# Capillary Action: Definition and Examples

A spontaneous flow of liquid that doesn't require gravity to work

Capillary action is defined as the spontaneous flow of a liquid into a narrow tube or porous material. This movement does not require the force of gravity to occur. In fact, it often acts in opposition to gravity. Capillary action is sometimes called capillary motion, capillarity, or wicking.

Capillary action is caused by the combination of cohesive forces of the liquid and the adhesive forces between the liquid and tube material. Cohesion and adhesion are two types of intermolecular forces. These forces pull the liquid into the tube. In order for wicking to occur, a tube needs to be sufficiently small in diameter.

Examples of capillary action include the uptake of water in paper and plaster (two porous materials), the wicking of paint between the hairs of a paintbrush, and the movement of water through sand.

### Fast Facts: History of Capillary Action Study

• Capillary action was first recorded by Leonardo da Vinci.
• Robert Boyle performed experiments on capillary action in 1660, noting a partial vacuum had no effect on the height a liquid could obtain via wicking.
• A mathematical model of the phenomenon was presented by Thomas Young and Pierre-Simon Laplace in 1805.
• Albert Einstein's first scientific paper in 1900 was written on the subject of capillarity.

## See Capillary Action Yourself

An excellent and easy demonstration of capillary action is done by placing a celery stalk in water. Color the water with food coloring and observe the progress of the dye up the celery stalk.

The same process may be used to color white carnations. Trim the bottom of a carnation stem to make sure it can absorb water. Place the flower in dyed water. The color will migrate via capillary action all the way to the flower petals.

A less dramatic but more familiar example of capillary action is the wicking behavior of a paper towel used to wipe up a spill.

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