What Are Newtonian and Non-Newtonian Fluids?

Newtonian fluid, non Newtonian fluid, rheopectic, thixotropic, dilatant... what's the difference between these liquid properties? More importantly, what does it matter? If you're sizing or selecting pumps, mixers, or any other type of equipment that applies shear to fluid, these are all terms you should know. 

All fluids can be broken down into two basic types, Newtonian, and non-Newtonian.

NEWTONIAN FLUIDS

A Newtonian fluid's viscosity remains constant, no matter the amount of shear applied for a constant temperature. These fluids have a linear relationship between viscosity and shear stress.

Examples:

• Water
• Mineral oil
• Gasoline
• Alcohol

NON-NEWTONIAN FLUIDS

You can probably guess that non-Newtonian fluids are the opposite of Newtonian fluids. When shear is applied, the viscosity of non-Newtonian fluids decreases or increases, depending on the fluid. The behavior of the fluid can be described in one of four ways:

• Dilatant - Viscosity of the fluid increases when shear is applied. For example:
  • Quicksand
  • Cornflour and water
  • Silly putty
• Pseudoplastic - Pseudoplastic is the opposite of dilatant; the more shear applied, the less viscous it becomes. For example:
  • Ketchup

This chart shows how viscosity changes with respect to the amount of shear or stress applied to the fluid.

• Rheopectic - Rheopectic is very similar to dilatant in that when shear is applied, viscosity increases. The difference here is that viscosity increase is time-dependent. For example:
  • Gypsum paste
  • Cream
• Thixotropic - Fluids with thixotropic properties decrease in viscosity when shear is applied. This is a time-dependent property as well. For example:
  
  • Paint
  • Cosmetics
  • Asphalt
  • Glue

This chart shows how viscosity changes in respect to shear applied over time to the fluid.

Why do you need to know the difference? It's important to fully understand the properties of the fluids you're transferring, mixing, or pumping because viscosity plays a major role in sizing and selecting equipment. Understanding how it reacts to shear will help you properly size and select all the equipment it touches. 

Here's a brief video to recap:

Need help with a viscous situation? Ask us about it! We gladly provide technical assistance to businesses in Wisconsin and upper Michigan.