Capillary action

Capillary action(sometimes calledcapillarity,capillary motion,capillary rise,capillary effect,orwicking) is the process of aliquidflowing in a narrow space without the assistance of external forces likegravity.

Capillary water flow up a 225 mm-high porous brick after it was placed in a shallow tray of water. The time elapsed after first contact with water is indicated. From the weight increase, the estimated porosity is 25%.

Capillary action ofwater(polar) compared tomercury(non-polar), in each case with respect to a polar surface such as glass (≡Si–OH)

The effect can be seen in the drawing up of liquids between the hairs of a paint-brush, in a thin tube such as astraw,in porous materials such as paper and plaster, in some non-porous materials such as clay and liquefiedcarbon fiber,or in abiological cell.

It occurs because ofintermolecular forcesbetween the liquid and surrounding solid surfaces. If the diameter of the tube is sufficiently small, then the combination ofsurface tension(which is caused bycohesionwithin the liquid) andadhesive forcesbetween the liquid and container wall act to propel the liquid.

Etymology

Capillary comes from the Latin word capillaris, meaning "of or resembling hair". The meaning stems from the tiny, hairlike diameter of a capillary.

History

The first recorded observation of capillary action was byLeonardo da Vinci.A former student ofGalileo,Niccolò Aggiunti,was said to have investigated capillary action.In 1660, capillary action was still a novelty to the Irish chemistRobert Boyle,when he reported that "some inquisitive French Men" had observed that when a capillary tube was dipped into water, the water would ascend to "some height in the Pipe". Boyle then reported an experiment in which he dipped a capillary tube into red wine and then subjected the tube to a partial vacuum. He found that the vacuum had no observable influence on the height of the liquid in the capillary, so the behavior of liquids in capillary tubes was due to some phenomenon different from that which governed mercury barometers.

Others soon followed Boyle's lead.Some (e.g.,Honoré Fabri,Jacob Bernoulli) thought that liquids rose in capillaries because air could not enter capillaries as easily as liquids, so the air pressure was lower inside capillaries. Others (e.g.,Isaac Vossius,Giovanni Alfonso Borelli,Louis Carré,Francis Hauksbee,Josia Weitbrecht) thought that the particles of liquid were attracted to each other and to the walls of the capillary.

Although experimental studies continued during the 18th century,a successful quantitative treatment of capillary actionwas not attained until 1805 by two investigators:Thomas Youngof the United KingdomandPierre-Simon Laplaceof France.They derived theYoung–Laplace equationof capillary action. By 1830, the German mathematicianCarl Friedrich Gausshad determined the boundary conditions governing capillary action (i.e., the conditions at the liquid-solid interface).In 1871, the British physicistSir William Thomson(later Lord Kelvin) determined the effect of themeniscuson a liquid'svapor pressure—a relation known as theKelvin equation.German physicistFranz Ernst Neumann(1798–1895) subsequently determined the interaction between two immiscible liquids.

Albert Einstein's first paper, which was submitted toAnnalen der Physikin 1900, was on capillarity.

Phenomena and physics

Moderate rising damp on an internal wall

Capillary flow experiment to investigate capillary flows and phenomena aboard theInternational Space Station

Capillary penetration in porous media shares its dynamic mechanism with flow in hollow tubes, as both processes are resisted by viscous forces.Consequently, a common apparatus used to demonstrate the phenomenon is thecapillary tube.When the lower end of a glass tube is placed in a liquid, such as water, a concavemeniscusforms.Adhesionoccurs between the fluid and the solid inner wall pulling the liquid column along until there is a sufficient mass of liquid forgravitational forcesto overcome these intermolecular forces. The contact length (around the edge) between the top of the liquid column and the tube is proportional to the radius of the tube, while the weight of the liquid column is proportional to the square of the tube's radius. So, a narrow tube will draw a liquid column along further than a wider tube will, given that the inner water molecules cohere sufficiently to the outer ones.

Examples

In the built environment, evaporation limited capillary penetration is responsible for the phenomenon ofrising dampinconcreteandmasonry,while in industry and diagnostic medicine this phenomenon is increasingly being harnessed in the field ofpaper-based microfluidics.

In physiology, capillary action is essential for the drainage of continuously producedtearfluid from the eye. Two canaliculi of tiny diameter are present in the inner corner of theeyelid,also called thelacrimal ducts;their openings can be seen with the naked eye within the lacrymal sacs when the eyelids are everted.

Wicking is the absorption of a liquid by a material in the manner of a candle wick.

Paper towelsabsorb liquid through capillary action, allowing afluidto be transferred from a surface to the towel. The small pores of aspongeact as small capillaries, causing it to absorb a large amount of fluid. Some textile fabrics are said to use capillary action to "wick" sweat away from the skin. These are often referred to aswicking fabrics,after the capillary properties ofcandleand lampwicks.

Capillary action is observed inthin layer chromatography,in which a solvent moves vertically up a plate via capillary action. In this case the pores are gaps between very small particles.

Capillary action drawsinkto the tips offountain pennibsfrom a reservoir or cartridge inside the pen.

With some pairs of materials, such asmercuryand glass, theintermolecular forceswithin the liquid exceed those between the solid and the liquid, so aconvexmeniscus forms and capillary action works in reverse.

