View source for Triboelectric effect

{{Short description|Charge transfer due to contact or sliding}}
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{{Use dmy dates|date=August 2023}}
[[File:Cat demonstrating static cling with styrofoam peanuts.jpg|thumb|upright=1.35|[[Styrofoam peanut]]s clinging to a cat's fur due to [[static electricity]].]]
{{electromagnetism|cTopic=Electrostatics}}

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,<ref name=":19" /><ref name=":34" /><ref name=":10" /><ref name=":28" /> or to the static electricity that results from it.<ref>{{cite web |url= https://education.mrsec.wisc.edu/triboelectricity/ |author=<!--Staff writer(s); no by-line.--> |title=Triboelectricity |date=2020 |work=Education.MRSEC.Wisc.edu |publisher=Materials Research Science and Engineering Centers Education Group / University of Wisconsin–Madison |access-date=21 July 2023}}</ref><ref>{{Cite web |date=23 July 2023 |title=Collins English Dictionary |url=https://www.collinsdictionary.com/us/dictionary/english/triboelectricity |access-date=23 July 2023}}</ref> 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,<ref name=":10">{{Cite journal |last1=Watanabe |first1=H. |last2=Ghadiri |first2=M |last3=Matsuyama |first3=T. |last4=Diing |first4=Y. |last5=Pitt |first5=K. |last6=Maruyama |first6=H. |last7=Matsusaka |first7=S. |last8=Masuda |first8=H. |date=2007 |title=Triboelectrification of pharmaceutical powders by particle impact |journal=International Journal of Pharmaceutics |volume=334 |issue=1–2 |pages=149–155 |doi=10.1016/j.ijpharm.2006.11.005 |pmid=17141989 |hdl=2433/194296 |issn=0378-5173|hdl-access=free }}</ref><ref>{{Cite journal |last1=Wong |first1=Jennifer |last2=Kwok |first2=Philip Chi Lip |last3=Chan |first3=Hak-Kim |date=2015 |title=Electrostatics in pharmaceutical solids |url= https://linkinghub.elsevier.com/retrieve/pii/S0009250914002632 |journal=Chemical Engineering Science |volume=125 |pages=225–237 |doi=10.1016/j.ces.2014.05.037|bibcode=2015ChEnS.125..225W }}</ref> and in many processes such as [[dust storm]]s<ref name=":24">{{Cite journal |last1=Kok |first1=Jasper F. |last2=Renno |first2=Nilton O. |date=2008 |title=Electrostatics in Wind-Blown Sand |journal=Physical Review Letters |volume=100 |issue=1 |page=014501 |doi=10.1103/physrevlett.100.014501 |pmid=18232774 |arxiv=0711.1341 |bibcode=2008PhRvL.100a4501K |s2cid=9072006 |issn=0031-9007}}</ref> and [[planetary formation]].<ref name=":36">{{Cite journal |last1=Blum |first1=Jürgen |last2=Wurm |first2=Gerhard |date=2008 |title=The Growth Mechanisms of Macroscopic Bodies in Protoplanetary Disks |journal=Annual Review of Astronomy and Astrophysics |volume=46 |issue=1 |pages=21–56 |doi=10.1146/annurev.astro.46.060407.145152 |bibcode=2008ARA&A..46...21B |issn=0066-4146}}</ref> It can also increase [[Triboelectric effect#Friction and adhesion from tribocharging|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 ==
{{See also|Timeline of electromagnetism and classical optics|History of electromagnetic theory|Electric charge#History}}
The historical development of triboelectricity is interwoven with work on static electricity and electrons themselves. Experiments involving triboelectricity and static electricity occurred before the discovery of the electron. The name [[wiktionary:ἤλεκτρον|ēlektron]] (ἤλεκτρον) is Greek for [[amber]],<ref name="DictOrigins">{{cite book |last=Shipley |first=J. T. |title=Dictionary of Word Origins |page=133 |publisher=[[The Philosophical Library]] |date=1945 |isbn=978-0-88029-751-6 |url= https://archive.org/details/dictionaryofword00ship/page/133 |url-access=registration}}</ref><ref name=":23">{{Cite book |last=Benjamin |first=Park |title=A history of electricity (the intellectual rise in electricity) from antiquity to the days of Benjamin Franklin by Park Benjamin ... |date=1898 |publisher=J. Wiley |location=New York |pages=1-45 ,Chapters 1-2 |doi=10.5962/bhl.title.19628|url=https://www.biodiversitylibrary.org/item/59453 }}</ref> which is connected to the recording of electrostatic charging by [[Thales of Miletus]] around 585 BCE,<ref name="Lacks">{{Cite journal |last1=Iversen |first1=Paul |last2=Lacks |first2=Daniel J. |date=2012 |title=A life of its own: The tenuous connection between Thales of Miletus and the study of electrostatic charging |url= https://www.sciencedirect.com/science/article/pii/S0304388612000216 |journal=Journal of Electrostatics |volume=70 |issue=3 |pages=309–311 |doi=10.1016/j.elstat.2012.03.002 |issn=0304-3886}}</ref> and possibly others even earlier.<ref name="Lacks" /><ref name="Roller 1953 343–356">{{Cite journal |last1=Roller |first1=Duane |last2=Roller |first2=Duane H. D. |date=1953 |title=The Prenatal History of Electrical Science |url= https://pubs.aip.org/aapt/ajp/article/21/5/343-356/1035214 |journal=American Journal of Physics |volume=21 |issue=5 |pages=343–356 |doi=10.1119/1.1933449 |bibcode=1953AmJPh..21..343R |issn=0002-9505}}</ref> The prefix ''{{transliteration|grc|tribo-}}'' (Greek for 'rub') refers to sliding, friction and related processes, as in [[tribology]].<ref>{{Citation |title=tribo- |date=2023-08-26 |url=https://en.wiktionary.org/w/index.php?title=tribo-&oldid=75791868 |work=Wiktionary, the free dictionary |access-date=2023-09-05 |language=en}}</ref>

From the [[Axial Age|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.<ref name=":23" /> There are indications that it was known both in Europe and outside, for instance China and other places.<ref name=":23" /> Syrian women used amber whorls in weaving and exploited the triboelectric properties, as noted by [[Pliny the Elder]].<ref name=":23" /><ref>{{Cite web |title=The Properties of Amber |url=https://www.getty.edu/publications/ambers/intro/6/ |access-date=16 August 2023 |website=Ancient Carved Ambers in the J. Paul Getty Museum |language=en}}</ref>

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]].<ref name="EncyclopediaAmericana">{{cite book |last=Maver |first=William Jr. |contribution=Electricity, Its History and Progress |url= https://archive.org/stream/encyclopediaame21unkngoog#page/n210/mode/1up |title=The Encyclopedia Americana: A Library of Universal Knowledge |volume=X |pages=172 ''ff.'' |date=1918 |location=New York |publisher=Encyclopedia Americana Corp. |via=Internet Archive}}</ref>

