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The triboelectric effect is an electrical phenomenon where certain materials become electrically charged after coming into contact with another, different, material. The polarity and strength of the charges produced differ according to material and surface smoothness. Amber, for example, can acquire an electric charge by friction. This property, first recorded by Thales of Miletus, suggested the word "electricity", from the Greek word for amber, elektron. Other examples of materials that can acquire a charge when rubbed together include glass rubbed with silk, and hard rubber rubbed with fur.
Series
Materials are often listed in order of the polarity of charge separation when they are touched with another object. A material towards the bottom of the series, when touched to a material near the top of the series, will attain a more negative charge, and vice versa. The further away two materials are from each other on the series, the greater the charge transferred. Materials near to each other on the series may not exchange any charge, or may exchange the opposite of what is implied by the list. This depends more on the presence of rubbing, the presence of contaminants or oxides, or upon other properties than the type of material. Lists vary somewhat as to the exact order of some materials, since the charge also varies for nearby materials.
Effect Although the word comes from the Greek for "rubbing", tribos, the two materials only need to come into contact and then separate for electrons to be exchanged. After coming into contact, a chemical bond is formed between some parts of the two surfaces, called adhesion. When separated, some of the bonded atoms have a tendency to keep extra electrons, and some a tendency to give them away. This is what creates the net charge imbalance between the objects. In addition, some materials may exchange ions of differing mobility, or exchange charged fragments of larger molecules. The triboelectric effect is only related to friction because they both involve adhesion. However, the effect is greatly enhanced by rubbing the materials together, as they touch and separate many times. For surfaces with differing geometry, rubbing may lead to heating of protrusions, causing pyroelectric charge separation which may add to the existing contact electrification, or which may oppose the existing polarity. Surface nano-effects are not well understood, and the atomic force microscope has made sudden progress possible in this field of physics.
Because the surface of the material is now electrically charged, either negatively or positively, any contact with an uncharged conductive object or with an object having substantially different charge may cause a discharge of the built-up static electricity; a spark. A person simply walking across a carpet may build up a charge of many thousands of volts, enough to cause a spark a centimeter long or more (this type of discharge is usually harmless, as the current, though very large, typically exists for far less than a millionth of a second).
Utilization The effect is of considerable industrial importance both in terms of safety and also potential damage to manufactured goods. The spark produced is fully capable of igniting inflammable vapours, for example, petrol or ether fumes. Means have to be found to discharge hospital trolleys which may carry such liquids. Even where only a small charge is produced, this can result in dust particles being attracted to the rubbed surface. In the case of textile manufacture this can lead to a permanent grimy mark where the cloth has been charged.
This effect is the principle behind the charge build-up in a triboelectric-type Van de Graaff generator.
See also
External links - "Triboelectrics (http://www.cleanairsystems.com/__data/page/1560/Triboelectrics.pdf)". Clean Air Systems (http://www.cleanairsystems.com/) (PDF)
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