Thixotropy
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2008-05-26T11:05:48Z
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robot Adding: [[hu:Tixotrópia]]
'''Thixotropy''' is the property of some [[Non-Newtonian fluid|non-Newtonian]] [[Power-law fluid|pseudoplastic]] fluids to show a time-dependent change in [[viscosity]]; the longer the fluid undergoes [[shear stress]], the lower its viscosity. A thixotropic fluid is a fluid which takes a finite amount of time to attain equilibrium viscosity when introduced to a step change in shear rate. However, this is not a universal definition; the term is sometimes applied to [[pseudoplastic]] fluids without a viscosity/time component. Many [[gel]]s and [[colloids]] are thixotropic materials, exhibiting a stable form at rest but becoming fluid when agitated.
It is important to note the distinction between a thixotropic fluid and a [[shear thinning]] fluid. The former displays a decrease in viscosity ''over time at a constant shear rate'', while the latter displays decreasing viscosity with ''increasing shear rate''. Fluids which exhibit the opposite property, in which constant shear stress for a time, such as shaking or mixing, causes an increase in viscosity or even solidification, are called [[rheopecty|rheopectic]], sometimes called anti-thixotropic, and are much less common.
==Natural examples==
Some [[clay]]s are thixotropic, with their behavior of great importance in [[structural engineering|structural]] and [[geotechnical engineering]]. In [[earthquake]] zones, clay-like ground can exhibit characteristics of [[soil liquefaction|liquefaction]] under the shaking of a [[tremor]], greatly affecting earth structures and buildings. [[Landslide]]s, such as those common in the [[cliff]]s around [[Lyme Regis]], [[Dorset]] and in the [[Aberfan]] [[slag heap]] disaster in [[Wales]] are evidence of this phenomenon. Similarly, a [[lahar]] is a mass of earth liquefied by a [[volcano|volcanic]] event, which rapidly solidifies once coming to a rest.
[[Drilling mud]]s used in geotechnical applications can be thixotropic. Honey from honey bees may also exhibit this property under certain conditions.
Another example of a thixotropic fluid is the [[synovial fluid]] found in joints between some bones. The [[ground substance]] in the human body is thixotropic. <ref>Hendrickson, T: "Massage for Orthopedic Conditions", page 9. Lippincott Williams & Wilkins, 2003.</ref>
Some clay deposits found in the process of exploring [[caves]] exhibit thixotropism: an initially solid-seeming mudbank will turn soupy and yield up moisture when dug into or otherwise disturbed. These clays were deposited in the past by low-velocity streams which tend to deposit fine-grained sediment.
==Applications==
[[Image:Organic Heinz Tomato Ketchup.jpg|thumb|200px|Ketchup displays thixotropic properties.]]
Examples of applications for thixotropic fluids are the thickening of food stuffs and medical products. Toothpaste is thixotropic, which allows it to be squeezed out of the tube, yet retain a solid shape on the brush. The ink developed for the Fisher [[space pen]] is thixotropic so that the ink flows only when the roller ball is pressed on paper.<ref>[http://www.spacepen.com/Public/Technology/index.cfm Technology - Fisher Space Pen<!-- Bot generated title -->]</ref> [[Ketchup]] is frequently thixotropic.
Modern [[alkyd]] and [[latex]] [[paint]] varieties are often thixotropic and will not run off the painter's [[brush]], but will still spread easily and evenly, since the gel-like paint "liquefies" when brushed out. Many clutch-type [[automatic transmissions]] use [[Automatic transmission fluid|fluids]] with thixotropic properties, to engage the different clutch plates inside the transmission housing at specific pressures, which then changes the gearset.
<!-- [[Laponite]], a mixture of water and synthetic clay, is used in the lubrication of robotic snails/slugs ([[gastropod]]s), which rely on Laponite's thixotropic property for locomotion.n <fact?> -->
==Etymology==
The word comes from [[Greek language|Greek]] ''thixis'', touch (from ''thinganein'', to touch) + ''-tropy'', ''-tropous'', from Greek ''-tropos'', of turning, from ''tropos'', changeable, from ''trepein'', to turn.
==See also==
*[[Aberfan#The_Aberfan_disaster|Aberfan disaster]]
*[[Rheopexy]] (antonym)
*[[Kaye effect]]
*[[polymer]]
*[[Silly putty]]
*[[Shear thinning]]
*[[viscosity]]
==References==
*Reiner, M., and Scott Blair, Rheology terminology, in ''Rheology'', Vol. 4 pp. 461, (New York: Achedemic Press, 1967)
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{{Geotechnical engineering|state=collapsed}}
[[Category:Continuum mechanics]]
[[Category:Fluid dynamics]]
[[Category:Non-Newtonian fluids]]
[[Category:Soil mechanics]]
[[Category:Soil physics]]
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