Contact electrification

Contact electrification is a phrase that describes the phenomenon whereby two surfaces become electrically charged when they contact and then separate. As such it is a subtopic of the more general area of triboelectricity, which includes sliding; often the two terms are used interchangeably.^[1][2][3] It can be a boon or a bane in industries  ranging from xerography^[4] to packing of pharmaceutical powders,^[5] and plays a critical role in many processes such as dust storms^[6] and planetary formation.^[7]

While many aspects of contact electrification are now understood, and consequences have been extensively documented, there remain disagreements in the current literature about the underlying mechanisms. Indeed, whether it is different from triboelectricity is not clear. As mentioned above contact electrification is when two bodies contact then separate; triboelectricity includes sliding. Very early it was suggested by Volta and Helmholtz (see ^[8]) that the role of sliding was to produce more contacts per second, so the two are the same. The idea here is that electrons move many times faster than atoms, so the electrons are always in equilibrium when atoms move, what is called the Born–Oppenheimer approximation. Then again, a recent paper has suggested that the sliding can act as a pump which can excite electrons to go from one material to another,^[9] and another has suggested that local heating during sliding matters.^[10] Other papers have considered that local bending at the nanoscale dominates.^[11] The jury is still out.

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