A non-Newtonian fluid is a fluid in which the viscosity changes with the applied shear force. As a result, 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
|
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, nylon, and Silly Putty
|
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
Last updated: 05-03-2005 17:50:55