Capillarity is sometimes referred to as capillary action. Capillary action is the ability of a liquid, such as water, to move up or down a narrow space such as tube without the assistance of external forces and against the force of gravity. The capillarity of water through the xylem tissue of plants is important because it allows plants to transport water and nutrients from their roots to structures that are located all the way at the top of the plant. Here, other uses of capillarity are discussed.
In water, capillary action is due to the hydrogen bonds amongst the molecules of water. Hydrogen bonds exist between a hydrogen atom that is attached to an electronegative atom in one molecule and an electronegative atom within a second molecule.
Hydrogen bonds result from electronegativity differences. Electronegativity is the likelihood that an atom will attract a pair of bonded electrons. The single oxygen atom within water is much more electronegative than the hydrogen atoms. Thus, the oxygen in a water molecule pulls the electrons towards it more than the hydrogen atoms do. Therefore, oxygen becomes slightly negative. Likewise, the hydrogens in a water molecule become slightly positive.
As mentioned above, the partially positive and negative charges that occur because of the hydrogen bonds within a water molecule result in water molecules attracting other another. When molecules of a substance attract one another it is called cohesion.
Additionally, the partial charges within a water molecules also cause them to attract other substances that also have slightly positive and negative charges. This is called adhesion.
Capillary action is a result of the adhesion of water molecules to the sides of a small tube and the cohesion of water molecules to one another. As a result of these forces, water “sticks” to the side of a small tube and to one another. When the adhesion of water to the surface of a tube is stronger than the cohesion of water molecules to one another, the water molecules move up the tube. This upward movement is called capillary action.