[[File:Hauksbee Generator.JPG|thumb|upright|Generator built by [[Francis Hauksbee]]<ref name=":22">{{Cite journal |last=Hauksbee |first=Francis |url= https://cir.nii.ac.jp/crid/1130282268698739328 |title=Physico-mechanical experiments |journal=(No Title) |publisher=J. Senex & W. Taylor |date=1719 |edition=2nd |location=London}}</ref>]]
It is generally considered<ref name="Roller 1953 343–356" /> that the first major scientific analysis was by [[William Gilbert (physician)|William Gilbert]] in his publication [[De Magnete]] in 1600.<ref name="EncyclopediaAmericana" /><ref>{{Cite book |last1=Gilbert |first1=William |last2=Mottelay |first2=Paul Fleury |title=De magnete |date=1991 |orig-date=1893 |publisher=Dover publ |isbn=978-0-486-26761-6 |edition=Facsimile |location=New York}}</ref> 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]].<ref>{{Cite book |last=Knight |first=Thomas Brown |url= http://penelope.uchicago.edu/pseudodoxia/pseudodoxia.shtml |title=Pseudodoxia epidemica: or, Enquiries into very many received tenents and commonly presumed truths (6th and last ed., corr. and enl.). Book II Chapter IV |date=1672 |pages=82–86 |doi=10.1037/13887-000}}</ref> 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<ref>{{Cite journal |last=de V. Heathcote |first=N.H. |date=1950 |title=Guericke's sulphur globe |url=http://www.tandfonline.com/doi/abs/10.1080/00033795000201981 |journal=Annals of Science |language=en |volume=6 |issue=3 |pages=293–305 |doi=10.1080/00033795000201981 |issn=0003-3790}}</ref> a machine that could automate triboelectric charge generation, making it much easier to produce more tribocharge; other [[electrostatic generator]]s followed.<ref name="EncyclopediaAmericana" /> 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''.<ref>{{Cite journal |date=1733 |title=V. A letter from Mons. Du Fay, F. R. S. and of the Royal Academy of Sciences at Paris, to his Grace Charles Duke of Richmond and Lenox, concerning electricity. Translated from the French by T. S. M D |url= https://royalsocietypublishing.org/doi/10.1098/rstl.1733.0040 |journal=Philosophical Transactions of the Royal Society of London |language=la |volume=38 |issue=431 |pages=258–266 |doi=10.1098/rstl.1733.0040 |s2cid=186208701 |issn=0261-0523}}</ref><ref>{{Cite book |last=Keithley |first=Joseph F.|title=The story of electrical and magnetic measurements: from 500 BC to the 1940s |date=1999 |publisher=IEEE Press |isbn=978-0-7803-1193-0 |location=New York}}</ref> These names corresponded to the glass (vitreous) rods and bituminous coal, amber, or sealing wax (resinous) used in du Fay's experiments.<ref name="Whittaker">{{Cite book |last=Whittaker |first=Edmund T. |title=A history of the theories of aether & electricity. 2: The modern theories, 1900–1926 |date=1989 |publisher=Dover Publ |isbn=978-0-486-26126-3 |edition=Repr |location=New York}}</ref>{{rp|I:44}} 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.<ref name="Whittaker" />{{rp||pages=43–48}}

At about the same time [[Johan Carl Wilcke]] published in his 1757 PhD thesis a <em>triboelectric series</em>.<ref name=":35">{{Cite book |last=Wilcke |first=Johan Carl |url=https://books.google.com/books?id=Mld_nQEACAAJ |title=Disputatio physica experimentalis, de electricitatibus contrariis ... |date=1757 |publisher=Typis Ioannis Iacobi Adleri |language=la}}</ref><ref name="Dictionary of Scientific Biography">{{cite book |last=Gillispie |first=C. C. |title=Dictionary of Scientific Biography |pages=352–353 |publisher=Scribner |location=New York |date=1976}}</ref> 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.<ref>{{Cite journal |last=Peclet |first=M. E. |date=1834 |title=Memoire sur l'Electricite produit par le Frottement |url= https://gallica.bnf.fr/ark:/12148/bpt6k6568974z/f343.item.r=Peclet |journal=[[Annales de chimie et de physique]] |volume=lvii |pages=337–416}}</ref> 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<ref name=":37">{{Cite journal |last=Owen |first=Morris |date=1909 |title=XLII. On frictional electricity |url= https://www.tandfonline.com/doi/full/10.1080/14786440408636622 |journal=The London, Edinburgh, and Dublin Philosophical Magazine and Journal of Science |volume=17 |issue=100 |pages=457–465 |doi=10.1080/14786440408636622 |issn=1941-5982}}</ref> and Jones in 1915.<ref name=":38">{{Cite journal |last=Jones |first=W. Morris |date=1915 |title=XXX. Frictional electricity on insulators and metals |url= https://www.tandfonline.com/doi/full/10.1080/14786440208635305 |journal=The London, Edinburgh, and Dublin Philosophical Magazine and Journal of Science |volume=29 |issue=170 |pages=261–274 |doi=10.1080/14786440208635305 |issn=1941-5982}}</ref> 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;<ref name=":33">{{Cite journal |last=Shaw |first=P. E. |date=1914 |title=The Electrification of Surfaces as Affected by Heat |url= https://iopscience.iop.org/article/10.1088/1478-7814/27/1/317 |journal=Proceedings of the Physical Society of London |volume=27 |issue=1 |pages=208–216 |doi=10.1088/1478-7814/27/1/317 |bibcode=1914PPSL...27..208S |issn=1478-7814}}</ref> analyzed in detail where different materials would fall in a triboelectric series, at the same time pointing out anomalies;<ref name=":19" /> separately analyzed glass and solid elements<ref name=":27">{{Cite journal |last1=Shaw |first1=P. E. |last2=Jex |first2=C. S. |date=1928 |title=Tribo-electricity and friction. II.—Glass and solid elements |journal=Proceedings of the Royal Society of London. Series A, Containing Papers of a Mathematical and Physical Character |volume=118 |issue=779 |pages=97–108 |doi=10.1098/rspa.1928.0037 |bibcode=1928RSPSA.118...97S |issn=0950-1207|doi-access=free }}</ref> and solid elements and textiles,<ref name="Shaw 1928 108–113">{{Cite journal |last1=Shaw |first1=P. E. |last2=Jex |first2=C. S. |date=1928 |title=Tribo-Electricity and Friction. III. Solid Elements and Textiles |journal=Proceedings of the Royal Society of London. Series A, Containing Papers of a Mathematical and Physical Character |volume=118 |issue=779 |pages=108–113 |doi=10.1098/rspa.1928.0038 |jstor=94891 |bibcode=1928RSPSA.118..108S |issn=0950-1207}}</ref> carefully measuring both tribocharging and friction; analyzed charging due to air-blown particles;<ref>{{Cite journal |last=Shaw |first=P. W. |date=1929 |title=Tribo-electricity and friction. IV.—Electricity due to air-blown particles |journal=Proceedings of the Royal Society of London. Series A, Containing Papers of a Mathematical and Physical Character |volume=122 |issue=789 |pages=49–58 |doi=10.1098/rspa.1929.0004 |bibcode=1929RSPSA.122...49S |issn=0950-1207|doi-access=free }}</ref> demonstrated that surface strain and relaxation played a critical role for a range of materials,<ref name="royalsocietypublishing.org">{{Cite journal |last1=Shaw |first1=P. E. |last2=Hanstock |first2=R. F. |date=1930 |title=Triboelectricity and friction. —V. On surface strain and relaxation of like solids |journal=Proceedings of the Royal Society of London. Series A, Containing Papers of a Mathematical and Physical Character |volume=128 |issue=808 |pages=474–480 |doi=10.1098/rspa.1930.0125 |bibcode=1930RSPSA.128..474S |s2cid=137932809 |issn=0950-1207|doi-access=free }}</ref><ref>{{Cite journal |last1=Shaw |first1=P. E. |last2=Hanstock |first2=R. F. |date=1930 |title=Triboelectricity and friction.—VI. On surface strain and relaxation for unlike solids |journal=Proceedings of the Royal Society of London. Series A, Containing Papers of a Mathematical and Physical Character |volume=128 |issue=808 |pages=480–487 |doi=10.1098/rspa.1930.0126 |bibcode=1930RSPSA.128..480S |issn=0950-1207|doi-access=free }}</ref> and examined the tribocharging of many different elements with silica.<ref>{{Cite journal |last1=Shaw |first1=P. E. |last2=Leavery |first2=E. W. |date=1932 |title=Triboelectricity and friction. VII.—Quantitative results for metals and other solid elements, with silica |journal=Proceedings of the Royal Society of London. Series A, Containing Papers of a Mathematical and Physical Character |volume=138 |issue=836 |pages=502–514 |doi=10.1098/rspa.1932.0199 |bibcode=1932RSPSA.138..502S |s2cid=136574422 |issn=0950-1207|doi-access=free }}</ref>

