van der Waals – the unseen force that holds objects together. What is van der Waals? you make ask, and why the fuss?
van der Waals is a type of intermolecular force – a force that exists between two neighbouring molecules.
First postulated by the Dutch physicist Johannes Diederick van der Waals in 1873, van der Waals forces are today widely used to explain the (unexpected) behaviour of liquids and gases. van der Waals increases the ‘stickiness’ between molecules. Thus,for example, liquids may have higher than expected boiling points because extra heat energy is need to break the van der Waals forces between the liquid molecules.
van der Waals forces are created between molecules by a domino-type effect. The movement of electrons in an atom creates dipoles in the atom – regions that are either positively (electron-poor) or negatively (electron-rich) charged.
Some dipoles are permanent, and other temporary. van der Waals arises due to temporary dipoles on atoms. The positively charged region of an atom attracts electrons (which have a negative charge) from the neighbouring atom, setting up a dipole in that atom. This sets off a domino-like cascade of dipoles throughout the atoms of a substance.
As electrons are continuously in motion, the dipole around the first atom shifts when the electrons move to the opposite side, setting off a cascade effect in the other direction. Though weak, the temporary effect created by attraction between dipoles is strong enough to influence the behaviour of solids, liquids and gases.
Iodine is an example of a molecule held together as a solid by large number of van der Waals forces. When a solid piece of iodine is exposed to a room temperature, the heat energy available in the air is sufficient to break the van der Waals forces between iodine molecules. The iodine ‘sublimes’ into purple vapour – i.e. turns directly from solid to gas. Dry ice is another example of this phenomenon.
Did you know…..?
Geckos use van der Waals to walk on vertical surfaces and even upside-down (now you know where Spiderman gets his superpowers from!) The thousands of microscopic hairz on their toepads can attach to surfaces using van der Waals attractions, allowing the gecko to climb up or down any surface with ease.
If you are interested in understanding how this works, here is a great article: How Geckos Stick on der Waals
and look… you can even buy dirt-repelling Gecko-inspired tape!
DAPRA makes Gecksin – can you imagine how useful this could be? The next Spiderman might actually be real!
Bristol Science Tutor 