology is the difference between a solid and 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.”
He said the real bodies that people encounter daily are often neither ideal solid nor ideal liquid.
“For example, a piece of steel may be considered a solid under normal usage, but a high forces, we can make in flow to form the shapes of car panels. Paint is easily layered onto walls, but when we stop painting, it remains on the way and does not drip.”
Goh said rheology is concerned with these materials that are neither ideal solids nor ideal liquids.
He said 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.
He said how the material behaves depends on the amount of force applied to it and the time that the force is applied.
“Depending on how fast we shear our materials, we may get different types of behaviors. For instance, ‘shearing thinning’ 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 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.”
Goh said some materials show an opposite behavior compared to shear thinning. In these ‘shear thickening’ materials, the faster we shear the material, the higher is its viscosity.
Guests to the 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.
“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,” Goh said.
He said one of the important materials on which rheology has a large influence is food.
“We are basically thinking rheology whenever we ask whether a certain type of margarine can be spread easily on bread, or whether a sauce can be easily poured out of the bottle or jar, or whether the noodles are spring 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.
“In the case of chilli sauce, insufficient control of the manufacturing processes may lead to unfavorable 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.”
There are no specific degree programmes on 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 degree programmes in specific fields such as food science.
At Curtin Sarawak, the study of rheology is embedded in its engineering programmes and hands-on exposure to the field is obtained by conducting undergraduate or postgraduate research in relevant fields. For better information, please visit http://www.curtin.edu.my
---------------------------------------------------------------------------------------------------------------------------------------------------------------
