Non-Newtonian fluid
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2008-07-16T22:14:38Z
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/* Common examples */
{{Continuum mechanics}}
A '''non-Newtonian fluid''' is a [[fluid]] whose flow properties cannot be described by a single constant [[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, 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 [[rheology|rheological]] properties which relate the relations between the [[Stress (physics) | stress]] and [[strain rate]] tensors under many different flow conditions, such as [[Oscillation | 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, [[Toxicity | 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 his feet can literally walk across such a liquid.<ref name="Brainiac">[http://www.youtube.com/watch?v=ygtX2miwU5I Brainiac - Jon Tickle Walks on Custard]</ref>
[[Dilatant|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|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.
==Classification types==
[[Image:Non-Newtonian fluid.PNG|369px|right| Classification of fluids based on the stress vs. rate of strain relationship.]]
{| class="wikitable"
|-
|[[Kelvin material]]
|"Parallel" linear combination of elastic and viscous effects
|-
|[[Viscoelasticity#Types of viscoelasticity|Anelastic]]
|Material returns to a well-defined "rest shape"
|-
|rowspan=2|Time-dependent viscosity
|[[Rheopecty|Rheopectic]]
|Apparent [[viscosity]] increases with duration of stress
|Some [[lubricant]]s
|-
|[[Thixotropy|Thixotropic]]
|Apparent [[viscosity]] decreases with duration of stress
|Non-drip [[paint]]s and tomato [[ketchup]] and most [[honey]] varieties.
|-
|colspan=2|[[Generalized Newtonian fluid]]s
|Stress depends on normal and shear strain rates and also the pressure applied on it
|[[Blood]], [[Custard]]
|}
==See also==
* [[Bingham plastic]]
* [[Complex fluid]]
* [[Dissipative particle dynamics]]
* [[Newtonian fluid]]
* [[Navier-Stokes equations]]
* [[Oobleck]]
* [[Pseudoplastic]]
* [[Quicksand]]
* [[Rheology]]
* [[Superfluids]]
* [[Viscosity]]
==External links==
* [http://www.research-equipment.com/viscosity%20chart.html Viscosity Chart]
* [http://www.youtube.com/watch?v=f2XQ97XHjVw A pool filled with non-newtonian fluid (youtube video)]
* [http://www.youtube.com/watch?v=lDzampa3xrc&NR Narrated scientific experiments on the effects of vibrations on a non-newtonian fluid (youtube video)]
* [http://www.youtube.com/watch?v=NKxKVpHZe5Q Cornstarch Lifeforms (youtube video)]
==References==
<references/>
{{Physics-footer}}
[[Category:Continuum mechanics]]
[[Category:Fluid dynamics]]
[[Category:Non-Newtonian fluids]]
[[Category:viscosity]]
[[Category:Polymers]]
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