Newtonian Fluids

The viscosity of a Newtonian fluid is dependent only on temperature but not on shear rate and time.
Examples:

• Water
• Milk
• Sugar solution
• Mineral oil

Non-Newtonian fluids, time dependent

The viscosity of the fluid is dependent on temperature, shear rate and time.
Depending on how viscosity changes with time the flow behaviour is characterised as:

• Thixotropic (time thinning, i.e. viscosity decreases with time)
• Rheopetic (time thickening, i.e. viscosity increases with time)

Thixotropic fluids are quite common in chemical as well as in food industry. Rheopectic fluids are very rare.

Note: some fluids show time thinning behaviour due to breakdown of structure. This phenomenon is somtimes known as rheomaiaxis.

Examples

• thixotropic: yoghurt, paint
• rheopectic: gypsum paste

Non-Newtonian fluids, time independent

The viscosity of a Non-Newtonian time independent fluid is dependent not only on temperature but also on shear rate.

Depending on how viscosity changes with shear rate the flow behaviour is characterised as:

• shear thinning - the viscosity decreases with increased shear rate.
• shear thickening - the viscosity increases with increased shear rate.
• plastic - exhibits a so-called yield value, i.e. a certain shear stress must be applied before flow occurs.

Sear thinning fluids are also called pseudoplastic and shear thickening fluids are also called dilatant.

Examples of shear thinning fluids:

• Paint
• Shampoo
• Slurries
• Fruit juice concentrates
• ketchup

Examples of shear thickening fluids:

• Wet sand
• Concentrated starch suspensions

Examples of plastic fluids

• Quark
• Tomato
• Tooth paste
• Hand cream
• Some ketchups
• grease

Viscosity: A measure of the resistance of a fluid to flow.

Thixotropic: Describes materials that are gel-like at rest but fluid when agitated.

Centipoise: Water is the standard at 1 cps.