capillarity

cap·il·lar·i·ty

(kăp'ə-lăr'ĭ-tē) pronunciation

n., pl., -ties.
The interaction between contacting surfaces of a liquid and a solid that distorts the liquid surface from a planar shape. Also called capillary action.

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capillarity

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Rise or fall of liquid in a small passage or tube. When a glass tube of small internal diameter is inserted into water, the surface water molecules are attracted to the glass and the water level in the tube rises. The narrower the tube, the higher the water rises. The water is said to "wet" the tube. Water will also be drawn into the fibres of a towel, even if the towel is in a horizontal position. Conversely, if a glass tube is inserted into mercury, the level of the liquid in the tube falls. The mercury does not wet the tube. Capillarity is caused by the difference in attraction of the liquid molecules to each other and the attraction of the liquid molecules to those of the tube.

For more information on capillarity, visit Britannica.com.

Columbia Encyclopedia:

capillarity

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capillarity or capillary action, phenomenon in which the surface of a liquid is observed to be elevated or depressed where it comes into contact with a solid. For example, the surface of water in a clean drinking glass is seen to be slightly higher at the edges, where it contacts the glass, than in the middle. Capillarity can be explained by considering the effects of two opposing forces: adhesion, the attractive (or repulsive) force between the molecules of the liquid and those of the container, and cohesion, the attractive force between the molecules of the liquid (see adhesion and cohesion). Adhesion causes water to wet a glass container and thus causes the water's surface to rise near the container's walls. If there were no forces acting in opposition, the water would creep higher and higher on the walls and eventually overflow the container. The forces of cohesion act to minimize the surface area of the liquid (see surface tension); when the cohesive force acting to reduce the surface area becomes equal to the adhesive force acting to increase it (e.g., by pulling water up the walls of a glass), equilibrium is reached and the liquid stops rising where it contacts the solid. In some liquid-solid systems, e.g., mercury and glass or water and polyethylene plastic, the liquid does not wet the solid, and its surface is depressed where it contacts the solid. Capillarity is one of the causes of the upward flow of water in the soil and in plants.

Wiley Dictionary of Flavors:

Capillary Action

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The perceived effect of a liquid on a substrate. An example is when a liquid is pulled across a solid due to attractive forces between the two. This effect is what produces the meniscus in a graduated cylinder and what draws a liquid up a perfume blotter. Colors can be identified by this manner. In a few days or hours, a blotter dipped in a mixture of colors will draw the liquid up the blotter and separate the colors so that they can be identified. The name for this technique is paper chromatography and is used for separating water-soluble components like food colors. See Meniscus.

Oxford Dictionary of Biochemistry:

capillarity

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the phenomenon, resulting from surface tension, in which liquids rise up capillary tubes and which also causes them to form a concave or convex meniscus at their surface where it contacts a solid.

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Saunders Veterinary Dictionary:

capillarity

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The action by which the surface of a liquid where it is in contact with a solid, as in a capillary tube, is elevated or depressed.

Mosby's Dental Dictionary:

capillarity

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(kap′iler′itē)
n

The phenomenon by which a film of fluid is drawn and held between two closely approximating surfaces.

Wikipedia on Answers.com:

Capillary action

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This article is about the physical phenomenon. For the band, see Capillary Action (band).

Capillary action, or capillarity, is the ability of a liquid to flow in narrow spaces without the assistance of, and in opposition to external forces like gravity. The effect can be seen in the drawing up of liquids between the hairs of a paint-brush, in a thin tube, in porous materials such as paper, in some non-porous materials such as liquified carbon fiber, or in a cell. It occurs because of inter-molecular attractive forces between the liquid and solid surrounding surfaces; If the diameter of the tube is sufficiently small, then the combination of surface tension (which is caused by cohesion within the liquid) and adhesive forces between the liquid and container act to lift the liquid.^[1]

Contents

1 Etymology
2 Phenomena and physics of capillary action
3 Examples
4 Height of a meniscus
5 Liquid transport in porous media
6 Miscellaneous
7 See also
8 References

Etymology

The word comes from the Latin adjective capillaris ("pertaining to the hair") from the noun capillus ("the hair of the head") ultimately derived from caput ("head").^[2] This would suggest the scientific phenomenon was first observed between contiguous hairs, for example within a paint-brush. In medicine and biology, it usually refers to the smallest blood vessels. The word "capillary," in the non-medical sense, means narrow tube.

Phenomena and physics of capillary action

Capillary Flow Experiment to investigate capillary flows and phenomena aboard the International Space Station

Capillary action, capillarity, capillary motion, or wicking refers to two phenomena:

A common apparatus used to demonstrate the first phenomenon is the capillary tube. When the lower end of a vertical glass tube is placed in a liquid such as water, a concave meniscus forms. Adhesion pulls the liquid column up until there is a sufficient mass of liquid for gravitational forces to overcome the intermolecular forces. The contact length (around the edge) between the top of the liquid column and the tube is proportional to the diameter of the tube, while the weight of the liquid column is proportional to the square of the tube's diameter, so a narrow tube will draw a liquid column higher than a wide tube.

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 potential ( $Ψ m$ ) drive capillary action in soil.

