A non-Newtonian fluid is a fluid whose flow properties differ in any way from those of
Newtonian fluids. Most commonly the viscosity (the measure of a fluid's ability
to resist gradual deformation by shear or tensile stresses) of non-Newtonian
fluids is dependent on shear rate or shear rate history. Some non-Newtonian
fluids with shear-independent viscosity, however, still exhibit normal
stress-differences or other non-Newtonian behavior. Many salt solutions and
molten polymers are non-Newtonian fluids, as are many commonly found substances
such as ketchup, custard, toothpaste, starch suspensions, paint, blood, and
shampoo. In a Newtonian fluid, the relation between the shear stress and the
shear rate is linear, passing through the origin, the constant of
proportionality being the coefficient of viscosity. In a non-Newtonian fluid, the
relation between the shear stress and the shear rate is different and can even
be time-dependent (Time Dependent Viscosity). Therefore, a constant coefficient
of viscosity cannot be defined.
Although the concept of viscosity is commonly used in fluid mechanics
to characterize the shear properties of a fluid, it can be inadequate to
describe non-Newtonian fluids. They are best studied through several other
rheological properties that relate stress and strain rate tensors under many
different flow conditions--such as oscillatory shear or extensional flow--which
are measured using different devices or rheometers. The properties are better
studied using tensor-valued constitutive equations, which are common in the
field of continuum mechanics.
(See also: http://en.wikipedia.org/wiki/Non-Newtonian_fluid)
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