The movement of a liquid along the surface of a solid caused by the attraction of molecules of the liquid to the molecules of the solid.
A Closer Look The paper towel industry owes its existence to capillary action, both for the way paper towels soak up liquids and for the trees out of which the towels are made. Molecules of water are naturally attracted to each other and form temporary hydrogen bonds with each other; their attraction for each other on the surface of a liquid, for example, gives rise to surface tension. But they are also attracted in a similar way to other molecules, called hydrophilic molecules, such as those in the sides of a narrow glass tube inserted into a cup of water, in the fibers of a towel, or in the cells of tree tissue known as xylem. These attractive forces can draw water upward against the force of gravity to a certain degree. However, they are not strong enough to draw water from the roots of a tree to its highest leaves. An additional, related force, referred to as transpiration pull, is required to do that. As water evaporates from the tiny pores, or stomata, of leaves, water from adjacent cells is drawn in to replace it by osmosis. Again, intermolecular attractive forces cause other water molecules to follow along, ultimately drawing water up from the roots of the tree.