Much of this work predates an understanding of [[Band-bending diagram|solid state variations]] of energies levels with position, and also [[band bending]].<ref name=":16" /> It was in the early 1950s in the work of authors such as Vick<ref name=":1" /> 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 (physicist)|David Tabor]].<ref name=":18" />

== Basic characteristics ==
Triboelectric charging occurs when two materials are brought into contact then separated, or slide against each other. An example is rubbing a plastic pen on a shirt sleeve made of cotton, wool, polyester, or the blended fabrics used in modern clothing.<ref>{{Citation |title=A Plastic Comb Rubbed With a Cotton Cloth Attracts Small Pieces of Paper | date=6 September 2012 |url=https://www.youtube.com/watch?v=rtl9TyMZSP8 |access-date=2023-09-05 |language=en}}</ref> An electrified pen will attract and pick up pieces of paper less than a square centimeter, and will repel a similarly electrified pen. This repulsion is detectable by hanging both pens on threads and setting them near one another. Such experiments led to the theory of two types of electric charge, one being the negative of the other, with a simple sum respecting signs giving the total charge. The electrostatic attraction of the charged plastic pen to neutral uncharged pieces of paper (for example) is due to induced [[dipole]]s<ref name=":16" />{{Rp|location=Chapter 27}} in the paper.

The triboelectric effect can be unpredictable because many details are often not controlled.<ref>{{Cite journal |last1=Lowell |first1=J. |last2=Akande |first2=A. R. |date=1988 |title=Contact electrification-why is it variable? |url= https://iopscience.iop.org/article/10.1088/0022-3727/21/1/018 |journal=Journal of Physics D: Applied Physics |volume=21 |issue=1 |pages=125–137 |doi=10.1088/0022-3727/21/1/018 |bibcode=1988JPhD...21..125L |s2cid=250782776 |issn=0022-3727}}</ref> Phenomena which do not have a simple explanation have been known for many years. For instance, as early as 1910, Jaimeson observed that for a piece of cellulose, the sign of the charge was dependent upon whether it was bent concave or convex during rubbing.<ref name=":8">{{Cite journal |last=Jamieson |first=Walter |date=1910 |title=The Electrification of Insulating Materials |journal=Nature |volume=83 |issue=2111 |page=189 |doi=10.1038/083189a0 |bibcode=1910Natur..83..189J |s2cid=3954491 |issn=0028-0836|doi-access=free }}</ref> The same behavior with curvature was reported in 1917 by Shaw,<ref name=":19" /> who noted that the effect of curvature with different materials made them either more positive or negative. In 1920, Richards pointed out that for colliding particles the velocity and mass played a role, not just what the materials were.<ref name=":6">{{Cite journal |last=Richards |first=Harold F. |date=1920 |title=Electrification by Impact |url= https://link.aps.org/doi/10.1103/PhysRev.16.290 |journal=Physical Review |volume=16 |issue=4 |pages=290–304 |doi=10.1103/PhysRev.16.290 |bibcode=1920PhRv...16..290R |issn=0031-899X}}</ref> In 1926, Shaw pointed out that with two pieces of identical material, the sign of the charge transfer from "rubber" to "rubbed" could change with time.<ref name=":7">{{Cite journal |last=Shaw |first=P. E. |date=1926 |title=Electrical separation between identical solid surfaces |journal=Proceedings of the Physical Society |volume=39 |issue=1 |pages=449–452 |doi=10.1088/0959-5309/39/1/344 |bibcode=1926PPS....39..449S |issn=0959-5309}}</ref>

