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!