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| − | :''This article is about the physical phenomenon of capillary action.''
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| − | [[Image:180126main CFE12.jpg|thumb|Capillary Flow Experiment to investigate capillary flows and phenomena aboard the [[International Space Station]].]]
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| − | '''Capillary action''', '''capillarity''', '''capillary motion''', or '''wicking''' is the ability of a substance to draw another substance into it. The standard reference is to a tube in plants but can be seen readily with porous paper. It occurs when the [[adhesion|adhesive]] [[intermolecular force]]s between the [[liquid]] and a [[Chemical substance|substance]] are stronger than the [[cohesion (chemistry)|cohesive]] intermolecular forces inside the liquid. The effect causes a concave [[meniscus]] to form where the substance is touching a vertical surface. The same effect is what causes [[porous]] materials such as [[sponge (tool)|sponges]] to soak up liquids.
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| − | A common [[apparatus]] used to demonstrate capillary action is the ''capillary tube''. When the lower end of a [[vertical direction|vertical]] [[glass]] tube is placed in a liquid such as [[water]], a concave [[meniscus]] forms. [[Surface tension]] pulls the liquid column up until there is a sufficient [[mass]] of liquid for [[gravitational force]]s to overcome the intermolecular forces. The [[contact]] [[length]] (around the edge) between the liquid and the tube is proportional to the diameter of the tube, while the weight of the liquid column is [[Proportionality (mathematics)|proportional]] to the [[Square (algebra)|square]] of the tube's [[diameter]], so a narrow tube will draw a liquid column higher than a wide tube. For example, a [[glass]] capillary tube 0.5 [[millimeter|mm]] in diameter will lift approximately a 2.8 mm column of water.
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| − | With some pairs of [[materials]], such as [[mercury (element)|mercury]] and glass, the interatomic forces within the liquid exceed those between the solid and the liquid, so a convex meniscus forms and capillary action works in reverse.
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| − | The term capillary flow is also used to describe the flow of carrier gas in a silica capillary column of a [[Gas-liquid chromatography|GC]] system. This flow can be calculated by [[Hagen-Poiseuille equation#Poiseuille's equation for compressible fluids|Poiseuille's equation for compressible fluids]].
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| − | ==Examples==
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| − | In [[hydrology]], capillary action describes the attraction of [[water]] molecules to [[soil]] particles. Capillary action is responsible for moving [[groundwater]] from wet areas of the soil to dry areas. Differences in soil matric [[water potential|potential]] (<math>\Psi_m</math>) drive capillary action in soil.
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| − | Capillary action is also essential for the drainage of constantly produced [[tears|tear]] fluid from the [[eye]]. Two canalicula of tiny diameter are present in the inner corner of the eyelid, also called the lacrymal ducts; their openings can be seen with the naked eye within the lacrymal sacs when the eyelids are everted.
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| − | [[Paper towel]]s absorb liquid through capillary action, allowing a fluid to be transferred from a surface to the towel. The small pores of a [[sponge (tool)|sponge]] act as small capillaries, causing it to absorb a comparatively large amount of fluid.
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| − | Some old sport and exercise [[textile|fabrics]], such as [[Coolmax]], use capillary action to "wick" sweat away from the skin. These are often referred to as [[layered clothing#wicking-materials|wicking fabrics]], presumably after the capillary properties of a [[candle wick]].
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| − | Chemists utilize capillary action in [[thin layer chromatography]], in which a solvent moves vertically up a plate via capillary action. Dissolved solutes travel with the solvent at various speeds depending on their polarity.
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| − | Capillary action is NOT responsible for water transport in plants. Instead cohesion between the water molecules and transpiration work together to draw up water.
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| − | ==Formula==
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| − | [[Image:Capillary Attraction Repulsion (PSF) (bjl).svg|250px|thumb|Demonstration of capillary attraction and repulsion in water and mercury.]]
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| − | With notes on the dimension in SI units, the height ''h'' of a liquid column ([[Metre|m]]) is given by:<ref name="Batchelor">[[George Batchelor|G.K. Batchelor]], 'An Introduction To Fluid Dynamics', Cambridge University Press (1967) ISBN 0521663962</ref>
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| − | ::<math>h={2{ \gamma \cos{\theta}}\over{\rho g r}}</math>
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| − | where:
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| − | :*<math>\scriptstyle \gamma </math> is the liquid-air [[surface tension]] (J/m² or N/m)
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| − | :*''θ'' is the [[contact angle]]
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| − | :*''ρ'' is the [[density]] of liquid (kg/m<sup>3</sup>)
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| − | :*''g'' is [[acceleration]] due to [[gravity]] (m/s²)
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| − | :*''r'' is [[radius]] of tube (m).
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| − | For a water-filled glass tube in [[air]] at [[sea level]],
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| − | :''<math>\scriptstyle \gamma </math>'' is 0.0728 J/m² at 20 °[[celsius|C]]
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| − | :''θ'' is 20° (0.35 [[radians|rad]])
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| − | :''ρ'' is 1000 kg/m<sup>3</sup>
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| − | :''g'' is 9.8 m/s²
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| − | therefore, the height of the water column is given by:
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| − | :<math>h\approx {{1.4 \times 10^{-5}\ \mbox{m}^2}\over r}</math>.
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| − | Thus for a 2 m wide (1 m radius) tube, the water would rise an unnoticeable 0.014 mm. However, for a 2 cm wide (0.01 m radius) tube, the water would rise 1.4 mm, and for a 0.2 mm wide (0.0001 m radius) tube, the water would rise 140 mm (about 5.5 [[inch]]es).
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| − | ==Miscellaneous==
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| − | [[Albert Einstein]]'s first paper<ref>[http://www.einstein-website.de/z_physics/wisspub-e.html List of Scientific Publications of Albert Einstein]</ref> submitted to [[Annalen der Physik]] was on capillarity. It was titled ''Folgerungen aus den Capillaritätserscheinungen'', which translates as ''Conclusions from the capillarity phenomena'', found in volume 4, page 513.<ref>[http://www.physik.uni-augsburg.de/annalen/history/papers/1901_4_513-523.pdf Folgerungen aus den Capillaritätserscheinungen] (in German)</ref> It was submitted in late 1900 and was published in 1901. In 1905 [[List of scientific publications by Albert Einstein|Einstein published]] four seminal papers in the same journal; these four papers are known as the [[Annus Mirabilis Papers]].
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| − | ==See also==
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| − | *[[Frost flowers]]
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| − | *[[Washburn's equation]]
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| − | *[[Wick effect]]
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| − | *[[Capillary fringe]]
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| − | *[[Capillary wave]]
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| − | == Notes ==
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| − | <references/>
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| − | ==References==
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| − | == External links ==
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| − | [[Category:Physical sciences]]
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| − | [[Category:Physics]]
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| − | [[Category:Earth science]]
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| − | [[Category:Environmental science]]
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| − | {{credit|248041684}}
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