There are other more recent experimental results which also do not have a simple explanation. For instance the work of Burgo and [[Ali Erdemir|Erdemir]],<ref name=":12">{{Cite journal |last1=Burgo |first1=Thiago A. L. |last2=Erdemir |first2=Ali |date=2014 |title=Bipolar Tribocharging Signal During Friction Force Fluctuations at Metal–Insulator Interfaces |url= https://onlinelibrary.wiley.com/doi/10.1002/anie.201406541 |journal=Angewandte Chemie International Edition |volume=53 |issue=45 |pages=12101–12105 |doi=10.1002/anie.201406541|pmid=25168573 }}</ref> which showed that the sign of charge transfer reverses between when a tip is pushing into a substrate versus when it pulls out; the detailed work of Lee et al<ref>{{Cite journal |last1=Lee |first1=Victor |last2=James |first2=Nicole M. |last3=Waitukaitis |first3=Scott R. |last4=Jaeger |first4=Heinrich M. |date=2018 |title=Collisional charging of individual submillimeter particles: Using ultrasonic levitation to initiate and track charge transfer |url= https://link.aps.org/doi/10.1103/PhysRevMaterials.2.035602 |journal=Physical Review Materials |volume=2 |issue=3 |page=035602 |doi=10.1103/PhysRevMaterials.2.035602 |arxiv=1801.09278 |bibcode=2018PhRvM...2c5602L |s2cid=118904552 |issn=2475-9953}}</ref> and Forward, Lacks and Sankaran<ref name="Shinbrot 2008 24004">{{Cite journal |last1=Shinbrot |first1=T. |last2=Komatsu |first2=T. S. |last3=Zhao |first3=Q. |date=2008 |title=Spontaneous tribocharging of similar materials |url= https://iopscience.iop.org/article/10.1209/0295-5075/83/24004 |journal=EPL (Europhysics Letters) |volume=83 |issue=2 |page=24004 |doi=10.1209/0295-5075/83/24004 |bibcode=2008EL.....8324004S |s2cid=40379103 |issn=0295-5075}}</ref> and others measuring the charge transfer during collisions between particles of [[zirconia]] of different size but the same composition, with one size charging positive, the other negative; the observations using sliding<ref name="Shinbrot 2008 24004" /> or [[Kelvin probe force microscope]]<ref name=":9" /> of inhomogeneous charge variations between nominally identical materials.
[[File:Asperities charged.svg|thumb|Illustration of triboelectric charging from contacting asperities|upright=1.5]]