Examples

Capillary action of water compared to mercury, in each case with respect to a polar surface e.g. glass

Capillary action is also essential for the drainage of constantly produced tear fluid from the eye. Two canaliculi of tiny diameter are present in the inner corner of the eyelid, also called the lacrimal ducts; their openings can be seen with the naked eye within the lacrymal sacs when the eyelids are everted.

Wicking is to absorb something and then drain like a wick. Paper towels absorb liquid through capillary action, allowing a fluid to be transferred from a surface to the towel. The small pores of a sponge act as small capillaries, causing it to absorb a comparatively large amount of fluid. Some modern sport and exercise fabrics use capillary action to "wick" sweat away from the skin. These are often referred to as wicking fabrics, after the capillary properties of a candle and lamp wicks.

Capillary action is observed 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 affinity for the solvent (the mobile phase) or the absorbent coating on the plate (the stationary phase).

With some pairs of materials, such as mercury and glass, the intermolecular forces within the liquid exceed those between the solid and the liquid, so a convex meniscus forms and capillary action works in reverse.

Height of a meniscus

The height h of a liquid column is given by:^[3]

$h={{2 \gamma \cos{\theta}}\over{\rho g r}},$

where $\scriptstyle \gamma$ is the liquid-air surface tension (force/unit length), θ is the contact angle, ρ is the density of liquid (mass/volume), g is local gravitational field strength (force/unit mass), and r is radius of tube (length).

For a water-filled glass tube in air at standard laboratory conditions, γ = 0.0728 N/m at 20 °C, θ = 20° (0.35 rad), ρ is 1000 kg/m³, and g = 9.81 m/s². For these values, the height of the water column is

$h\approx {{1.48 \times 10^{-5}}\over r} \ \mbox{m}.$

Thus for a 4 m (13 ft) diameter glass tube in lab conditions given above (radius 2 m (6.6 ft)), the water would rise an unnoticeable 0.007 mm (0.00028 in). However, for a 4 cm (1.6 in) diameter tube (radius 2 cm (0.79 in)), the water would rise 0.7 mm (0.028 in), and for a 0.4 mm (0.016 in) diameter tube (radius 0.2 mm (0.0079 in)), the water would rise 70 mm (2.8 in).

Liquid transport in porous media

Capillary flow in a brick, with a sorptivity of 5.0 mm min^-1/2 and a porosity of 0.25.

When a dry porous medium, such as a brick or a wick, is brought into contact with a liquid, it will start absorbing the liquid at a rate which decreases over time. For a bar of material with cross-sectional area A that is wetted on one end, the cumulative volume V of absorbed liquid after a time t is

$V = AS\sqrt{t},$

where S is the sorptivity of the medium, with dimensions m/s^1/2 or mm/min^1/2. The quantity

$i = \frac{V}{A}$

is called the cumulative liquid intake, with the dimension of length. The wetted length of the bar, that is the distance between the wetted end of the bar and the so-called wet front, is dependent on the fraction f of the volume occupied by liquid. This number f is the porosity of the medium; the wetted length is then

$x = \frac{i}{f} = \frac{S}{f}\sqrt{t}.$

Some authors use the quantity S/f as the sorptivity.^[4] The above description is for the case where gravity and evaporation do not play a role.

Sorptivity is a relevant property of building materials, because it affects the amount of rising dampness. Some values for the sorptivity of building materials are in the table below.

Material	Sorptivity (mm min^-1/2)	Source
Aerated concrete	0.54	^[5]
Gypsum plaster	3.50	^[5]
Clay brick	1.16	^[5]

Miscellaneous

Albert Einstein's first paper^[6] submitted in 1900 to Annalen der Physik was on capillarity. It was titled Folgerungen aus den Capillaritätserscheinungen, which was translated as Conclusions from the capillarity phenomena, found in volume 4, page 513 (published in 1901).

References

^ http://science.jrank.org/pages/1182/Capillary-Action.html
^ http://en.wiktionary.org/wiki/capillary
^ G.K. Batchelor, 'An Introduction To Fluid Dynamics', Cambridge University Press (1967) ISBN 0-521-66396-2
^ C. Hall, W.D. Hoff, Water transport in brick, stone, and concrete. (2002) page 131 on Google books
^ ^a ^b ^c Hall and Hoff, p. 122
^ List of Scientific Publications of Albert Einstein

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		Wiley Dictionary of Flavors. Copyright © 2008 by Wiley-Blackwell. Wiley and the Wiley logo are registered trademarks of John Wiley & Sons, Inc. and/or its affiliates in the United States and other countries. Used here by license. Read more
		Oxford Dictionary of Biochemistry. Oxford University Press. Oxford Dictionary of Biochemistry and Molecular Biology © 1997, 2000, 2006 All rights reserved. Read more
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		Wikipedia on Answers.com. This article is licensed under the Creative Commons Attribution/Share-Alike License. It uses material from the Wikipedia article Capillary action. Read more

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capillarity

cap·il·lar·i·ty

capillarity

capillarity

Capillary Action

capillarity

capillarity

capillarity

Capillary action

Etymology

Phenomena and physics of capillary action

Examples

Height of a meniscus

Liquid transport in porous media

Miscellaneous

See also

References