A non-Newtonian fluid is a fluid whose flow properties are not described by a single constant value of viscosity. Many
polymer solutions and molten polymers are non-Newtonian fluids, as are many commonly found substances such as
ketchup, starch suspensions, paint, blood and shampoo. In a Newtonian fluid, the relation between the shear stress and
the strain rate is linear (and if one were to plot this relationship, it would pass 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
strain rate is nonlinear, and can even be time-dependent. Therefore a constant coefficient of viscosity can not be defined.
A ratio between shear stress and rate of strain (or shear-dependent viscosity) can be defined, this concept being more
useful for fluids without time-dependent behavior.

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 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.

An inexpensive, non-toxic example of a non-Newtonian fluid is a suspension of corn starch (corn flour) in water,
sometimes called oobleck (uncooked imitation custard, being a suspension of primarily corn flour, has the same
properties). 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. This is the "shear thickening" property of this non-
Newtonian fluid. 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 applying
sufficient force with their feet can literally walk across such a liquid.

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. Some shear thickening fluids are
also used in all wheel drive systems utilising a viscous coupling unit for power transmission.

A familiar example of the opposite, a shear thinning fluid, or pseudoplastic 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.

There are fluids which have a linear shear stress, shear strain relationship, that requires a finite yield stress before they
begin to flow. That is the shear stress, shear strain curve doesn't pass through the origin. These fluids are called
Bingham plastics. Several examples are clay suspensions, drilling mud, toothpaste, mayonnaise, chocolate, and
mustard. The classic case is ketchup which will not come out of the bottle until you stress it by shaking.

There are also fluids whose strain rate is a function of time. Fluids that require a gradually increasing shear stress to
maintain a constant strain rate are referred to as rheopectic. An opposite case of this, is a fluid that thins out with time
and requires a decreasing stress to maintain a constant strain rate (thixotropic).

Source: WikipediA