What do these two things have in common – the amazing ability of the gecko to stay attached to a window with only one toe and the ability of a spider to stay on the ceiling? Give up? Well, the answer is amazing forces called van der Waals forces, named after Dutch scientist Johannes Diderik van der Waals.
The cool thing about adhesive power that is based on van der Waals forces is that it is not affected by rain, sunlight, etc. In fact, the only thing that really affects this adhesive is the distance between the two objects. Therefore, for it to work, there must be close contact between the two surfaces. This force acts between individual molecules that are within a nanometer of each other.
The tiny hairs on the gecko’s feet (setae) are split into as many as 1,000 branches whose tips are only 200 nanometers wide. When you look at the van der Waals forces that are actually acting on an individual tip, the force seems inconsequential; however, when you consider the adhesion force of a billion or more tips, you can understand why that gecko can stick to the window. The movement of electrons in atoms and molecules is what causes these forces to work because they become dipolar (having a positive pole and a negative pole). These poles are attracted to each other (similar to the attraction between the north and south poles of magnets).
There are actually three kinds of van der Waals forces: dipole-dipole, dispersion and hydrogen bonding. We will talk about dipole-dipole first. Although the molecules of some substances are electrically neutral, they may in fact be permanent electric dipoles. This happens when fixed distortion in the distribution of electric charge causes one side of the molecule to be somewhat positive and the other side of the molecule to be somewhat negative. Let’s look at a water molecule, for example. Water is a fairly neutral substance. However, because the charge in the molecule is not symmetrical, this leads to a microscopic separation of negative and positive charge centers (a dipole moment).
Dispersion forces exist between nonpolar molecules, i.e. chlorine gas which has two chlorine atoms in which the electrons are equally shared. Even though there may be a tiny instance when the electrons are dominate on one side, this situation disappears quickly as the electrons are moving very fast. It is these temporary dipoles that allow the temporary negative side of one molecule to attract the temporary positive side of another molecule.
Hydrogen bonding is actually just dipole-dipole force with a special name. This bonding occurs between any molecule with a bond between a hydrogen atom and any of oxygen/fluorine/nitrogen. The bond is extremely strong (at least compared to the other van der Waals forces) and extremely one-sided.
The dipole-dipole is a little weaker than hydrogen bonding, and the dispersion force is the weakest of all. The truth is, though, that all van der Waals forces are weaker than chemical bonds and can easily be overcome or disrupted by random thermal motion. However, although van de Waals forces are weak, life would be quite different without them, as these forces affect such things as water condensing from vapor into solid or liquid forms, snowflakes coming in such amazing shapes, melting points and boiling points, surface tension, etc. Therefore, van de Waals forces are weak forces with a very strong impact on our lives.
