Newtonian fluids vs non-Newtonian fluids

 

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

 

 

NEWTONIAN FLUIDS

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

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 to non-Newtonian fluids, the viscosity of the fluid changes. The behavior of the fluid can be described in following ways:

Time Independent:-

·Dilatant - Apparent viscosity of the fluid increases when shear is applied.

(Apparent viscosity= Slope of Shear stress & shear rate curve)

 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

 

Bingham plastic is a material that behaves as a rigid body at low stresses but flows as a viscous fluid at high stress. This behavior is exhibited by slurries, suspensions of solids in liquids, paints, emulsions, foams, etc.

Bingham's model is described by the following relation.
τ = τ_o + μ_pγ

where τ is shear stress, γ is the shear rate, τ_o is called minimum yield stress and  μ_p is called plastic viscosity.

 

 

 

Time Dependant:-

·Rheopectic -Rheopectic is very similar to dilatant in that when shear is applied, apparent viscosity increases. The difference here is that viscosity the 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

 

 

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.