Capillary action

Capillary action

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Capillary action, or capilarity, is the ability of a liquid to flow against gravity where liquid spontanously rise in a narrow space such as between the hair of a paint-brush, in a thin tube, or in porous material such as paper or in some non-porous material such as liquified carbon fiber, or in a cell. This effect can cause liquids to flow against the force of gravity, sun or the magnetic field induction. It occurs because of inter-molecular attractive forces between the liquid and solid surrounding surface; If the diameter of the tube is sufficiently small, then the combination of surface tension
Surface tension
Surface tension is a property of the surface of a liquid that allows it to resist an external force. It is revealed, for example, in floating of some objects on the surface of water, even though they are denser than water, and in the ability of some insects to run on the water surface...

 (which is caused by cohesion
Cohesion (chemistry)
Cohesion or cohesive attraction or cohesive force is the action or property of like molecules sticking together, being mutually attractive...

 within the liquid) and force of adhesion
Adhesion is any attraction process between dissimilar molecular species that can potentially bring them in close contact. By contrast, cohesion takes place between similar molecules....

 between the liquid and container act to lift the liquid.


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"). 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 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
The meniscus is the curve in the upper surface of a liquid close to the surface of the container or another object, caused by surface tension. It can be either convex or concave. A convex meniscus occurs when the molecules have a stronger attraction to each other than to the material of the...

 forms. Adhesion
Adhesion is any attraction process between dissimilar molecular species that can potentially bring them in close contact. By contrast, cohesion takes place between similar molecules....

 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
Hydrology is the study of the movement, distribution, and quality of water on Earth and other planets, including the hydrologic cycle, water resources and environmental watershed sustainability...

, capillary action describes the attraction of water molecules to soil particles. Capillary action is responsible for moving groundwater
Groundwater is water located beneath the ground surface in soil pore spaces and in the fractures of rock formations. A unit of rock or an unconsolidated deposit is called an aquifer when it can yield a usable quantity of water. The depth at which soil pore spaces or fractures and voids in rock...

 from wet areas of the soil to dry areas. Differences in soil potential
Water potential
Water potential is the potential energy of water per unit volume relative to pure water in reference conditions. Water potential quantifies the tendency of water to move from one area to another due to osmosis, gravity, mechanical pressure, or matrix effects such as surface tension...

 () drive capillary action in soil.


Capillary action is also essential for the drainage of constantly produced tear
Tears are secretions that clean and lubricate the eyes. Lacrimation or lachrymation is the production or shedding of tears....

 fluid from the eye. Two canaliculi of tiny diameter are present in the inner corner of the eyelid
An eyelid is a thin fold of skin that covers and protects an eye. With the exception of the prepuce and the labia minora, it has the thinnest skin of the whole body. The levator palpebrae superioris muscle retracts the eyelid to "open" the eye. This can be either voluntarily or involuntarily...

, also called the lacrimal ducts
Nasolacrimal duct
The nasolacrimal duct carries tears from the lacrimal sac into the nasal cavity. Excess tears flow through nasolacrimal duct which drains into the inferior nasal meatus...

; 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 towel
Paper towel
A paper towel is an absorbent textile made from paper instead of cloth. Unlike cloth towels, paper towels are disposable and intended to be used only once. Paper towels soak up water because they are loosely woven which enables water to travel between them, even against gravity...

s absorb liquid through capillary action, allowing a fluid to be transferred from a surface to the towel. The small pores of a sponge
Sponge (tool)
A sponge is a tool, implement, utensil or cleaning aid consisting of porous material. Sponges are used for cleaning impervious surfaces. They are especially good absorbers of water and water-based solutions....

 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
A candle is a solid block or cylinder of wax with an embedded wick, which is lit to provide light, and sometimes heat.Today, most candles are made from paraffin. Candles can also be made from beeswax, soy, other plant waxes, and tallow...

 and lamp wicks
Candle wick
A candle wick is a string, cord, or wooden object that holds the flame of a candle. A candle wick works by capillary action, drawing the fuel to the flame. When the liquid fuel, typically melted candle wax, reaches the flame it then vaporizes and combusts. The candle wick influences how the...


Capillary action is observed in thin layer chromatography
Thin layer chromatography
Thin layer chromatography is a chromatography technique used to separate mixtures. Thin layer chromatography is performed on a sheet of glass, plastic, or aluminum foil, which is coated with a thin layer of adsorbent material, usually silica gel, aluminium oxide, or cellulose...

, 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
Mercury (element)
Mercury is a chemical element with the symbol Hg and atomic number 80. It is also known as quicksilver or hydrargyrum...

 and glass, the intermolecular force
Intermolecular force
Intermolecular forces are forces of attraction or repulsion which act between neighboring particles: atoms, molecules or ions. They are weak compared to the intramolecular forces, the forces which keep a molecule together...

s 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:
where is the liquid-air surface tension
Surface tension
Surface tension is a property of the surface of a liquid that allows it to resist an external force. It is revealed, for example, in floating of some objects on the surface of water, even though they are denser than water, and in the ability of some insects to run on the water surface...

