Why does water bead up better on a well waxed car?
(Lansing State Journal, August 10, 1994)
Water molecules also can be attracted to other substances, such as metal or dirt, especially if they have some static charge on them. This is called adhesion.
Lastly, some substances are not at all attracted to water, and even repel it. These include oils, fats and waxes; all of which are called non-polar substances.
When water falls on an unwaxed car, the forces of adhesion are almost as strong as the forces of cohesion, and the water spreads out. Furthermore, if the painted surface is not perfectly smooth, water can be channeled for some distance along tiny ridges and valleys. This is particularly true if there is dirt on the car. The dirt itself may be charged, and attract water even more. These tiny flaws may not be readily visible without a magnifying glass, but you can sense a rough surface when you run your hand over it. On such a rough surface, drops of water appear flat and wide and often uneven.
Car wax, when applied properly to a clean car, fills in the larger scratches and layers the whole car. The chemical structure of the wax prevents water from penetrating to the surface of the car. Because the wax itself is hydrophobic (literally "fears water"), the forces of adhesion are much less than the forces of cohesion. So, water is more likely to stick to itself, and water beads up higher and rounder than on an unwaxed car.