A 'non-Newtonian fluid' is a
fluid in which the
viscosity changes with the applied strain rate. As a result, non-Newtonian fluids may not have a well-defined viscosity.
Although the concept of viscosity is commonly used to characterize a material, it can be inadequate to describe the mechanical behavior of a substance, particularly non-Newtonian fluids. They are best studied through several other
rheological properties which relate the relations between the stress and strain tensors under many different flow conditions, such as oscillatory shear, or extensional flow which are measured using different devices or rheometers. The rheological properties are better studied using
tensor-valued
constitutive equations, which are common in the field of
continuum mechanics.
Common examples
An inexpensive, non-toxic sample of a non-Newtonian fluid sometimes known as
oobleck can be made very easily by adding
corn starch (cornflour) to a cup of water. Add the starch in small portions and stir it in slowly. When the suspension nears the critical
concentration - becoming like single cream (light cream) in consistency - the so called "shear thickening" property of this non-Newtonian fluid becomes apparent. The application of force - for example by stabbing the surface with a finger, or rapidly inverting the container holding it - leads to the fluid behaving like a
solid rather than a liquid. More gentle treatment, such as slowly inserting a spoon, will leave it in its liquid state. Trying to jerk the spoon back out again, however, will trigger the return of the temporary solid state. A person moving quickly and/or applying sufficient force with his feet can literally walk across such a liquid, as shown
here and
here and
here
Shear thickening fluids of this sort are being researched for bullet resistant
body armor, useful for their ability to absorb the energy of a high velocity projectile impact but remain soft and flexible while worn.
A familiar example of the opposite, a shear-thinning fluid, is
paint: one wants
the paint to flow readily off the brush when it is being applied to the surface
being painted, but not to drip excessively.
Classification types

Classification of fluids based on the stress vs. rate of strain relationship.
See also
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Newtonian fluid
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Rheology
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Viscosity
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Superfluids
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Navier-Stokes equations
★
Dissipative particle dynamics
★
Finite deformation tensors
★
Oobleck
★
Quicksand
★
Bingham plastic
External links
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Viscosity Chart
★
A pool filled with non-newtonian fluid (youtube video)
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Narrated scientific experiments on the effects of vibrations on a non-newtonian fluid (youtube video)
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Amateur experiments on the effects of vibrations on a non-newtonian fluid (google video)
★
Cornstarch Lifeforms (youtube video)