By Jo Stuart, UNE Discovery

Have you ever wondered how geckos can climb walls and stick to ceilings? You may be thinking of suction cups or some sort of sticky serum on their toes, but it’s actually a much more elegant solution, using intermolecular attraction and teeny tiny toe pad forests!

Gecko – feet on glass. Photo credit: Wikimedia: Bjørn Christian Tørrissen

It’s all to do with van der Waals forces. Johannes Diderik van der Waals was a Dutch physicist who, in 1873, proposed a theory of weak intermolecular electrical forces (that is, forces that act between molecules because of their electrical charges) to explain the properties of gases. There are few different forces that fall under the ‘van der Waals’ label, but the precise nature of the one that’s relevant to geckos was investigated and explained further by Polish-born physicist Fritz London in 1930.

The basic principle is that within all molecules there are both positive and negative charges and under certain circumstances those charges can attract molecules to each other. The positive charges in a molecule come from the nuclei of the component atoms, and the negative charges come from their electrons. The molecular structure of some molecules means that there is always one side of the molecule that is positively and one that is negatively charged. These are called permanent dipoles and they’re a bit like magnets with a north and a south end. Most molecules, however, become temporary dipoles under various conditions, including being really close to another molecule such that the positive charge in one molecule is attracted to the negative charge of the molecule next to it.

As you can imagine, van der Waals forces are everywhere, and they affect everything from the boiling point of water to how microfibre cloths are able to pick up dust without scratching your glasses. However, these forces are quite weak and have a very short range, which means that two molecules have to be really close together for them to have a measurable effect.

Image credit:  University of Wisconsin Chem 103 text – modification of work by “JC*+A!”/Flickr

So, what does all this have to do with geckos? Well, each of their toe pads is designed to maximise the surface area that is in contact with the surface they’re sticking to. As you can see in the diagram and photo, their toes have rows and rows of tiny little keratin tubes called setae (approx. 25 millionths of a metre wide), each of which branches out at the tip into even smaller spatulae (approx. 150 billionths of a metre wide), like a teensy little forest on each tiny toe. This creates a huge amount of surface area available to press onto whatever a gecko is climbing on.

When those tiny spatulae spread out on a surface – even one as smooth as glass or Teflon – their molecules and those of the surface get close enough to become temporary dipoles and attract one another. And the cumulative effect of all those weak forces acting on the forest of setae on each toe is enough to keep our little lizard friends safe and sound on our ceilings and walls.

Sources:

Van der Waals forces | Intermolecular Interactions & Applications | Britannica

Ge, Liehui, Sunny Sethi, Lijie Ci, Pulickel M. Ajayan, and Ali Dhinojwala. “Carbon Nanotube-Based Synthetic Gecko Tapes.” Proceedings of the National Academy of Sciences of the United States of America 104, no. 26 (2007): 10792–95. http://www.jstor.org/stable/25436017.

Autumn, Kellar, Metin Sitti, Yiching A. Liang, Anne M. Peattie, Wendy R. Hansen, Simon Sponberg, Thomas W. Kenny, Ronald Fearing, Jacob N. Israelachvili, and Robert J. Full. “Evidence for van Der Waals Adhesion in Gecko Setae.” Proceedings of the National Academy of Sciences of the United States of America 99, no. 19 (2002): 12252–56. http://www.jstor.org/stable/3073201.