VII. Shear thinning and Thixotropy

I think I am jumping a bit here. The words shear thinning are familier enough but not thixotropy.

What is the meaning of thixotropy? Googling it I found a few sites give the defination; a thixotropy material is a material that appears or feels like a gel but flows when forced. Wait a minute, isn't that the same as shear thinning? Yes, you are about right. No, you are just almost right, means you are wrong. Thixotropic materials will thin even the shear rate remains but the shear thinning materials do not become thinner at a fixed shear rate. If you shear the thixotropic materials, the viscosity depends on how long you shear it; the longer, the lower. So, time is an important factor when describing thixotropic materials. And, it is the time dependent behavior that distingushes the shear thinning and thixotropic materials.

Example for thixotropic materials? Er...ok found one: thixotropic clay. URL it by clicking below:

http://www.potters.org/subject18056.htm

The clay mentioned here should be a high concentration clay slurry that you can shape it into a pot. The material, if I am not mistaken, should appear like gel. It will flow only when certain amount of force applied to it (not gravity though). The spinning pot and your hand (also with the air) will create shearing to the clay, so shear thinning will occur. The constant rotating speed will provide the constant shear rate provided your hand didn't move much. Under this circumstances, the clay will thin (but I guess it would not thin very quickly and would not thin a lot) and make the shaping easier.

V. Shear Rate and Shear Stress

Shear rate and shear stress are the rate and the stress of shear (everybody knows that!). What is the definition for shear rate and shear stress?

Well, shear rate is how fast your hand slices/pushes at the top of the stack of paper mentioned previously. In rheology, it has an unit of reciprocal of second (means 1/s). So, it is about how fast the plate or cone is rotating. If you control the shear rate, then you control how fast you want to rotate the plate or cone on top of the liquid.

Shear stress is defined as the force per unit area acting on the direction of shear. The unit is Pascal second (Pa.s). You can measure the stress when applied shear to your sample. Alternatively, you can measure shear rate when applied shear stress to your sample. You can apply shear in two ways; rotational and oscillational.

I have here an example for you to imagine; say you have a piece of float board (similar to those used to learn swiming) and tied it with a long rope. Throw the board into a river the board will flow to the direction of the current. Now, pull the rope, the board will shear the water surface. Depending on the direction of shear you applied, the shear stress can be low (if you are pulling in the direction of the flow) or high (if you pull it against the flow).

IV. Shear

Shear is a very important word in rheology. So what is shear? Imagine you have a stack of A4 paper sitting nicely on your desk. Put your hand on the top of the stack and push the paper to a direction horizontally. The papers on the top will move according to the same direction. But not all the paper moved with same distance. The distance the paper moves decreases from the top of the stack. The papers at the bottom are likely to be static.

In rheology, they call the action of your hand 'shearing'.

Similarly, action of shear can be found in our daily life. For instance, rubbing eraser on a piece of paper.

The energy used to perform the shear is called shear stress. Your hand is the source of energy for both the cases mentioned above.

When you shear a liquid, the liquid moves. Of course the same direction! How the liquid moves is the heart of the rheology. If you are a person that do not always follow the rules (like me!) you might want to try to shear faster, slower or give it a shear then suddenly stop etc.. All these actions can be applied by a rheometer and the subsequent behaviours (just like human being, isn't it?) such as flow faster, flow slower, doesn't flow at all, fly away, screaming, shouting etc.. are measured and recorded by the rheometer itself. Then a lot of equations can be generated. A lot of graphs can be produced. What a wonderful instrument!

III. From Viscous to Elastic

As previously mentioned that rheology covers all materials, from very low viscosity gases, medium viscosity liquids to very high viscosity solids. There are specific terms that describe all these categories in rheology. Let's ignore the extremes; the very low and very high viscosities materials. It means that I don't want to include gas and some solids that do not flow in years (stone, glass etc..). I am interested, or perhaps most of the rheologists are interested, only in the middle range. I am talking about liquids to semi-solid materials.

In rheology, the most interesting and demanding area involves changes that can be observed immediately or within a reasonable observation period of time (Let you imagine what is the reasonable observation period of time). Gases respond to stress immediately right? (sorry I am not going to cover gas here). I am going to mention viscous liquids such as water, coffee, teh tarik etc.. and elastic materials such as spring made of metal. In between viscous and elastic is a cleverly defined term: viscoelastic (I bet a child 3 years old can do that!). This will be covered too.

Apparently, according to an article found in wikipedia, visco means 'creep' (will come back to this next time). As elastic means that a material can recover after the stress is removed. Spring for instance, elongates when pulled at both ends but returns to its original length after the stress is removed. Metal spring does that immediately but elastomer such as rubber will take some time to recover. I will discuss about rubber in the future.

Viscous liquids are also called Newtonian liquids. Examples given already.

Viscoelastic materials are non-Newtonian. Note this category covers anything in between viscous and elastic materials so expect a wide range of materials.

Elastic materials obey Hooke's law.

II. Rheology for All Materials

Just to summarise my previous post here; I have given examples of shear thinning materials in daily use products including butter, spreadable, shampoo, etc.. Infact, rheology not only cover shear thinning materials, as you already have guessed. Yes, you are right, rheology not just about materials that flow when sheared, it is also about materials that refuse to flow if sheared (stuborn! ready). It is also about materials that do not bother about how fast you shear them, they are what they are (what a personality!). Basically there are THREE categories of materials in rheology, shear thinning, shear thickening and constant viscosity (why not they call it shear remaining, shear unchanging??). Examples? As mentioned earlier, most of our daily products are shear thinning materials. What about shear thickening materials? It is rare but potato starch paste with high solid content is a shear thickening materials. Not convinced? Try it yourself at home. What about materials that do not change their viscosity no matter how fast they are sheared? The best example would be water. How? If you stir a cup of water (or coffee or tea), it will remain as easy no matter you stir it slowly or fast i.e. the viscosity does not change.

This time, I would like to talk about the wide range of materials that rheology covers. Let's start from very low viscosity. You might think of water, or alcohol? No, they are more viscous than gases. Gas does have viscosity, but very low. Liquids have higher viscosity if compared to gas. Solids have even higher viscosity. So, need examples? Gas is straight forward. Examples of liquids have been given. What about solids? Theoretically, solids like stone, metal, rubber etc.. can be defined rheologically. Confused? Yeah, me too initially!

Rheology is the study of flow of matter. How can stone flow? I am not talking about many stones flowing down from a mountain. I am talking about the flow within the inner structure of the stone! Then a stone can not flow, that is for sure! Wait a minute, if you observe the stone in minutes, hours, days or even years, I am sure you are absolutely right! But as many scientist will tell you, what about a million years? Or a billion years?

Can a mountain move? No, if you are coming to visit it again a few years later. Yes, if your grand grand grand children continue your observation.

Anyway, the stone and mountain sound ridicurous, don't they?

Let's use rubber as an example. Rubber, a solid materials. When you try to pull it apart, it elongates. When you let go, it pulls itself back to its original dimension. When under stress (pulling) the material flows (elongates). Isn't that fulfill the defination of rheology?

Now, do you think glass can flow? I guess now you are not 100% sure that glass can not flow.

Here I have an interesting article for you to read, it is about shear thinning beauty product:-

http://bestthingsinbeauty.blogspot.com/2009/11/blog-post.html

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.