Cast your mind back to GCSE chemistry and you might remember the van der Waals force: The weak bond between molecules, caused by the way their electrons shift at the atomic level. Now, for the first time, those tiny forces have been measured between two atoms.
Van der Waals forces are caused by fluctuating electric fields around molecules and atoms, which can cause small attractive or repulsive forces to be generated. While they’re considered weak at the atomic level—ionic and covalent bonds are much stronger—they can add up to be surprisingly useful, allowing geckos to climb walls and strapless bras to provide support.
But researchers from the Swiss Nanoscience Institute and the University of Basel wanted to measure these forces between single atoms. To do that, they embedded different noble gas atoms—including argon, krypton and xenon—into a molecular grid made of copper atoms. The team refers to the voids within these girds as “nano-beakers of copper atoms in which the noble gas atoms are held in place like a bird egg.”
Then, using an atomic force microscope, the team were able to measure the small changing forces between the noble gas atoms when they were placed at different distances from each other. When comparing their measurements to those predicted by theoretical calculations, the team found their results to be broadly consistent—though there were instances where the forces between xenon atoms were twice what was expected. The team reckons this could be the result of weak covalent bonds forming, rather than unusually high van der Waals forces being generated.
These are the smallest forces ever detected between individual atoms, and the team will be able to use the results to understand the physical behaviour of atoms better than ever.