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).