The very weak attractive forces between neutral atoms and molecules are known as van der Waals forces. These forces are important for holding molecules together and influencing materials properties such as boiling and melting points, solubility, viscosity, and surface tension. Until recently, there had never been a direct measurement of the force between two atoms. However, in June 2012, L. Béguin and colleagues at the National Center for Scientific Research (CNRS) in France reported they had measured the van der Waals force between two atoms at various controlled distances and found the results to be in very good agreement with theoretical calculations. See also: Intermolecular forces
At any given moment, the electrons in an atom or molecule may be concentrated at one end or the other, making either end slightly positive or slightly negative (dipole moment). A polarized atom will induce a dipole moment of opposite polarity in another nearby atom, and both atoms will be electrostatically attracted to each other. Larger atoms or molecules have greater van der Waal forces because they have more electrons that can move over greater distances. See also: Dipole; Dipole-dipole interaction; Dipole moment
In the experiment, the researchers used rubidium (Rb) Rydberg atoms. Rb, atomic number 37, is a relatively large atom to begin with, and a Rydberg atom, which is an atom with one electron that has been excited to a very high energy state far from the nucleus, is even larger. As a result, Rb Rydberg atoms have very large dipole moments, so a strong van der Waals interaction would be expected. See also: Rubidium; Ultralong-range Rydberg molecules
Using laser cooling and trapping methods as well as some sophisticated optical techniques, the researchers trapped two Rb atoms a few micrometers apart. The distance R between the two trapped atoms was controlled by changing the incidence angle of the laser beams. Laser light was also used to excite the Rb atoms to Rydberg atoms. Because Rb Rydberg atoms have a large dipole moment, researchers were able to put them some relatively long distance apart and still measure the force. At very short distances, only one atom could be excited to a Rydberg state and no force could be measured. What the researchers observed was that as they changed the distance R between the atoms, the force changed as 1/R6, as is expected theoretically for van der Waals force. See also: Laser cooling
Even if physics survived without a known measurement of the van der Waals force for 140 years, it may be important to be able to measure it in the future because in nanotechnology very weak attractive forces become significant, for example, in the function of nanomaterials and nanodevices. See also: Nanotechnology