Entry by Andrew Capulli, Fall 2011
Note "tribo" comes from the Greek word for "to rub".
Triboelectric charging (triboelectric generation or triboelectric effect) is caused by the rubbing of two materials together, ie contact electrification; the result of triboelectric charging is what's termed 'static electricity'. 'Static electricity' refers to just that: electricity that is static or stationary, it is accumulated on the surface of a material. The tribocharge (static charge) generated by the rubbing of two materials is dependent on surface roughness of the materials, temperature, and humidity. A tribocharge is often acquired when walking across a rug for example due to the 'rough' surface (the many fibers that make up the surface of the rug. As will be discussed, the generation of a tribocharge is related to the friction between the materials (as is evident already taking the surface roughness of the materials to be a contributing factor).
Essentially, the tribocharge is a result of unequal flow of electrons while the materials are in contact which leave as negative charge on the material that has net gained electrons and a positive charge on the material that has net lost electrons when the materials are separated. In more detail: Ungrounded materials, when in contact, exchange electrochemical charges; many sources refer to this "contact" as adherence of the materials in which charged ions can freely flow between the materials. The flow of ions between materials in contact is driven by the 'desire' of these charges to balance their chemical potential between the now "adherent" materials. Essentially, to reduce their energy state, some charged ions will be exchanged between the materials in contact. As the materials are separated, the exchanged charged ions are left in their new location, transferred onto the surface of the other material. For different materials, this will result in a charge imbalance... or what we call a triboelectric charge or static charge. Rubbing the materials together only increases the contact and separation between the materials which results in an even higher charge imbalance (higher tribocharge). One cannot obtain a tribocharge between two like materials because there is no electrochemical gradient between the materials (since they are the same) and thus no net ion movement.
Determining which material will acquire the negative charge and which will acquire the positive charge depends on the relative properties of the materials (ie which has a higher affinity for the addition of electrons). A triboelectric series "ranks" different materials according to their affinity to gain or tendency to lose electrons in comparison to other materials. It is important to note that the conductivity of a material is irrelevant in terms of its affinity for or tendency to lose electrons. The following triboelectric chart (from source 3) is an example of a typical triboelectric series:
It is important to understand that these rankings on a triboelectric series are relative to the materials listed; in a way one can relate the triboelectric series to the HLB Scale for surfactants where HLB numbers assigned to surfactants are not necessarily indicative of any physical/chemical/etc properties, just relative rankings of hydrophilicity of a surfactant.
Importance of Tribocharges
We all know the relatively harmless effects of the tribocharge (static charge). I'm sure at some point you have rubbed your shoes on a carpet and 'shocked' someone... or rubbed a balloon and stuck it to your (or another person's) head. However, the implications of the discharge of a tribocharge may be more severe. Fuel transport systems (piping) and even helicopters can acquire dangerous tribocharges. In the presence of flammable vapors, even a small discharge can cause very damaging (and life threatening) explosions. NASA even follows a strict "triboelectrification rule" which grounds any mission if the clouds a shuttle is predicted to fly through may result in a potentially unsafe surface charge on the vehicle. I highly recommend the Wikipedia article on the triboelectric effect which can be found at:http://en.wikipedia.org/wiki/Triboelectric_effect. This article goes into more detail examining the potential effects tribocharges and their real (dangerous) impact on what seems to be everyday processes.
The article summarized (found at the bottom of this page under Keyword in References) discusses surfaces modifications that reduce the triboelectric charge build up on moving systems by the patterned addition of permanent charges on the surface of materials.