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