What are Newtonian and non-Newtonian fluids?

Newtonian fluids, non-Newtonian fluids, rheology, thixotropy, dilation... What is the difference between these liquid properties? Remember: what does it matter? Here's what you should know if you're sizing or choosing a pump, mixer, or other device that applies shear to a fluid. All fluids can be divided into two basic types, Newtonian and non-Newtonian.

Newtonian fluid

The viscosity of a Newtonian fluid remains constant regardless of the amount of shear applied at a constant temperature. These fluids have a linear relationship between viscosity and shear stress.

for example:

• water

• mineral oil

• gasoline

• alcohol

non-newtonian fluid

As you might guess, non-Newtonian fluids are the opposite of Newtonian fluids. When a shear force is applied to a non-Newtonian fluid, the viscosity of the fluid changes. The behavior of fluids can be described in one of four ways:

Swelling - The viscosity of a fluid increases when a shear force is applied. For example:

• quicksand

• cornmeal and water

• Plasticine

Pseudoplasticity - Pseudoplasticity is the opposite of dilatancy; the more shear force is applied, the more viscous it becomes. For example:

• ketchup

This graph shows how viscosity changes relative to the amount of shear or stress applied to the fluid.

Rheological - Rheological is very similar to bulking agents in that viscosity increases when shear is applied. The difference here is that the viscosity increase is time dependent. For example:

• plaster paste

• cream

Thixotropy - Thixotropic fluids decrease in viscosity when shear is applied. This is also a time-dependent property. For example:

• coating

• cosmetic

• asphalt

• glue

This graph shows how viscosity changes over time as it is applied to the fluid.

Why do you need to know the difference? It is important to have a good understanding of the properties of the fluids to be transferred, mixed or pumped, as viscosity plays a large role in selecting and selecting equipment. Knowing how it responds to shear will help you properly size and select any equipment it will come in contact with.