Have you ever wondered how geckos manage to stick to walls? The answer is remarkably simple: the van der Waals force.
The van der Waals force exists in all molecules that exhibit polarity, that is to say, it exists in all atoms that are negative on one side, and positive on the other. The cause of this polarity it a chemical property called electronegativity, the affinity a particular atom has for electrons. In water, for example, hydrogen has a high electronegativity relative to oxygen. This means that the electrons in a water molecule cluster around the hydrogen atoms, leading the hydrogen side of the molecule to be slightly more negative, and the oxygen side to be slightly more positive.
The charge imbalance in the molecule leads to cohesion, or the tendency of the substance to stick to itself and to other things. What happens is that for tenths of a second, each water molecule is drawn to every other molecule, leading the entire mass of water molecules to stick together. This momentary attraction is the van der Waals force.
Of course, the water molecules at the edge of the clump find themselves attracted to whatever is immediately outside the clump. Try pouring some water down your arm – you’ll notice quickly that regardless of what you do (short of flailing it about to shake off the water) the water will not fall straight down. It will always try to follow your arm down.
Water is not the only polar molecule, and is therefore also not the only molecule that has van der Waals forces. Detergents, for example, are huge molecules that have a non-polar end, and a polar end. Oil, which cannot be dissolved in water, can be dissolved in detergent. The detergent, in turn, because it is partly polar, can dissolve in water. The net effect is to make oil soluble in water.
Of course, plants and animals take advantage of van der Waals forces, too. Trees guide sap up their internal veins by relying on adhesion (a property quite similar to cohesion) to drive the entire mass of water to the leaves. Proteins and enzymes often rely on having small, polar regions to retain their structure – which is critical for the continued usefulness of both. Damage to the structure, known as denaturing, destroys either. And, finally, there is the gecko. It relies on tens of thousands of polarized hairs on its limbs to keep it to the wall.
