Non-Newtonian fluid

A non-Newtonian fluid is a fluid which cannot be described by a single constant viscosity. Most commonly, the viscosity changes with the applied shear stress. Many polymer solutions and molten polymers are non-Newtonian fluids.

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.

Common examples
An inexpensive, non-toxic example of a non-Newtonian fluid is a solution of corn starch (corn flour) and water, sometimes called oobleck. 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, 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.