Drawn Together by the Casimir Effect
On average, an atom has no net charge. But at any moment in time, an uneven distribution of its electrons can induce it to become a dipole, with one end positive and the other negative (called a polarized state). Dipoles can then interact with other atoms, further inducing them to also become polarized. These short-range attractions are called van der Waals forces.
When many atoms and molecules interact this way, they create what's called the Casimir effect. In essence, this effect explains why two parallel objects are attracted to each other even in a vacuum. The animation interprets what happens to the electromagnetic field because of quantum effects and virtual photons, to show what results when two plates are brought close together in such an environment:
Active Sigma Xi members and American Scientist magazine subscribers can view the related article here: http://www.americanscientist.org/issues/feature/2014/4/engines-powered-by-the-forces-between-atoms/.