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Newtonian and non-Newtonian Fluids

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Newtonian fluids:

Fluids which obey the Newton's law of viscosity are called as Newtonian fluids. Newton's law of viscosity is given by

t = mdv/dy
where t = shear stress

m = viscosity of fluid

dv/dy = shear rate, rate of strain or velocity gradient

All gases and most liquids which have simpler molecular formula and low molecular weight such as
water, benzene, ethyl alcohol, CCl_{4}, hexane and most solutions of simple molecules are Newtonian fluids.

Non-Newtonian fluids:

Fluids which do not obey the Newton's law of viscosity are called as non-Newtonian fluids.

Generally non-Newtonian fluids are complex mixtures: slurries, pastes, gels, polymer solutions etc.,

Various non-Newtonian Behaviors:

Time-Independent behaviors:

Properties are independent of time under shear.

*Bingham-plastic*: Resist a small shear stress but flow easily under larger shear stresses. e.g. tooth-paste, jellies, and some slurries.

*Pseudo-plastic*: Most non-Newtonian fluids fall into this group.
Viscosity decreases with increasing velocity gradient. e.g. polymer solutions, blood.
Pseudoplastic fluids are also called as Shear thinning fluids.
At low shear rates(du/dy) the shear thinning fluid is more viscous than the Newtonian fluid, and at high shear rates it is less viscous.

*Dilatant fluids*: Viscosity increases with increasing velocity
gradient. They are uncommon, but suspensions of starch and sand behave in this way.
Dilatant fluids are also called as shear thickening fluids.

*Time dependent behaviors*:

Those which are dependent upon duration of shear.

*Thixotropic fluids*: for which the dynamic viscosity decreases with the time for which shearing forces are applied. e.g. thixotropic jelly paints.

*Rheopectic fluids*: Dynamic viscosity increases with the time for which shearing forces are applied. e.g. gypsum suspension in water.

**Visco-elastic fluids**: Some fluids have elastic properties, which allow them to spring back when a shear force is released. e.g. egg white.

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Last Modified on: 14-Sep-2014

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