Tendency for water to interact with and especially attach to polar substances.

Though adhesion by water to non-water is important to the functioning of various , the concept of adhesion is more general than just water-non-water interactions. Instead, adhesion is simply the of substances via to substances unlike themselves. See by contrast cohesion.

On surfaces, water will form smaller given greater adhesion and indeed will surfaces to which water is especially attracted. A consequence of substantial cohesion is a process known as , where water is drawn against other , particularly, , via its adherence to surfaces.

The force exerted by capillary action is relative small. As a consequence, it only provides substantial movement of water given relatively high surface to water-volume ratios, which indeed is why this works within – narrow vessels – versus larger-diameter tubes.