 (force/unit length), θ is the contact angle
Contact angle
The contact angle is the angle at which a liquid/vapor interface meets a solid surface. The contact angle is specific for any given system and is determined by the interactions across the three interfaces. Most often the concept is illustrated with a small liquid droplet resting on a flat...

, ρ is the density
The mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...

 of liquid (mass/volume), g is local gravitational field strength
Gravitational field
The gravitational field is a model used in physics to explain the existence of gravity. In its original concept, gravity was a force between point masses...

 (force/unit mass), and r is radius
In classical geometry, a radius of a circle or sphere is any line segment from its center to its perimeter. By extension, the radius of a circle or sphere is the length of any such segment, which is half the diameter. If the object does not have an obvious center, the term may refer to its...

 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 = 9.8 m/s2. For these values, the height of the water column is
Thus for a 4 m (13.1 ft) diameter glass tube in lab conditions given above (radius 2 m (6.6 ft)), the water would rise an unnoticeable 0.007 mm (0.000275590551181102 in). However, for a 4 cm (1.6 in) diameter tube (radius 2 cm (0.78740157480315 in)), the water would rise 0.7 mm (0.0275590551181102 in), and for a 0.4 mm (0.015748031496063 in) diameter tube (radius 0.2 mm (0.0078740157480315 in)), the water would rise 70 mm (2.8 in).

Liquid transport in porous media

When a dry porous medium, such as a brick
A brick is a block of ceramic material used in masonry construction, usually laid using various kinds of mortar. It has been regarded as one of the longest lasting and strongest building materials used throughout history.-History:...

 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
where S is the sorptivity
John Philip described sorptivity as a measure of the capacity of the medium to absorb or desorb liquid by capillarity.Calculation of the true sorptivity required numerical iterative procedures dependent on soil water content and diffusivity....

 of the medium, with dimensions m/s1/2 or mm/min1/2. The quantity
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
Porosity or void fraction is a measure of the void spaces in a material, and is a fraction of the volume of voids over the total volume, between 0–1, or as a percentage between 0–100%...

 of the medium; the wetted length is then
Some authors use the quantity S/f as 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 of rising dampness. Some values for the sorptivity of building materials are in the table below.
Material Sorptivity
(mm min-1/2)
Aerated concrete 0.54
Gypsum plaster 3.50
Clay brick 1.16


Albert Einstein
Albert Einstein
Albert Einstein was a German-born theoretical physicist who developed the theory of general relativity, effecting a revolution in physics. For this achievement, Einstein is often regarded as the father of modern physics and one of the most prolific intellects in human history...

's first paper submitted in 1900 to Annalen der Physik
Annalen der Physik
Annalen der Physik is one of the oldest physics journals worldwide. The journal publishes original, peer-reviewed papers in the areas of experimental, theoretical, applied and mathematical physics and related areas...

 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).

See also

  • Young–Laplace equation
    Young–Laplace equation
    In physics, the Young–Laplace equation is a nonlinear partial differential equation that describes the capillary pressure difference sustained across the interface between two static fluids, such as water and air, due to the phenomenon of surface tension or wall tension, although usage on the...

  • Capillary pressure
  • Damp-proof course
  • Capillary fringe
    Capillary fringe
    The capillary fringe is the subsurface layer in which groundwater seeps up from a water table by capillary action to fill pores. Pores at the base of the capillary fringe are filled with water due to tension saturation. This saturated portion of the capillary fringe is less than total capillary...

  • Capillary wave
    Capillary wave
    A capillary wave is a wave traveling along the phase boundary of a fluid, whose dynamics are dominated by the effects of surface tension.Capillary waves are common in nature and the home, and are often referred to as ripples...

  • Frost flowers
    Frost flowers
    Frost flowers is the name commonly given to a condition in which thin layers of ice are extruded from long-stemmed plants in autumn or early winter...

  • Frost heaving
    Frost heaving
    Frost heaving results from ice forming beneath the surface of soil during freezing conditions in the atmosphere. The ice grows in the direction of heat loss , starting at the freezing front or boundary in the soil...

  • Needle ice
    Needle ice
    Needle ice is a phenomenon that occurs when the temperature of the soil is above and the surface temperature of the air is below . The subterranean liquid water is brought to the surface via capillary action, where it freezes and contributes to a growing needle-like ice column.The ice needles are...

  • Washburn's equation
    Washburn's equation
    In physics, the Washburn's equation describes capillary flow in a bundle of parallel cylindrical tubes; it is extended with some issues also to imbibition into porous materials...

  • Wick effect
    Wick effect
    The wick effect is the name given to the partial destruction of a human body by fire, when the clothing of the victim soaks up melted human fat and acts like the wick of a candle. The wick effect is a phenomenon that has been proven to occur under certain conditions, and thoroughly observed...

  • Bound water
    Bound water
    In hydrology, bound water, is an extremely thin layer of water surrounding mineral surfaces.Water molecules have a strong electrical polarity, meaning that there is a very strong positive charge on one end of the molecule and a strong negative charge on the other. This causes the water molecules...

  • Surface Tension
    Surface tension
    Surface tension is a property of the surface of a liquid that allows it to resist an external force. It is revealed, for example, in floating of some objects on the surface of water, even though they are denser than water, and in the ability of some insects to run on the water surface...