A physical manifestation of the surface tension of a liquid. The molecules of some liquids—such as water, for example—have a greater attraction for solid surfaces, such as glass, than for each other, and as a result these liquids can "creep" up or down a thin tube. A thin glass tube placed in a pan of water will result in a water level higher in the tube than that of the water in the pan, the difference in height being equivalent to the difference in intermolecular forces. In the tube (and even in the pan itself) the water surface will have a pronounced concave (U-shaped) meniscus (or curvature of the surface of the liquid) as the water molecules in contact with the glass sides of the tube have a greater affinity for the glass than for other water molecules. (In contrast, a liquid such as mercury, whose molecules have a greater affinity for each other than for other surfaces, will, when subjected to the same glass-tube experiment as the water, have a convex meniscus and a level less than that of the mercury in the surrounding pan.) Capillary action—also known as capillarity'—is the primary means by which plants survive and is the force that allows water to rise to the top of tall trees.
In printing processes, the degree of an ink's ability to demonstrate capillary action is one determinant of the amount of ink penetration into the surface of a substrate (especially a porous one—such as paper, which has many tiny capillaries throughout its surface and structure). Excessive capillary action by an ink causes such printing defects as strike-through, and excessive capillarity by an ink vehicle causes drying problems. On the plus side, capillary action is also one of the means by which ink transfer is effected in gravure printing (as capillarity moves the liquid ink from the engraved gravure cells to the substrate) and flexography (capillarity causing ink to flow from the pits of the inking anilox roller to the surface of the plate).