Physicists at the Swiss Nanoscience Institute and the University of Basel have succeeded in measuring the very weak van der Waals forces between individual atoms for the first time. To do this, they fixed individual noble gas atoms within a molecular network and determined the interactions with a single xenon atom that they had positioned at the tip of an atomic force microscope. As expected, the forces varied according to the distance between the two atoms; but, in some cases, the forces were several times larger than theoretically calculated. These findings are reported by the international team of researchers in Nature Communications. Read more at: http://phys.org/news/2016-05-physicists-van-der-waals-individual.html#jCp http://phys.org/news/2016-05-physicists-van-der-waals-individual.html
Does it not show that more work is required to be done even to understand the basic atomic and sub atomic interactions ? For decades students were reading about the present prevalent forces theory, without any iota of doubt about their exactness. This can have substantial implications on many aspects of science.
http://www.nature.com/ncomms/2016/160513/ncomms11559/full/ncomms11559.html Abstract Van der Waals forces are among the weakest, yet most decisive interactions governing condensation and aggregation processes and the phase behaviour of atomic and molecular matter. Understanding the resulting structural motifs and patterns has become increasingly important in studies of the nanoscale regime. Here we measure the paradigmatic van der Waals interactions represented by the noble gas atom pairs Ar–Xe, Kr–Xe and Xe–Xe with a Xe-functionalized tip of an atomic force microscope at low temperature. Individual rare gas atoms were fixed at node sites of a surface-confined two-dimensional metal–organic framework. We found that the magnitude of the measured force increased with the atomic radius, yet detailed simulation by density functional theory revealed that the adsorption induced charge redistribution strengthened the van der Waals forces by a factor of up to two, thus demonstrating the limits of a purely atomic description of the interaction in these representative systems.
Very nice experiment and interesting that the magnitude of the force is apparently somewhat understated by the conventional model of atomic mutual polarisation. The article tantalisingly says the researchers are now suggesting there may be a hitherto unsuspected charge-transfer component, in addition to the electrostatic dipole-dipole attraction, and thus some sort of tenuous bond character (ionic? covalent?) may also contribute. That, presumably, is what is being referred to in the abstract PB has quoted, in which they observe that the results demonstrate "the limits of a purely atomic description of the interaction in these representative systems". Presumably they chose inert gas atoms to try to exclude any interatomic bonding effects. It's a pity we can't read that part of their paper in more detail.
As a trivial aside, I am reminded of Van de Waals forces every evening when we see our resident geckos chase insects or battle it out for territory on the walls of our living room after dark. When a gecko catches something, the "pounce" involves keeping all four feet on the wall (obviously) and is so fast that it seems instantaneous. I understand about as much about the forces as the geckos do.
