Triboelectric effect

The triboelectric effect (also known as triboelectricity, triboelectric charging, triboelectrification, or tribocharging) describes electric charge transfer between two objects when they contact or slide against each other. It can occur with different materials, such as the sole of a shoe on a carpet, or between two pieces of the same material. It is ubiquitous, and occurs with differing amounts of charge transfer (tribocharge) for all solid materials. There is evidence that tribocharging can occur between combinations of solids, liquids and gases, for instance liquid flowing in a solid tube or an aircraft flying through air. Often static electricity is a consequence of the triboelectric effect when the charge stays on one or both of the objects and is not conducted away. The term triboelectricity has been used to refer to the field of study or the general phenomenon of the triboelectric effect, or to the static electricity that results from it. When there is no sliding, tribocharging is sometimes called contact electrification, and any static electricity generated is sometimes called contact electricity. The terms are often used interchangeably, and may be confused. Triboelectric charge plays a major role in industries such as packaging of pharmaceutical powders, and in many processes such as dust storms and planetary formation. It can also increase friction and adhesion. While many aspects of the triboelectric effect are now understood and extensively documented, significant disagreements remain in the current literature about the underlying details.

History

The historical development of triboelectricity is interwoven with work on static electricity and electrons themselves. Experiments involving triboelectricity and static electricity occurred well before the discovery of the electron. The name ēlektron (ἤλεκτρον) is Greek for amber, which is connected to the recording of electrostatic charging by Thales of Miletus around 585 BC, and possibly others even earlier. The prefix tribo- (Greek for 'rub') refers to sliding, friction and related processes, as in tribology. From the axial age (8th to 3rd century BC) the attraction of materials due to static electricity by rubbing amber and the attraction of magnetic materials were considered to be similar or the same. There are indications that it was known both in Europe and outside, for instance China and other places. Syrian women used amber whorls in weaving and exploited the triboelectric properties, as noted by Pliny the Elder. The effect was mentioned in records from the medieval period. Archbishop Eustathius of Thessalonica, Greek scholar and writer of the 12th century, records that Woliver, king of the Goths, could draw sparks from his body. He also states that a philosopher was able, while dressing, to draw sparks from his clothes, similar to the report by Robert Symmer of his silk stocking experiments, which may be found in the 1759 Philosophical Transactions.

It is generally considered that the first major scientific analysis was by William Gilbert in his publication De Magnete in 1600. He discovered that many more materials than amber such as sulphur, wax, glass could produce static electricity when rubbed, and that moisture prevented electrification. Others such as Sir Thomas Browne made important contributions slightly later, both in terms of materials and the first use of the word electricity in Pseudodoxia Epidemica. He noted that metals did not show triboelectric charging, perhaps because the charge was conducted away. An important step was around 1663 when Otto von Guericke invented a machine that could automate triboelectric charge generation, making it much easier to produce more tribocharge; other electrostatic generators followed. For instance, shown in the Figure is an electrostatic generator built by Francis Hauksbee the Younger. Another key development was in the 1730s when C. F. du Fay pointed out that there were two types of charge which he named vitreous and resinous. These names corresponded to the glass (vitreous) rods and bituminous coal, amber, or sealing wax (resinous) used in du Fay's experiments. These names were used throughout the 19th century. The use of the terms positive and negative for types of electricity grew out of the independent work of Benjamin Franklin around 1747 where he ascribed electricity to an over- or under- abundance of an electrical fluid. At about the same time Johan Carl Wilcke published in his 1757 PhD thesis a triboelectric series. In this work, materials were listed in order of the polarity of charge separation when they are touched or slide against another. A material towards the bottom of the series, when touched to a material near the top of the series, will acquire a more negative charge. The first systematic analysis of triboelectricity is considered to be the work of Jean Claude Eugène Péclet in 1834. He studied triboelectric charging for a range of conditions such as the material, pressure and rubbing of surfaces. It was some time before there were further quantitative works by Owen in 1909 and Jones in 1915. The most extensive early set of experimental analyses was from 1914–1930 by the group of Professor Shaw, who laid much of the foundation of experimental knowledge. In a series of papers he: was one of the first to mention some of the failings of the triboelectric series, also showing that heat had a major effect on tribocharging; analyzed in detail where different materials would fall in a triboelectric series, at the same time pointing out anomalies; separately analyzed glass and solid elements and solid elements and textiles, carefully measuring both tribocharging and friction; analyzed charging due to air-blown particles; demonstrated that surface strain and relaxation played a critical role for a range of materials, and examined the tribocharging of many different elements with silica. Much of this work predates an understanding of solid state variations of energies levels with position, and also band bending. It was in the early 1950s in the work of authors such as Vick that these were taken into account along with concepts such as quantum tunnelling and behavior such as Schottky barrier effects, as well as including models such as asperities for contacts based upon the work of Frank Philip Bowden and David Tabor.

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