What is "static electricity"/"contact electrification"/triboelectricity?

An early physics demonstration that many of us see in elementary school is that of static electricity:  an electrical insulator like a wool cloth or animal fur is rubbed on a glass or plastic rod, and suddenly the rod can pick up pieces of styrofoam or little bits of paper.  Alternately, a rubber balloon is rubbed against a kid's hair, and afterward the balloon is able to stick to a wall with sufficient force that static friction keeps the balloon from sliding down the surface.  The physics here is that when materials are rubbed together, there can be a net transfer of electrical charge from one to the other, a phenomenon called triboelectricity.  The electrostatic attraction between net charge on the balloon and the polarizable surface of the wall is enough to hold up the balloon.  

Balloons electrostatically clinging to a wall, from here.

The big mysteries are, how and why do charges transfer between materials when they are rubbed together?  As I wrote about once before, this is still not understood, despite more than 2500 years of observations.  The electrostatic potentials that can be built up through triboelectricity are not small.  They can be tens of kV, enough to cause electrons accelerating across those potentials to emit x-rays when they smack into the positively charged surface.  Whatever is going on, it's a way to effectively concentrate the energy from mechanical work into displacing charges.  This is how Wimshurst machines and Van de Graaff generators work, even though we don't understand the microscopic physics of the charge generation and separation.

There are disagreements to this day about the mechanisms at work in triboelectricity, including the role of adsorbates, surface chemistry, whether the charges transferred are electrons or ions, etc.  From how electronic charge transfer works between metals, or between metals and semiconductors, it's not crazy to imagine that somehow this should all come down to work functions or the equivalent.  Depending on the composition and structure of materials, the electrons in there can be bound more tightly (energetically deeper compared to the energy of an electron far away, also called "the vacuum" level) or more loosely (energetically shallower, closer to the energy of a free electron).  It's credible that bringing two such materials in contact could lead to electrons "falling down hill" from the more loosely-binding material into the more tightly binding one.   That clearly is not the whole story, though, or this would've been figured out long ago.

This week, a new paper revealed an interesting wrinkle.  The net preference for picking up or losing charge seems to depend very clearly on the history of repeated contacts.  The authors used PDMS silicone rubber, and they find that repeated contacting can deterministically bake in a tendency for charge to flow one direction.  Using various surface spectroscopy methods, they find no obvious differences at the PDMS surface before/after the contacting procedures, but charge transfer is affected.  

My sneaking suspicion is that adsorbates will turn out to play a huge role in all of this.  This may be one of those issues like friction (see here too), where there is a general emergent phenomenon (net charge transfer) that can take place via multiple different underlying pathways.  Experiments in ultrahigh vacuum with ultraclean surfaces will undoubtedly show quantitatively different results than experiments in ambient conditions, but they may both show triboelectricity.