This Force Is Strong with Geckos

Humans have many scientific achievements, but when it comes to the van der Waals force, we have been bested by geckos.

Johannes Diderik van der Waals was a physicist and chemist working at the turn of the last century who was interested in how gases interacted. While there was a set of laws set up to explain the pressure, volume, and temperature of gases, it worked with ideal gases. The gas molecules were all points with no real volume, and they all moved independently of each other, experiencing no intermolecular forces. Van der Waals didn't buy that happening in the real world, and so with time and research he found ways to describe how actual gases — and consequently all kinds of other molecules — behaved.

One of the best-known results of his investigations is the van der Waals force, which explains why two seemingly normal and balanced molecules will suddenly stick together. Such behavior isn't strange in molecules like water, but water is polar. Polar molecules have equal numbers of electrons and protons, but have them in slightly odd arrangements. Water, famously, is a bit of a v-shape, with the oxygen at the apex of the v and the two hydrogen atoms forming the legs of the v. The oxygen holds the available electrons close to itself, and so is a bit negatively charged, while the hydrogen molecules are a bit positive. Since positive charge attracts negative charge, water molecules are lightly attracted to each other, which helps them condense into liquid.

This Force Is Strong with Geckos

Molecules that undergo the van der Waals force don't have any shape or distribution of charge that gives them a steady polarity. They have a polarity that can be temporarily induced. Electrons are flying around a molecule, randomly, at all times. At some point, they can aggregate on one side of the molecule. When they do, they give the molecule a bit of a negative charge on one side, and a positive charge on the other side.

That's no big deal, and should have no lasting effect, until another molecule comes in range. The electrons in the new molecule are influenced by the presence of a positive charge nearby. They all speed over to the side of the molecule nearest to the positive side of the original molecule. While doing so they repel the electrons in the first molecule — so instead of being a temporary state of polarity, the two molecules can become a relatively stable system. More and more temporarily polar molecules can build up around the first two. The positive sides of one molecule attract the negative sides of another molecule, and the molecules are drawn together. That's the van der Waals force.

Geckos use this force to scale walls; the pads of their feet are subdivided into tiny hairs that induce this state of temporary polarity in the molecules of the walls they climb. Forces like that are tough to turn on and off. The geckos can't manage that. They can only manage to control how much of the hairs are in contact with the surface they climb.

When they press their feet down, they draw them back, along the wall, as if they were brushing it. This causes the tiny hair on their footpads to get pulled sideways, exposing more surface area to the wall. As the gecko draws its foot back, and more of the surface of its hair comes into contact with the wall, it increases the amount of force holding the gecko to the wall. To let go of the wall, it just pushes its foot forward to decrease the surface area in contact with the wall. The force holding the foot to the wall decreases, and the gecko can pull its foot off the all. The van der Waals force, and the use the gecko makes of it, has the added bonus of keeping the gecko stable. The harder you shake the surface it's on, the more its feet will slip against the surface, and the more force it can use to cling on tight.

Top Image: Steve Evans.

[Via Mechanisms of Gecko Adhesion, How Geckos Stick to der Waals, Gecko Adhesion]