2. Capillarity

Surface tension: the greater attraction of water molecules for each other than the air above at liquid-air interfaces primarily due to cohesion.

● Adhesion and surface tension together cause the phenomenon called capillarity--the movement of water up a wick made of hydrophilic solid materials.

● Capillary movement takes place in any direction.

● The height of capillary rise in a tube is directly proportional to the liquid's surface tension and adhesion to the solid surface, but inversely proportional to the tube radius and the density of the liquid.

Everyday evidences of water’s surface tension (left) as insects walk on water and do not sink, and of forces of cohesion and adhesion (right) as a drop of water is held between the fingers. (Photos courtesy of R. Weil)