Capillary Action

Definition - What does Capillary Action mean?

Capillary action is the movement of water within the spaces of a porous material due to the forces of adhesion, cohesion and surface tension. It is the ability of a liquid to flow in narrow spaces without the assistance of, and sometimes in opposition to, external forces like gravity. Capillary action occurs when the adhesion to the walls is stronger than the cohesive forces between the liquid molecules.

Capillary action is also known as capillarity, capillary motion or wicking.

Corrosionpedia explains Capillary Action

Capillary action occurs because of intermolecular forces between the liquid and surrounding solid surfaces. It is caused by the pressure of cohesion and adhesion, which cause the liquid to work against gravity.

The effect of capillary action can be seen in:

  • Drawing up of liquids between the hairs of a paintbrush
  • Thin tubes
  • Porous materials such as paper
  • Some non-porous materials, such as liquefied carbon fiber
  • Cells

Capillary action has many significant applications like thin layer chromatography, in which a solvent moves vertically up a plate via capillary action. The small pores of a sponge act 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 as wicking fabrics, after the capillary properties of candle and lamp wicks. In brazing, capillary action causes a filler metal to be drawn into the space between workpieces.

Capillary action can cause corrosion. For example, rain water, laden with chloride, typically runs over a roof and is discharged into the gutters. Capillary action at the overlapped joint tends to pull water under the upper roof sheet, holding it there as it slowly dries out and concentrates the amount of chloride in the joint. With time, this can cause pitting corrosion, leading to failure of the roof panels.

Intruding water is the key problem in corrosion under insulation. Special care must be taken during design not to promote corrosion by permitting water to enter a system either directly or indirectly by capillary action.

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