ts a fluid whose viscosity changes when stress is applied to it

Newtonian fluids like water flow at a constant viscosity regardless of force. However, a non-Newtonian fluid can act like a liquid or a solid depending on the pressure or stress it undergoes.

So non-Newtonian fluids all have the same characteristics:

Altered viscosity

Their viscosity is not constant and depends on the amount of force, or shear rate applied to them

Multiple states

Due to their changing viscosity, they can switch between behaving like a solid and a liquid

Non-Newtonian behaviour

They do not follow Newton's Law of Viscosity which applies to simpler fluids like water where viscosity is constant regardless of stress applied

Complex internal structure

This unusual behaviour is caused by the presence of complex macromolecules or particles within the fluid that rearrange when the fluid is agitated