The details of how and why tribocharging occurs are not established science as of 2023. One component is the difference in the [[work function]] (also called the [[electron affinity]]) between the two materials.<ref name=":4">{{Cite journal |last=Harper |first=W. E. |date=1951 |title=The Volta effect as a cause of static electrification |url=https://royalsocietypublishing.org/doi/10.1098/rspa.1951.0019 |journal=Proceedings of the Royal Society of London. Series A. Mathematical and Physical Sciences |volume=205 |issue=1080 |pages=83–103 |bibcode=1951RSPSA.205...83H |doi=10.1098/rspa.1951.0019 |issn=0080-4630 |s2cid=110618773}}</ref> This can lead to charge transfer as, for instance, analyzed by Harper.<ref name=":2">{{Cite book |last=Harper |first=W. R. |title=Contact and frictional electrification |date=1998 |publisher=Laplacian Press |isbn=1-885540-06-X |oclc=39850726}}</ref><ref name=":13" /> As has been known since at least 1953,<ref name=":1">{{Cite journal |last=Vick |first=F.A.|date=1953 |title=Theory of contact electrification |url= https://iopscience.iop.org/article/10.1088/0508-3443/4/S2/301 |journal=British Journal of Applied Physics |volume=4 |issue=S2 |pages=S1–S5 |doi=10.1088/0508-3443/4/S2/301 |bibcode=1953BJAP....4S...1V |issn=0508-3443}}</ref><ref>{{Cite journal |last=Castle |first=G. S. P. |date=1997 |title=Contact charging between insulators |url= https://linkinghub.elsevier.com/retrieve/pii/S0304388697000090 |journal=Journal of Electrostatics |volume=40–41 |pages=13–20 |doi=10.1016/S0304-3886(97)00009-0}}</ref><ref>{{Cite journal |last=Bailey |first=Adrian G. |date=2001 |title=The charging of insulator surfaces |url= https://linkinghub.elsevier.com/retrieve/pii/S0304388601001061 |journal=Journal of Electrostatics |volume=51–52 |pages=82–90 |doi=10.1016/S0304-3886(01)00106-1}}</ref><ref>{{Cite journal |last=Schein |first=L. B. |date=2007 |title=Recent Progress and Continuing Puzzles in Electrostatics |url= https://www.science.org/doi/10.1126/science.1142325 |journal=Science |volume=316 |issue=5831 |pages=1572–1573 |doi=10.1126/science.1142325 |pmid=17569848 |s2cid=136500498 |issn=0036-8075}}</ref> the contact potential is part of the process but does not explain many results, such as the ones mentioned in the last two paragraphs.<ref name=":8" /><ref name=":7" /><ref name=":12" /><ref name=":9" /> Many studies have pointed out issues with the work function difference ([[Volta potential]]) as a complete explanation.<ref>{{Cite thesis |last=Elsdon |first=R. |url= https://www.repository.cam.ac.uk/items/41c4ca4e-53ba-451f-9e4d-f9f7e6cd5c2e |title=Fundamental and applied aspects of contact electrification |type=PhD |publisher=University of Cambridge |date=1975 |doi=10.17863/CAM.16064}}</ref><ref>{{Cite journal |last1=Akande |first1=A. R. |last2=Lowell |first2=J |date=1987 |title=Charge transfer in metal/polymer contacts |url= https://iopscience.iop.org/article/10.1088/0022-3727/20/5/002 |journal=Journal of Physics D: Applied Physics |volume=20 |issue=5 |pages=565–578 |doi=10.1088/0022-3727/20/5/002 |bibcode=1987JPhD...20..565A |s2cid=250812629 |issn=0022-3727}}</ref><ref name=":17">{{Cite journal |last1=Kok |first1=Jasper F. |last2=Lacks |first2=Daniel J. |date=2009 |title=Electrification of granular systems of identical insulators |url= https://link.aps.org/doi/10.1103/PhysRevE.79.051304 |journal=Physical Review E |volume=79 |issue=5 |page=051304 |doi=10.1103/PhysRevE.79.051304 |pmid=19518446 |arxiv=0902.3411 |bibcode=2009PhRvE..79e1304K |s2cid=2225090 |issn=1539-3755}}</ref><ref name=":28">{{Cite journal |last1=Galembeck |first1=Fernando |last2=Burgo |first2=Thiago A. L. |last3=Balestrin |first3=Lia B. S. |last4=Gouveia |first4=Rubia F. |last5=Silva |first5=Cristiane A. |last6=Galembeck |first6=André |date=2014 |title=Friction, tribochemistry and triboelectricity: recent progress and perspectives |url= http://xlink.rsc.org/?DOI=C4RA09604E |journal=RSC Adv. |volume=4 |issue=109 |pages=64280–64298 |doi=10.1039/C4RA09604E |bibcode=2014RSCAd...464280G |issn=2046-2069}}</ref> There is also the question of why sliding is often important. Surfaces have many nanoscale [[Asperity (materials science)|asperities]] where the contact is taking place,<ref name=":18">{{Cite book |last1=Bowden |first1=Frank Philip |title=The friction and lubrication of solids |last2=Tabor |first2=David |date=2001 |publisher=Clarendon Press |orig-date=1950 |isbn=978-0-19-850777-2 |edition=Repr |series="Oxford Classic Texts" series |location=Oxford}}</ref> which has been taken into account in many approaches to triboelectrification.<ref name=":2" /> [[Alessandro Volta]] and [[Hermann von Helmholtz]] suggested that the role of sliding was to produce more contacts per second.<ref name=":13">{{Cite journal |last=Harper |first=W. R. |date=1961 |title=Electrification following the contact of solids |journal=Contemporary Physics |volume=2 |issue=5 |pages=345–359 |doi=10.1080/00107516108205281 |bibcode=1961ConPh...2..345H |issn=0010-7514}}</ref> In modern terms, the idea is that electrons move many times faster than atoms, so the electrons are always in equilibrium when atoms move (the [[Born–Oppenheimer approximation]]). With this approximation, each asperity contact during sliding is equivalent to a stationary one; there is no direct coupling between the sliding velocity and electron motion.<ref>{{Cite journal |last1=Born |first1=M. |last2=Oppenheimer |first2=R. |date=1927 |title=Zur Quantentheorie der Molekeln |journal=Annalen der Physik |language=de |volume=389 |issue=20 |pages=457–484 |doi=10.1002/andp.19273892002|bibcode=1927AnP...389..457B |doi-access=free }}</ref> An alternative view (beyond the Born–Oppenheimer approximation) is that sliding acts as a quantum mechanical pump which can excite electrons to go from one material to another.<ref name=":11">{{Cite journal |last1=Alicki |first1=Robert |last2=Jenkins |first2=Alejandro |date=2020 |title=Quantum Theory of Triboelectricity |url= https://link.aps.org/doi/10.1103/PhysRevLett.125.186101 |journal=Physical Review Letters |volume=125 |issue=18 |page=186101 |doi=10.1103/PhysRevLett.125.186101 |pmid=33196235 |arxiv=1904.11997 |bibcode=2020PhRvL.125r6101A |s2cid=139102854 |issn=0031-9007}}</ref> A different suggestion is that local heating during sliding matters,<ref>{{Cite journal |last1=Liu |first1=Guangming |last2=Liu |first2=Jun |last3=Dou |first3=Wenjie |date=2022 |title=Non-adiabatic quantum dynamics of tribovoltaic effects at sliding metal–semiconductor interfaces |url= https://linkinghub.elsevier.com/retrieve/pii/S2211285522001185 |journal=Nano Energy |volume=96 |page=107034 |doi=10.1016/j.nanoen.2022.107034|arxiv=2112.04687 |bibcode=2022NEne...9607034L |s2cid=247006239 }}</ref> an idea first suggested by Frenkel in 1941.<ref>{{Cite journal |last=Frenkel |first=J. |date=1941 |title=On the electrification of dielectrics by friction |journal=Journal of Physics-USSR |volume=V |issue=1 |pages=25–29}}</ref> Other papers have considered that local bending at the nanoscale produces voltages which help drive charge transfer via the [[Flexoelectricity|flexoelectric]] effect.<ref name=":5">{{Cite journal |last1=Mizzi |first1=C. A. |last2=Lin |first2=A. Y. W. |last3=Marks |first3=L. D. |date=2019 |title=Does Flexoelectricity Drive Triboelectricity? |url= https://link.aps.org/doi/10.1103/PhysRevLett.123.116103 |journal=Physical Review Letters |volume=123 |issue=11 |page=116103 |doi=10.1103/PhysRevLett.123.116103 |pmid=31573269 |arxiv=1904.10383 |bibcode=2019PhRvL.123k6103M |s2cid=128361741 |issn=0031-9007}}</ref><ref name=":26" /> There are also suggestions that surface or trapped charges are important.<ref>{{Cite journal |last1=Fukada |first1=E. |last2=Fowler |first2=J. F. |date=1958 |title=Triboelectricity and Electron Traps in Insulating Materials: Some Correlations |url= https://www.nature.com/articles/181693b0 |journal=Nature |volume=181 |issue=4610 |pages=693–694 |doi=10.1038/181693b0 |bibcode=1958Natur.181..693F |s2cid=4269111 |issn=0028-0836}}</ref><ref>{{Cite journal |last1=Guerret-Piecourt |first1=Christelle |last2=Bec |first2=Sandrine |last3=Treheux |first3=Daniel |date=2001 |title=Electrical charges and tribology of insulating materials |url= https://linkinghub.elsevier.com/retrieve/pii/S1296214701012185 |journal= Comptes Rendus de l'Académie des Sciences, Série IV |volume=2 |issue=5 |pages=761–774 |doi=10.1016/S1296-2147(01)01218-5|arxiv=0707.2649 |bibcode=2001CRASP...2..761G }}</ref> More recently there have been attempts to include a full solid state description.<ref>{{Cite journal |last1=Pan |first1=Shuaihang |last2=Zhang |first2=Zhinan |date=2017 |title=Triboelectric effect: A new perspective on electron transfer process |url= https://pubs.aip.org/jap/article/122/14/144302/144925/Triboelectric-effect-A-new-perspective-on-electron |journal=Journal of Applied Physics |volume=122 |issue=14 |page=144302 |doi=10.1063/1.5006634 |bibcode=2017JAP...122n4302P |issn=0021-8979}}</ref><ref name=":39">{{Cite journal |last1=Olson |first1=Karl P. |last2=Mizzi |first2=Christopher A. |last3=Marks |first3=Laurence D. |date=2022 |title=Band Bending and Ratcheting Explain Triboelectricity in a Flexoelectric Contact Diode |url= https://pubs.acs.org/doi/10.1021/acs.nanolett.2c00107 |journal=Nano Letters |volume=22 |issue=10 |pages=3914–3921 |doi=10.1021/acs.nanolett.2c00107 |pmid=35521939 |arxiv=2201.04688 |bibcode=2022NanoL..22.3914O |s2cid=245906054 |issn=1530-6984}}</ref><ref>{{Cite journal |last1=Willatzen |first1=Morten |last2=Lin Wang |first2=Zhong |date=2018 |title=Theory of contact electrification: Optical transitions in two-level systems |url= https://linkinghub.elsevier.com/retrieve/pii/S2211285518305780 |journal=Nano Energy |volume=52 |pages=517–523 |doi=10.1016/j.nanoen.2018.08.015|bibcode=2018NEne...52..517W |s2cid=106380058 }}</ref><ref name=":11" />

== Explanations and mechanisms ==
From early work starting around the end of the 19th century<ref name=":37" /><ref name=":38" /><ref name=":33" /> a large amount of information is available about what, empirically, causes triboelectricity. While there is extensive experimental data on triboelectricity there is not as yet full scientific consensus on the source,<ref name=":14">{{Cite journal |last=Lacks |first=Daniel J. |date=2012 |title=The Unpredictability of Electrostatic Charging |url= https://onlinelibrary.wiley.com/doi/10.1002/anie.201202896 |journal=Angewandte Chemie International Edition |volume=51 |issue=28 |pages=6822–6823 |doi=10.1002/anie.201202896|pmid=22653881 }}</ref><ref name=":15">{{Cite journal |last1=Lacks |first1=Daniel J. |last2=Shinbrot |first2=Troy |date=2019 |title=Long-standing and unresolved issues in triboelectric charging |url= https://www.nature.com/articles/s41570-019-0115-1 |journal=Nature Reviews Chemistry |volume=3 |issue=8 |pages=465–476 |doi=10.1038/s41570-019-0115-1 |s2cid=197403212 |issn=2397-3358}}</ref> or perhaps more probably the sources. Some aspects are established, and will be part of the full picture:
* Work function differences between the two materials.<ref name=":2" />
* Local curvature, strain and roughness.<ref name=":8" /><ref name=":19" /><ref name=":21" />
* The forces used during sliding, and the velocities when particles collide as well as the sizes.<ref name=":10" /><ref name=":17" />
* The electronic structure of the materials, and the crystallographic orientation of the two contacting materials.<ref name=":1" />
* Surface or interface states, as well as environmental factors such as humidity.<ref name=":1" /><ref name=":2" />

=== Triboelectric series ===
[[File:Triboelectric-series_EN.svg|thumb|right|A simple triboelectric series]]

An empirical approach to triboelectricity is a triboelectric series''.'' This is a list of materials ordered by how they develop a charge relative to other materials on the list. [[Johan Carl Wilcke]] published the first one in a 1757 paper.<ref name=":35" /><ref name="Dictionary of Scientific Biography" /> The series was expanded by Shaw<ref name=":19" /> and Henniker<ref name=":32" /> by including natural and synthetic polymers, and included alterations in the sequence depending on surface and environmental conditions. Lists vary somewhat as to the order of some materials.<ref name=":19">{{Cite journal |last=Shaw |first=P. E. |date=1917 |title=Experiments on tribo-electricity. I.—The tribo-electric series |journal=Proceedings of the Royal Society of London. Series A, Containing Papers of a Mathematical and Physical Character |volume=94 |issue=656 |pages=16–33 |doi=10.1098/rspa.1917.0046 |bibcode=1917RSPSA..94...16S |issn=0950-1207|doi-access=free }}</ref><ref name=":32">{{Cite journal |vauthors=Henniker J |date=1962 |title=Triboelectricity in Polymers |journal=Nature |volume=196 |issue=4853 |page=474 |bibcode=1962Natur.196..474H |doi=10.1038/196474a0 |s2cid=4211729|doi-access=free }}</ref>

Another triboelectric series based on measuring the triboelectric charge density of materials was proposed by the group of [[Zhong Lin Wang]]. The triboelectric [[charge density]] of the tested materials was measured with respect to liquid mercury in a [[glove box]] under well-defined conditions, with fixed temperature, pressure and humidity.<ref name=":0">{{cite journal |display-authors=6 |vauthors=Zou H, Zhang Y, Guo L, Wang P, He X, Dai G, Zheng H, Chen C, Wang AC, Xu C, Wang ZL |date=2019 |title=Quantifying the triboelectric series |journal=Nature Communications |volume=10 |issue=1 |page=1427 |bibcode=2019NatCo..10.1427Z |doi=10.1038/s41467-019-09461-x |pmc=6441076 |pmid=30926850}}</ref><ref>{{Cite journal |last1=Zou |first1=Haiyang |last2=Guo |first2=Litong |last3=Xue |first3=Hao |last4=Zhang |first4=Ying |last5=Shen |first5=Xiaofang |last6=Liu |first6=Xiaoting |last7=Wang |first7=Peihong |last8=He |first8=Xu |last9=Dai |first9=Guozhang |last10=Jiang |first10=Peng |last11=Zheng |first11=Haiwu |last12=Zhang |first12=Binbin |last13=Xu |first13=Cheng |last14=Wang |first14=Zhong Lin |date=2020-04-29 |title=Quantifying and understanding the triboelectric series of inorganic non-metallic materials |journal=Nature Communications |language=en |volume=11 |issue=1 |page=2093 |doi=10.1038/s41467-020-15926-1 |issn=2041-1723 |pmc=7190865 |pmid=32350259|bibcode=2020NatCo..11.2093Z }}</ref>
[[File:Cyclic triboelectric series example.png|thumb|Cyclic triboelectric series example, illustrating that a linear approach does not work in practice]]
It is known that this approach is too simple and unreliable.<ref name=":1" /><ref name=":2" /><ref name=":3">{{Cite journal |last1=Lowell |first1=J. |last2=Rose-Innes |first2=A.C. |date=1980 |title=Contact electrification |journal=Advances in Physics |volume=29 |issue=6 |pages=947–1023 |doi=10.1080/00018738000101466 |bibcode=1980AdPhy..29..947L |issn=0001-8732}}</ref> There are many cases where there are triangles: material A is positive when rubbed against B, B is positive when rubbed against C, and C is positive when rubbed against A, an issue mentioned by Shaw in 1914.<ref name=":33" /> This cannot be explained by a linear series;  cyclic series are inconsistent with the empirical triboelectric series.<ref>{{Cite journal |last1=Pan |first1=Shuaihang |last2=Zhang |first2=Zhinan |date=2019 |title=Fundamental theories and basic principles of triboelectric effect: A review |journal=Friction |volume=7 |issue=1 |pages=2–17 |doi=10.1007/s40544-018-0217-7 |s2cid=256406551 |issn=2223-7690|doi-access=free }}</ref> Furthermore, there are many cases where charging occurs with contacts between two pieces of the same material.<ref>{{Cite journal |last1=Lowell |first1=J. |last2=Truscott |first2=W. S. |date=1986 |title=Triboelectrification of identical insulators. I. An experimental investigation |url= https://iopscience.iop.org/article/10.1088/0022-3727/19/7/017 |journal=Journal of Physics D: Applied Physics |volume=19 |issue=7 |pages=1273–1280 |doi=10.1088/0022-3727/19/7/017 |bibcode=1986JPhD...19.1273L |s2cid=250769950 |issn=0022-3727}}</ref><ref>{{Cite journal |last1=Lowell |first1=J. |last2=Truscott |first2=W. S. |date=1986 |title=Triboelectrification of identical insulators. II. Theory and further experiments |url= https://iopscience.iop.org/article/10.1088/0022-3727/19/7/018 |journal=Journal of Physics D: Applied Physics |volume=19 |issue=7 |pages=1281–1298 |doi=10.1088/0022-3727/19/7/018 |bibcode=1986JPhD...19.1281L |s2cid=250811149 |issn=0022-3727}}</ref><ref name=":9">{{Cite journal |last1=Baytekin |first1=H. T. |last2=Patashinski |first2=A. Z. |last3=Branicki |first3=M. |last4=Baytekin |first4=B. |last5=Soh |first5=S. |last6=Grzybowski |first6=B. A. |date=2011 |title=The Mosaic of Surface Charge in Contact Electrification |journal=Science |volume=333 |issue=6040 |pages=308–312 |doi=10.1126/science.1201512 |pmid=21700838 |bibcode=2011Sci...333..308B |s2cid=18450118 |issn=0036-8075|doi-access=free |hdl=20.500.11820/f416715b-eaa4-4051-a054-a6cd527a6066 |hdl-access=free }}</ref> This has been modelled as a consequence of the electric fields from local bending ([[flexoelectricity]]).<ref name=":5" /><ref name=":26" /><ref name=":25">{{Cite journal |last=Persson |first=B. N. J. |date=2020 |title=On the role of flexoelectricity in triboelectricity for randomly rough surfaces |url= https://iopscience.iop.org/article/10.1209/0295-5075/129/10006 |journal=EPL (Europhysics Letters) |volume=129 |issue=1 |page=10006 |doi=10.1209/0295-5075/129/10006 |arxiv=1911.06207 |bibcode=2020EL....12910006P |s2cid=208615180 |issn=1286-4854}}</ref>

=== Work function differences ===
[[File:Work function mismatch gold aluminum.svg|thumb|When the two metals depicted here are in thermodynamic equilibrium with each other as shown (equal [[Fermi level]]s), the vacuum [[electrostatic potential]] ''ϕ'' is not flat due to a difference in [[work function]].]]
In all materials there is a positive electrostatic potential from the positive atomic nuclei, partially balanced by a negative electrostatic potential of what can be described as a sea of electrons.<ref name=":16">{{Cite book |last1=Ashcroft |first1=Neil W. |last2=Mermin |first2=N. David |title=Solid State Physics |date=1976 |publisher=Cengage Learning |isbn=978-0-03-083993-1}}</ref> The average potential is positive, what is called the ''mean inner potential'' (MIP). Different materials have different MIPs, depending upon the types of atoms and how close they are. At a surface the electrons also spill out a little into the vacuum, as analyzed in detail by [[Walter Kohn|Kohn]] and Liang.<ref name=":16" /><ref name=":20">{{Cite journal |last1=Lang |first1=N. D. |last2=Kohn |first2=W. |date=1971 |title=Theory of Metal Surfaces: Work Function |url= https://link.aps.org/doi/10.1103/PhysRevB.3.1215 |journal=Physical Review B |volume=3 |issue=4 |pages=1215–1223 |doi=10.1103/PhysRevB.3.1215 |bibcode=1971PhRvB...3.1215L |issn=0556-2805}}</ref> This leads to a [[dipole]] at the surface. Combined, the dipole and the MIP lead to a potential barrier for electrons to leave a material which is called the [[work function]].<ref name=":16" />

A rationalization of the triboelectric series is that different members have different work functions, so electrons can go from the material with a small work function to one with a large.<ref name=":1" /> The potential difference between the two materials is called the [[Volta potential]], also called the ''contact potential''. Experiments have validated the importance of this for metals and other materials.<ref name=":4" />  However, because the surface dipoles vary for different surfaces of any solid<ref name=":16" /><ref name=":20" /> the contact potential is not a universal parameter. By itself it cannot explain many of the results which were established in the early 20th century.<ref name=":6" /><ref name=":7" /><ref name=":8" />

=== Electromechanical contributions ===
Whenever a solid is strained, electric fields can be generated. One process is due to linear strains, and is called [[piezoelectricity]], the  second depends upon how rapidly strains are changing with distance (derivative) and is called [[flexoelectricity]]. Both are established science, and can be both measured and calculated using [[density functional theory]] methods. Because flexoelectricity depends upon a gradient it can be much larger at the nanoscale during sliding or contact of asperity between two objects.<ref name=":18" />

There has been considerable work on the connection between piezoelectricity and triboelectricity.<ref>{{Cite journal |last=Peterson |first=John W. |date=1949 |title=The Influence of Piezo-Electrification on Tribo-Electrification |url= https://link.aps.org/doi/10.1103/PhysRev.76.1882.2 |journal=Physical Review |volume=76 |issue=12 |pages=1882–1883 |doi=10.1103/PhysRev.76.1882.2 |bibcode=1949PhRv...76.1882P |issn=0031-899X}}</ref><ref>{{Cite journal |last=Harper |first=W. R. |date=1955 |title=Adhesion and charging of quartz surfaces |url= https://royalsocietypublishing.org/doi/10.1098/rspa.1955.0182 |journal=Proceedings of the Royal Society of London. Series A. Mathematical and Physical Sciences |volume=231 |issue=1186 |pages=388–403 |doi=10.1098/rspa.1955.0182 |bibcode=1955RSPSA.231..388H |s2cid=137276822 |issn=0080-4630}}</ref> While it can be important, piezoelectricity only occurs in the small number of materials which do not have inversion symmetry,<ref name=":16" /> so it is not a general explanation. It has recently been suggested that flexoelectricity may be very important<ref name=":5" /> in triboelectricity as it occurs in all insulators and semiconductors.<ref name=":29">{{Cite journal |last1=Zubko |first1=Pavlo |last2=Catalan |first2=Gustau |last3=Tagantsev |first3=Alexander K. |date=2013 |title=Flexoelectric Effect in Solids |url= https://www.annualreviews.org/doi/10.1146/annurev-matsci-071312-121634 |journal=Annual Review of Materials Research |volume=43 |issue=1 |pages=387–421 |doi=10.1146/annurev-matsci-071312-121634 |bibcode=2013AnRMS..43..387Z |hdl=10261/99362 |issn=1531-7331|hdl-access=free }}</ref><ref>{{Cite journal |last1=Arias |first1=Irene |last2=Catalan |first2=Gustau |last3=Sharma |first3=Pradeep |date=2022 |title=The emancipation of flexoelectricity |url= https://pubs.aip.org/aip/jap/article/2836249 |journal=Journal of Applied Physics |volume=131 |issue=2 |page=020401 |doi=10.1063/5.0079319 |bibcode=2022JAP...131b0401A |s2cid=245897525 |issn=0021-8979|doi-access=free |hdl=10261/280763 |hdl-access=free }}</ref> Quite a few of the experimental results such as the effect of curvature can be explained by this approach, although full details have not as yet been determined.<ref name=":26">{{Cite journal |last1=Mizzi |first1=Christopher A. |last2=Marks |first2=Laurence D. |date=2022 |title=When Flexoelectricity Drives Triboelectricity |url= https://pubs.acs.org/doi/10.1021/acs.nanolett.2c00240 |journal=Nano Letters |volume=22 |issue=10 |pages=3939–3945 |doi=10.1021/acs.nanolett.2c00240 |pmid=35575563 |bibcode=2022NanoL..22.3939M |s2cid=225070213 |issn=1530-6984}}</ref>  There is also early work from Shaw and Hanstock,<ref name="royalsocietypublishing.org"/> and from the group of Daniel Lacks demonstrating that strain matters.<ref>{{Cite journal |last1=Sow |first1=Mamadou |last2=Lacks |first2=Daniel J. |last3=Mohan Sankaran |first3=R. |date=2012 |title=Dependence of contact electrification on the magnitude of strain in polymeric materials |url= https://pubs.aip.org/aip/jap/article/375942 |journal=Journal of Applied Physics |volume=112 |issue=8 |pages=084909–084909–5 |doi=10.1063/1.4761967 |bibcode=2012JAP...112h4909S |issn=0021-8979}}</ref><ref>{{Cite journal |last1=Sow |first1=Mamadou |last2=Lacks |first2=Daniel J. |last3=Sankaran |first3=R. Mohan |date=2013 |title=Effects of material strain on triboelectric charging: Influence of material properties |url= https://linkinghub.elsevier.com/retrieve/pii/S0304388612001350 |journal=Journal of Electrostatics |volume=71 |issue=3 |pages=396–399 |doi=10.1016/j.elstat.2012.11.021}}</ref><ref name=":21">{{Cite journal |last1=Xie |first1=L. |last2=He |first2=P. F. |last3=Zhou |first3=J. |last4=Lacks |first4=D. J. |date=2014 |title=Correlation of contact deformation with contact electrification of identical materials |url= https://iopscience.iop.org/article/10.1088/0022-3727/47/21/215501 |journal=Journal of Physics D: Applied Physics |volume=47 |issue=21 |page=215501 |doi=10.1088/0022-3727/47/21/215501 |bibcode=2014JPhD...47u5501X |s2cid=121319419 |issn=0022-3727}}</ref>

