A new paper seem to contradict a common understanding on how static electricity on objects come about.
Abstract: When dielectric materials are brought into contact and then separated, they develop static electricity. For centuries, it has been assumed that such contact charging derives from the spatially homogeneous material properties (along the material's surface) and that within a given pair of materials, one charges uniformly positively and the other negatively. We demonstrate that this picture of contact charging is incorrect. While each contact-electrified piece develops a net charge of either positive or negative polarity, each surface supports a random “mosaic” of oppositely charged regions of nanoscopic dimensions. These mosaics of surface charge have the same topological characteristics for different types of electrified dielectrics and accommodate significantly more charge per unit area than previously thought.
Wired has a review of this work.
For many of us, static electricity is one of the earliest encounters we have with electromagnetism, and it’s a staple of high school physics. Typically, it’s explained as a product of electrons transferred in one direction between unlike substances, like glass and wool, or a balloon and a cotton T-shirt (depending on whether the demo is in a high school class or a kids’ party). Different substances have a tendency to pick up either positive or negative charges, we’re often told, and the process doesn’t transfer a lot of charge, but it’s enough to cause a balloon to stick to the ceiling, or to give someone a shock on a cold, dry day.
Nearly all of that is wrong, according to a paper published in today’s issue of Science. Charges can be transferred between identical materials, all materials behave roughly the same, the charges are the product of chemical reactions, and each surface becomes a patchwork of positive and negative charges, which reach levels a thousand times higher than the surfaces’ average charge.
From what I've gathered, there is an actual transfer of material, not charges, from one object to another, when they touch or rub against each other.
So, what causes these charges to build up? It’s not, apparently, the transfer of electrons between the surfaces. Detailed spectroscopy of one of the polymers (PDMS) suggests that chemical reactions may be involved, as many oxidized derivatives of the polymer were detected. In addition, there is evidence that some material is transferred from one surface to another. Using separate pieces of fluorine- and silicon-containing polymers allowed the authors to show that signals consistent with the presence of fluorine were detected in the silicon sample after contact.
It will be interesting to see if this is the predominant cause of static electricity.
 H.T. Baytekin et al., Science Online DOI: 10.1126/science.1201512 (Publication citation not available yet).