The traditional explanation states that when two objects come in contact, friction knocks the electrons off the atoms in one of the objects and transfers them to the other object. This leaves the first object with an excess of positively charged atoms and the second with an excess of negative electrons. Both objects can then be said to be statically charged. But why do electrons flow from one object to the other, instead of moving in both directions?
As Northwestern University researcher Bartosz Grzybowski detailed last year in the journal Science, patches of both excess positive and excess negative charge exist on statically charged objects. In addition, he found that entire molecules, not just electrons, seemed to migrate between objects as they are rubbed together. Therefore, it would seem the forces involved in generating this patchwork of particle migration are still a little bit mysterious.