Exploring the Viscosity of Non-Newtonian Fluids

Mary Canlas
Oct 20, 2021
2 min read

What is Viscosity?

Viscosity is the measure of a fluid's resistance to flow. A fluid with a large viscosity resists motion, due to a large internal friction between the layers of molecules. Moreover, a fluid with low viscosity flows easily due to little internal friction when in motion.

Newton's Law of Viscosity: τ = μ du/dy

τ = Shearing Stress
μ = Dynamic Viscosity
du/dy = Rate of the Shear Strain

This equation shows that the shearing stress is linearly related to the rate of shearing strain - hence, Newtonian Fluids follow this rule.

But...

Non-Newtonian fluids do not follow this rule. In other words, the shearing stress is NOT linearly related to the rate of shearing strain. Instead, the viscosity of Non-Newtonian fluids depends on the shear rate.

There are 2 types of Non-Newtonian Fluids I will be exploring:

Shear thinning fluid
Shear thickening fluids

Shear Thinning Fluids (Pseudoplastic)

This is when the fluid viscosity decreases with an increasing rate of shear strain. So, the harder the fluid is sheared, the less viscous it becomes.

For example, in ketchup, when the shear force applied is small, the shear stress is small, resulting in a small shear rate, resulting in a large viscosity. This small shear rate means the displacement of the ketchup is small, so the resistance to motion is large (large viscosity): this can be observed as the ketchup will not come out of the bottle (or will pour extremely slowly) when it is turned upside down.

Conversely, when the shear force applied is large (e.g hard tap), the shear stress is large, resulting in a large shear rate, resulting in a small viscosity. Therefore, there is less resistance to motion; this can be observed as the ketchup can now pour out of the bottle.

Shear Thickening Fluids (Dilatant)

This is when the fluid viscosity increases with an increasing rate of shear strain. So, the harder the fluid is sheared, the more viscous it becomes.

For example, in quicksand, when the shear force applied is large, the shear stress is large, resulting in a large shear rate, resulting in a large viscosity. This large shear rate means that the difficulty removing an object from quicksand increases because the viscosity (resistance to motion) is so large.

To conclude, the things I have learned are: