Surface Tension

Surface tension is the result of the cohesive forces that attract water molecules to one another. This surface force keeps objects which are more dense than water (meaning they should not float) from sinking into it. The surface tension of water makes it puddle on the ground and keeps it in a droplet shape when it falls.

If you use a table fork to carefully place a paper clip on the surface of some clean water, you will find that the paper clip, although more dense than water, will remain on the water's surface. If you look closely, you will see that the surface is bent by the weight of the paper clip much as your skin bends when you push on it with your finger.

A molecule inside a volume of water is pulled equally in all directions by the other molecules of water that surround it. A molecule on the water's surface, on the other hand, is pulled by the molecules below it and to its sides. The net force on this surface molecule is inward. The result is a surface that behaves as if it were under tension. If a glob of water with an irregular shape is created, the inward forces acting on the molecules at its surface quickly pull it into the smallest possible volume it can have, which is a sphere.

A simple apparatus can be used to measure the forces on a liquid's surface. A force is applied to a wire of known length which forms a circle parallel to the surface of the water. The force balances the surface forces acting on each side of the wire. The surface tension of the liquid, g, is defined as the ratio of the surface force to the length of the wire (the length along which the force acts). For this kind of measurement, g = F/2L. The force, F, applied to the wire is that required to balance the surface forces; The long legs of a waterstrider (Gerris paludum) allow it to distribute its weight across the water and be supported by surface tension. © Hermann Eisenbeiss, National Audubon Society Collection/Photo Researchers, Inc. Reproduced by permission. L is the length of the wire. The 2 appears in the denominator because there is a surface film on each side of the wire. Thus, surface tension has units of force per length—dynes/cm, newtons/m, ounces/inch, etc.

Water has a relatively high surface tension that, not surprisingly, decreases with increasing temperature. The increased kinetic energy of the molecules at higher temperature would oppose the forces of cohesion. In fact, at the boiling temperature, the kinetic energy of molecules is sufficient to overcome their cohesive forces of attraction and the molecules separate to form a gas.