Fill some water in a glass and dye it with ink. Then put a thin, colourless straw into the water. If it is thin enough, the liquid in the straw will rise a little and the level will be higher than in the rest of the water.

This effect, where water rises in a tube with a very small diameter (so-called capillaries), is called the capillary effect. The thinner the capillary is, the higher the water is dragged upwards.

Attracting forces, called adhesion forces, act between the water molecules and the wall of the vessel. These forces are more powerful than the forces between the water molecules, therefore the water is dragged upwards along the vessel wall until the weight of the water column is in balance with the adhesion force. Consequently, the thinner the straw is, the higher the water column that can be retained.

In bigger vessels, the adhesion forces can only be noticed at the vessel wall. There the water forms an edge that is slightly curved upwards. It is said that the water wets the vessel. You can observe this phenomenon when you watch the edge of the water surface closely.

Not all liquids behave like water. In liquid mercury, for example, the forces between the particles are stronger than the forces between the particles and the vessel wall. As a result, mercury has a lower liquid level than the rest of the liquid in a capillary. Mercury does not wet the vessel.

In the picture you can see a vessel filled with mercury on the left side and one filled with water on the right side. By contrast, in the pointed corner the water is dragged upwards and the mercury is dragged downwards.

Many substances have small hollow spaces that act like capillaries and in which the water can rise. A sponge, a piece of blotting-paper, or a piece of fabric that is dipped into water with only one corner will be soaked completely after some time because of the capillary effect. If the walls of a house are not made watertight, the water can rise in the bricks and dampen the wall.

Place a lump of sugar in a flat bowl with raspberry juice and watch how the liquid rises due to the capillary effect!

The capillary effect is also very important for plants. Underground water rises up to the roots through hollow spaces in the ground. From there, it is transported upwards to the leaves through thin, tube-like vascular bundles. Not only the capillary effect, but also (and mainly) a pull, which is generated as water constantly evaporates from the leaves, is responsible for transporting the water against gravity.