Surface Tension

To understand surface tension, you should make sure that you understand floating and sinking first.

As we know, something that is more dense than water should sink. However, some things can stay on th top of water even though they are more dense. This includes water striders, pepper grains and even pins and needles. It is important to notice, however, that they are NOT floating. In other words they did not sink until they had displaced their weight in water. These things are actually resting on the surface of the water itself. This can be seen in the picture below of a water strider.

The distortions around the feet of the water strider show the indentations it is making in the surface of the water. The water strider is standing on the water, it is NOT floating.

This is possible because water, and some other liquids, have a “skin” that is denser and stronger than the rest of the liquid. It is this skin that holds up the water strider. The name of this “skin” is surface tension.

Remember that water is shaped as a tetrahedron and is polar with two of the four points of the tetrahedron somewhat positive and two somewhat negative.

Under the surface of water, a molecule is pulled in all directions because it is surrounded by other polar water molecules in all directions.

On the surface of water, however, the molecule is pulled down into the liquid, but is not pulled up into the air.

The result of this uneven force is that surface molecules are pulled in toward the liquid, making the surface denser than the rest of the liquid. The closeness of these molecules means that the attractions between them are stronger than normal and this is what causes the skin on which the water strider stands.