Inhydrology,capillary action describes the attraction of water molecules to soil particles. Capillary action is responsible for movinggroundwaterfrom wet areas of the soil to dry areas. Differences in soilpotential($\Psi _{m}$) drive capillary action in soil.

A practical application of capillary action is the capillary action siphon. Instead of utilizing a hollow tube (as in most siphons), this device consists of a length of cord made of a fibrous material (cotton cord or string works well). After saturating the cord with water, one (weighted) end is placed in a reservoir full of water, and the other end placed in a receiving vessel. The reservoir must be higher than the receiving vessel.A related but simplified capillary siphon only consists of two hook-shaped stainless-steel rods, whose surface is hydrophilic, allowing water to wet the narrow grooves between them.Due to capillary action and gravity, water will slowly transfer from the reservoir to the receiving vessel. This simple device can be used to water houseplants when nobody is home. This property is also made use of in thelubrication of steam locomotives:wicks ofworsted woolare used to draw oil from reservoirs into delivery pipes leading to thebearings.

In plants and animals

Capillary action is seen in many plants, and plays a part intranspiration.Water is brought high up in trees by branching; evaporation at the leaves creating depressurization; probably byosmotic pressureadded at the roots; and possibly at other locations inside the plant, especially when gathering humidity withair roots.

Capillary action for uptake of water has been described in some small animals, such asLigia exoticaandMoloch horridus.

Height of a meniscus

Capillary rise of liquid in a capillary

Water height in a capillary plotted against capillary diameter

The heighthof a liquid column is given byJurin's law

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

where$\scriptstyle \gamma$is the liquid-airsurface tension(force/unit length),θis thecontact angle,ρis thedensityof liquid (mass/volume),gis the localacceleration due to gravity(length/square of time), andris theradiusof tube.

Asris in the denominator, the thinner the space in which the liquid can travel, the further up it goes. Likewise, lighter liquid and lower gravity increase the height of the column.

For a water-filled glass tube in air at standard laboratory conditions,γ= 0.0728 N/mat 20°C,ρ= 1000 kg/m³,andg= 9.81 m/s².Because waterspreadson clean glass, the effective equilibrium contact angle is approximately zero.For these values, the height of the water column is

$h\approx {{1.48\times 10^{-5}\ {\mbox{m}}^{2}} \over r}.$

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

Capillary rise of liquid between two glass plates

The product of layer thickness (d) and elevation height (h) is constant (d·h= constant), the two quantities areinversely proportional.The surface of the liquid between the planes ishyperbola.

• Water between two glass plates
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Liquid transport in porous media

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

When a dry porous medium is brought into contact with a liquid, it will absorb the liquid at a rate which decreases over time. When considering evaporation, liquid penetration will reach a limit dependent on parameters of temperature, humidity and permeability. This process is known as evaporation limited capillary penetrationand is widely observed in common situations including fluid absorption into paper and rising damp in concrete or masonry walls. For a bar shaped section of material with cross-sectional areaAthat is wetted on one end, the cumulative volumeVof absorbed liquid after a timetis

$V=AS{\sqrt {t}},$

whereSis thesorptivityof the medium, in units of m·s^−1/2or mm·min^−1/2.This time dependence relation is similar toWashburn's equationfor the wicking in capillaries and porous media.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-calledwet front,is dependent on the fractionfof the volume occupied by voids. This numberfis theporosityof the medium; the wetted length is then

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

Some authors use the quantityS/fas the sorptivity.

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 ofrising dampness.Some values for the sorptivity of building materials are in the table below.

Sorptivity of selected materials (source:)

Material	Sorptivity (mm·min−1/2)
Aerated concrete	0.50
Gypsum plaster	3.50
Clay brick	1.16
Mortar	0.70
Concrete brick	0.20

See also

• Bond number– Dimensionless number in fluid dynamicsPages displaying short descriptions of redirect targets
• Bound water– Thin layer of water surrounding mineral surfaces.
• Capillary fringe– Subsurface layer in which groundwater seeps up from a water table by capillary action
• Capillary pressure– Pressure between two fluids from forces between the fluids and tube walls
• Capillary wave– Wave on the surface of a fluid, dominated by surface tension
• Capillary bridges– Minimised surface of liquid commecting two wetted objects
• Damp proofing– Type of moisture control in building construction
• Darcy's law– Equation describing the flow of a fluid through a porous medium
• Frost flower– Thin layer of ice extruded from a plant
• Frost heaving– Upwards swelling of soil during freezing
• Hindu milk miracle– 1995 alleged miracle incidentsPages displaying short descriptions of redirect targets
• Krogh model
• Porosimetry– Measurement and characterization of the porosity of a material
• Needle ice– Ice column formed when liquid groundwater rises into freezing air
• Surface tension– Tendency of a liquid surface to shrink to reduce surface area
• Washburn's equation– Equation describing the penetration length of a liquid into a capillary tube with time
• Young–Laplace equation– Describing pressure difference over an interface in fluid mechanics

Further reading

• de Gennes, Pierre-Gilles; Brochard-Wyart, Françoise; Quéré, David (2004).Capillarity and Wetting Phenomena.Springer New York.doi:10.1007/978-0-387-21656-0.ISBN978-1-4419-1833-8.