A non-Newtonian fluid is a fluid in which the viscosity changes with the applied shear force. 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.
An inexpensive, non-toxic sample of a non-Newtonian fluid can be made quite easily. Just add corn starch to a cup of water. Add the starch in small portions and stir in slowly. When the suspension nears the critical concentration, the so called "shear thickening" property of this non-Newtonian fluid becomes apparent. The application of force from the spoon, your fingers etc. cause the fluid to behave in a more solid like fashion. If left at rest it will recover its liquid like behavior.
Shear thickening fluids of this sort are actually being researched for bullet resistant body armor due to their capacity of absorbing the energy of projectiles impacting them at high velocity, while being soft and flexible at low velocities.
Principal types of non-Newtonian fluid include:
| Type of fluid
| Behaviour
| Characteristics
| Examples
|
| Plastic solids
| Perfectly plastic
| Strain does not result in opposing stress
| Ductile metals
|
| Bingham plastic
| Linear relationship between shear stress and rate of strain once threshold shear stress exceeded
| Mud, some colloids
|
| Yield pseudo-plastic
| Pseudo-plastic above some threshold shear stress
|
| Yield dilatent
| Dilatent above some threshold shear stress
|
| Power-law fluids
| Pseudo-plastic or "shear thinning"
| Apparent viscosity reducing with rate of shear
| Some colloids, clay, milk, gelatine, blood and liquid cement
|
| Dilatant or "shear thickening"
| Apparent viscosity increasing with rate of shear
| Concentrated solution of sugar in water, suspensions of rice starch or corn starch
|
| Viscoelastic - having both viscous and elastic properties
| Maxwell material
| "Series" linear combination of elastic and viscous effects
| metals, composite materials
|
| Oldroyd-B fluid
| Linear combination of Maxwell and Newtonian behaviour
| Bitumen, dough, nylon, and Silly Putty
|
| 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
|
| Generalised Newtonian fluids
| Stress depends on normal and shear strain rates
| Blood, Custard
|
See also
Jargon Dictionary definition of ooblick: http://www.jargon.net/jargonfile/o/ooblick.html