Ver el código de «Tixotropía»

{{traducción|inglés|Thixotropy|en}}

'''Tixotropía''' es la propiedad de algunos [[Fluido no-newtoniano|fluidos no newtonianos]] y [[pseudoplásticos]] que muestran un cambio dependiente del tiempo en su [[viscosidad]]; cuanto más se someta el fluido a esfuerzos de [[Tensión cortante|cizalla]], más disminuye su viscosidad. Un fluido tixotrópico es un fluido que tarda un tiempo finito en alcanzar una viscosidad de equilibrio cuando hay un cambio instantáneo en el ritmo de cizalla. Sin embargo no existe una definición universal; el término a veces se aplica a los fluidos pseudoplásticos que no muestran una relación viscosidad/tiempo. Es importante tener en cuenta la diferencia entre un fluido tixotrópico y otro pseudoplástico. El primero muestra una disminución de la viscosidad a lo largo del tiempo a una velocidad de corte constante, mientras que el último muestra esta disminución al aumentar la velocidad de corte. A los fluidos que exhiben la propiedad opuesta, en la que la agitación a lo largo del tiempo provoca la solidificación, se les llama [[reopexia|reopécticos]], a veces anti-tixotrópicos, y son mucho menos comunes.

== Ejemplos y aplicaciones ==
Algunos [[gel]]es y [[coloide]]s se consideran materiales tixotrópicos, pues muestran una forma estable en reposo y se tornan fluidos al ser agitados. Variedades modernas de recubrimientos alcalinos, de látex y pinturas son materiales por lo general tixotrópicos que no caen de la brocha del pintor pero se pueden aplicar fácil y uniformemente pues el gel se liquidifica cuando se aplica. La salsa de tomate es frecuentemente tixotrópica.

Durante el [[Moldeo por colada|proceso de colado]] la barbotina tiene que tener la suficiente facilidad de fluir para llenar un molde y para que pasado un tiempo luego de tomar espesor se pueda vaciar sin que queden choreaduras o espesores falso. <ref>[http://www.manises.com/forum/topic.asp?TOPIC_ID=1440 Curvas reológicas]</ref>


== Véase También ==

* [[Reología]]
* [[Reología en barbotina]]
* [[Viscosidad]]



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<includeonly><!-- ==Examples==

==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, as is [[semen]]. <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==
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>

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.

Thixotropy has been proposed as a scientific explanation of blood liquification "miracles" such as that of [[Saint Januarius]] in [[Naples]].<ref>[http://www.cicap.org/new/articolo.php?id=101014 Garlaschelli, Ramaccini, Della Sala, "The Blood of St. Januarius", ''Chemistry in Britain'' '''30'''.2, (1994:123)]</ref>

Semi-solid casting process such as thixomoulding uses thixotropic property of some alloys (mostly light metals). At certain temperature range, with an appropriate preparation, an alloy can be put into a semi-solid state, which can be injected with less shrinkage and better overall property than by normal injection moulding.

Many kinds of inks—used in [[silkscreen]] [[textile printing]]—made from [[plastisol]], exhibit thixotropic qualities.  Some, such as those used in [[CMYK]]-type process printing, are designed to quickly regain viscosity once they are applied to protect the structure of the dots for accurate color reproduction. This is a sort of reverse thixotropy.
<!-- [[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?> -->

==Misconceptions==

[[Toothpaste]], [[ketchup]], and [[paint]] are frequently mis-labeled as being thixotropic materials. In reality, these are often [[pseudoplastic]] or "shear thinning". The viscosity of these materials decrease under increasing shear rate, not increasing time. When squeezed out of a tube, toothpaste flows easily but will set-up on the toothbrush. When shaken or squeezed out of a bottle, ketchup will thin and flow readily but will retain its shape on a burger or plate. When modern paints are applied the shear created by the brush or roller will allow them to thin and wet out the surface evenly. Once applied the paints regain their higher viscosity which avoids drips and runs.

==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==

*[[Pseudoplastic]]
*[[Aberfan#Aberfan_disaster|Aberfan disaster]]
*[[Rheopexy]] (antonym)
*[[Kaye effect]]
*[[polymer]]
*[[Silly putty]]
*[[Shear thinning]]
*[[viscosity]]

==References==
{{Reflist}}
{{refbegin}}
*Reiner, M., and Scott Blair, Rheology terminology, in ''Rheology'', Vol. 4 pp. 461, (New York: Achedemic Press, 1967)
*[http://espace.library.uq.edu.au/view/UQ:78856 Dam break wave of thixotropic fluid] in ''Journal Hydraulic Engineering'', Vol. 132, No. 3, pp. 280-293
{{refend}}

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[[Category:Continuum mechanics]]
[[Category:Fluid dynamics]]
[[Category:Non-Newtonian fluids]]
[[Category:Soil mechanics]]
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[[Categoría:Mecánica de fluidos]]
[[Categoría:barbotina]]