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	Psuedo-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
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