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Non-Newtonian fluid
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Non-Newtonian fluid

A non-Newtonian fluid is a fluid in which shear stress is not simply proportional solely to the velocity gradient, perpendicular to the plane of shear. Non-Newtonian fluids may not have a well-defined viscosity.

Rheological properties are better studied using tensor valued constitutive equations, which are common in the field of continuum mechanics .

Principal types of non-Newtonian fluid include:

Type of fluid Behaviour Characteristics Examples
Plastic solids Perfectly plastic Strain does not result in opposing stress Ductile metals
Bingham plastic Linear relationship between shear stress and rate of strain once threshold shear stress exceeded Mud, some colloids
Yield pseudo-plastic Pseudo-plastic above some threshold shear stress
Yield dilatent Dilatent above some threshold shear stress
Power-law fluids Pseudo-plastic Apparent viscosity reducing with rate of shear Some colloids, clay, milk, gelatine, blood and liquid cement
Dilatant Apparent viscosity increasing with rate of shear Concentrated solution of sugar in water, suspensions of rice starch or corn starch, Silly Putty
Viscoelastic - having both viscous and elastic properties Maxwell material "Series" linear combination of elastic and viscous effects metals, composite materials
Oldroyd-B fluid Linear combination of Maxwell and Newtonian behaviour Bitumen, dough and nylon
Kelvin material "Parallel" linear combination of elastic and viscous effects
Anelastic Material returns to a well-defined "rest shape"
Time-dependent viscosity Rheopectic Apparent viscosity increases with duration of stress Some lubricants
Thixotropic Apparent viscosity decreases with duration of stress Non-drip paints and tomato ketchup
Generalised Newtonian fluids Stress depends on normal and shear strain rates Blood

See also: Newtonian fluid | rheology | viscosity