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.
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.
|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 and most honey varieties.|
|Generalized Newtonian fluids||Stress depends on normal and shear strain rates and also the pressure applied on it||Blood, Custard|