I. What is rheology?

I like rheology. I like it but don't know much about it (not yet!). After consulting the world largest search engine, google, I obtained many articles regarding rheology. As a starter, I read the introductional articles (not the scientific journal papers yet!). I am not if this is common but I tend to do it this way, I call it 'my way'. Perhaps this isn't something extraordinary, after all! If I don't like it, I will stop at this level. No further reading! If I do like it, I will proceed and demand something deeper about it. I didn't like rheology before but that doesn't mean I won't like it now! Yes, I started to like it. Not only that, I think I have fallen in love with it!

So, what sparks my interest in rheology?

I kept this to the end of the story.....

Doesn't matter. Let's talk about rheology now.

Rheology is a subject that studies the flow of matter. Sound simple? Yes, it does 'sound' simple, depends on what you want to know, really. It can be as simple as 'look, the toothpaste flow out from the tube but stay on the brush'. The toothpaste flows when stress applied to it, through the tube, from your fingers. Rheologically, the toothpaste is experiencing a 'thinning' effect under stress. It is similar to many other daily products such as yogurt, butter, honey, shampoo, shower gel (to me, shampoo = shower gel). Sounds easy? It did to me then no.

So, what do these daily products have to do with rheology? Well, like the toothpaste, these products have been characterised by rheometry. Butter for example, stay quietly in the pot, or sometimes in a paper wrapper. It looks like solid. It feels like solid. But when you press on it using your finger, preferably your thumb, you can make a permanent dent. The butter has flown down because of the pressure from your thumb. If you think this example is a bit too 'hard', let's consider bread spread. Bread spreads feature effortless spreading on bread, or toast, or even on chicken for roast. You can dig out the fat using a knife without any effort but the fat won't change its shape if you left it on your breakfast slice (unless you have just toasted it! The heat will melt the fat. Well, that will be another issue, also part of the rheology study and I shall talk about it later). So, your spreable fat flown when you dig it out from the pot. Digging is essentially appliying stress. The fat flown from the pot to your knife, that is flowing. Another words, the fat flows under stress. That's rheological behaviour! It is termed shear thinning behaviour in rheology.

I should give another examples. What about ketchup and mayonese? Too common? Ok, let me use DURIAN as an example. Durian is revered as 'King of Fruits' in South East Asia. When unripe, the flesh of durian is as hard as stone and it is incrediblely smooth. Lovely to touch though. When ripened, the flavour is simply irresistable and the flesh will be tender when fresh but goes waterly after a few days..... Wait, what is it to do with rheology? Seriously, it is an excellent natural product that exhibits shear thinning behaviour. Unlike watermelon, or other melons, which the flesh does not deform, but snap if you apply stress to it (you must apply considerable stress in order to snap the flesh though). Offhand I can't think of another fruit that the flesh texture similar to durian. Please do tell me if you know one.

The examples mentioned above, butter, spreadable fat, shampoo (or shower gel) yogurt, mayonese, ketchup and durian are of same category in term of rheological behaviour. They are shear thinning materials. You can call them lazy materials as they only move (or flow) when you push (apply stress) them. They are lazy, aren't they? One might ask why these materials do not move when left alone? In fact, the gravity is pulling them down all the time. The reason why these products stay as they are is the stress acting on them by the gravity force isn't powerful enough to make them flow. Here I am going to introduce an important term in rheology; the yield stress. All the examples given have an yield stress that is higher than the gravity force can produce but lower than the power of your hand. So, you don't have to sweat in order to spread fat on a slice of bread.