Non-Newtonian fluid
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| Continuum mechanics |
| Key topics |
| Conservation of mass Conservation of momentum Navier-Stokes equations |
| Classical mechanics |
| Stress · Strain · Tensor |
| Solid mechanics |
| Solids · Elasticity Plasticity · Hooke's law |
| Fluid mechanics |
| Fluids · Fluid statics Fluid dynamics · Viscosity · Newtonian fluids Non-Newtonian fluids Surface tension |
| Scientists |
| Newton · Stokes · others |
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A non-Newtonian fluid is a fluid in which the viscosity changes with the applied strain rate. 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.
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[edit] Common examples
An inexpensive, non-toxic sample of a non-Newtonian fluid sometimes known as oobleck can be made very easily by adding corn starch (cornflour) to a cup of water. Add the starch in small portions and stir it in slowly. When the suspension nears the critical concentration - becoming like single cream (light cream) in consistency - the so called "shear thickening" property of this non-Newtonian fluid becomes apparent. 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. 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/or applying sufficient force with their feet can literally walk across such a liquid, as shown here, here and here
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.
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.
[edit] Classification types
| 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 | |