What is the role of insulators in the accumulation of static electricity?

Question

The Wikipedia article for static electricity says

A static electric charge can be created whenever two surfaces contact and have worn and separated, and at least one of the surfaces has a high resistance to electric current (and is therefore an electrical insulator).

Why is it important for one of the surfaces to be an insulator? I have read some high-level descriptions of the triboelectric effect that talk about electrons being more attracted to one surface upon contact but it is not clear to me where the insulator vs. conductor distinction comes into play. Why couldn't two conductors with different electron energy levels experience a net transfer of charge, for example? (This example might be completely off-base, I don't have a good understanding of the underlying mechanism.)

[update: clarifying questions]

The triboelectric pump effect is strong enough to overcome the high voltage generated because it moved the electrons in the first place. So shouldn't an equilibrium be reached where the static electric potential energy is balanced by the triboelectric potential energy, and electrons don't "want" to migrate back, regardless of how conductive the materials are?

If it were true that between conductors the triboelectric effect can move a small charge but that charge leaks back, then wouldn't you expect there to be a persistent (possibly oscillating) current loop between the two surfaces? Current from which energy could be extracted, e.g. by resistance heating? The Wikipedia article on the triboelectric effect says that only contact is necessary, with friction just helping to establish contact, so it seems like this would be steady-state.

[update: the following paragraph has been resolved]

In particular, how does the mechanism by which insulation is required relate to everyday experience suggesting that statically charged objects easily distribute the charge across themselves? If I rub my feet on a carpet and accumulate static charge, that charge quickly spreads over my entire body (my arm hairs might stand up) and if I then touch a doorknob my entire body is discharged almost instantly, despite the charge having originated at my feet. So if my body is acting like a conductor in this situation (because of the high voltage?) why does it not also act like a conductor when initially accumulating the charge?

Answer 1

The triboelectric effect acts like a pump. It moves charges from one place to another. The very act of moving the charges creates a field that will attempt to restore the charges.

If both substances are conductors, this field will return the charges. It's like putting stuff into a bucket that has a hole inside. You can't accumulate a large separation because the charges are allowed to flow back together.

Only when the charges are prevented from flowing can they accumulate. As the two substances are in direct contact, at least one of them must be an insulator.

In your scenario, both the shoes and the carpet (which is where the separation is happening) are good insulators. They're not perfect (some of the charge does move from the shoe into your body), but they're good enough that the charge separation doesn't immediately dissipate.

You're correct that your body and the doorknob are poor insulators. You're not going to be able to develop a large charge imbalance by rubbing a piece of metal on your skin.

I am still not clear on why the insulator matters though. The triboelectric pump effect is strong enough to overcome the high voltage generated because it moved the electrons in the first place. So shouldn't an equilibrium be reached where the static electric potential energy is balanced by the triboelectric potential energy, and electrons don't "want" to migrate back, regardless of how conductive the materials are?

If the triboelectric pump were working all the time and could push a significant amount of charge, sure. That's how a voltage source like a battery or a generator can use conductors.

But triboelectric charging is intermittent and usually only small amounts of charge move at a time. If the separation is being made to good conductors, the charge simply couldn't keep up.

A static charge that you feel from a carpet is probably in the order of millijoules of energy (see this other question on static charge) and therefore nanocoulombs of charge. Something that probably took several seconds of shuffling to produce.

That's far less than even a coin cell could produce, and it would not able to maintain any significant voltage across a good conductor.