=== Capacitor charge compensation model ===

[[File:Capacitor schematic with dielectric.svg|thumb|Capacitor schematic with dielectric]]
An explanation that has appeared in different forms is analogous to charge on a capacitor. If there is a potential difference between two materials due to the difference in their work functions (contact potential), this can be thought of as equivalent to the potential difference across a capacitor. The charge to compensate this is that which cancels the electric field. If an insulating dielectric is in between the two materials, then this will lead to a [[polarization density]] <math>\mathbf P</math> and a [[Maxwell's equations#Bound charge and current|bound surface charge]] of <math>\mathbf P \cdot \mathbf n</math>, where <math>\mathbf n</math> is the surface normal.<ref>{{Cite journal |last=Fisher |first=L. H. |date=1951 |title=On the Representation of the Static Polarization of Rigid Dielectrics by Equivalent Charge Distributions |url= https://pubs.aip.org/aapt/ajp/article/19/2/73-78/1034513 |journal=American Journal of Physics |volume=19 |issue=2 |pages=73–78 |doi=10.1119/1.1932714 |bibcode=1951AmJPh..19...73F |issn=0002-9505}}</ref><ref>{{Cite book |last=Griffiths |first=David |title=Introduction to Electrodynamics |date=29 June 2017 |publisher=Cambridge University Press |isbn=978-1-108-33351-1 |pages=296–354 |doi=10.1017/9781108333511.008}}</ref> The total charge in the capacitor is then the combination of the bound surface charge from the polarization and that from the potential.

The triboelectric charge from this compensation model has been frequently considered as a key component.<ref>{{Cite journal |last=Ireland |first=Peter M. |date=2010 |title=Triboelectrification of particulate flows on surfaces: Part II — Mechanisms and models |url= https://linkinghub.elsevier.com/retrieve/pii/S0032591009006081 |journal=Powder Technology |volume=198 |issue=2 |pages=199–210 |doi=10.1016/j.powtec.2009.11.008}}</ref><ref>{{Cite journal |last1=Matsusaka |first1=S. |last2=Maruyama |first2=H. |last3=Matsuyama |first3=T. |last4=Ghadiri |first4=M. |date=2010 |title=Triboelectric charging of powders: A review |url= https://linkinghub.elsevier.com/retrieve/pii/S0009250910004239 |journal=Chemical Engineering Science |volume=65 |issue=22 |pages=5781–5807 |doi=10.1016/j.ces.2010.07.005|bibcode=2010ChEnS..65.5781M |hdl=2433/130693 |hdl-access=free }}</ref><ref>{{Cite journal |last1=Xie |first1=Li |last2=Li |first2=Junjie |last3=Liu |first3=Yakui |date=2020 |title=Review on charging model of sand particles due to collisions |journal=Theoretical and Applied Mechanics Letters |volume=10 |issue=4 |pages=276–285 |doi=10.1016/j.taml.2020.01.047 |s2cid=225960006 |issn=2095-0349|doi-access=free |bibcode=2020TAML...10..276X }}</ref><ref>{{Cite journal |last1=Han |first1=Chun |last2=Zhou |first2=Qun |last3=Hu |first3=Jiawei |last4=Liang |first4=Cai |last5=Chen |first5=Xiaoping |last6=Ma |first6=Jiliang |date=2021 |title=The charging characteristics of particle–particle contact |url= https://linkinghub.elsevier.com/retrieve/pii/S0304388621000322 |journal=Journal of Electrostatics |volume=112 |page=103582 |doi=10.1016/j.elstat.2021.103582|s2cid=235513618 }}</ref> If the additional polarization due to strain ([[piezoelectricity]]) or bending of samples ([[flexoelectricity]]) is included<ref name=":5" /><ref name=":26" /> this can explain observations such as the effect of curvature<ref name=":8" /> or inhomogeneous charging.<ref name=":25" />

=== Electron and/or ion transfer ===
There is debate about whether electrons or ions are transferred in triboelectricity. For instance, Harper<ref name=":2" /> discusses both possibilities, whereas Vick<ref name=":1" /> was more in favor of electron transfer. The debate remains to this day with, for instance, [[George M. Whitesides]] advocating for ions,<ref>{{Cite journal |last1=McCarty |first1=Logan S. |last2=Whitesides |first2=George M. |date=2008 |title=Electrostatic Charging Due to Separation of Ions at Interfaces: Contact Electrification of Ionic Electrets |url= https://onlinelibrary.wiley.com/doi/10.1002/anie.200701812 |journal=Angewandte Chemie International Edition |volume=47 |issue=12 |pages=2188–2207 |doi=10.1002/anie.200701812|pmid=18270989 }}</ref> while Diaz and Fenzel-Alexander<ref>{{Cite journal |last1=Diaz |first1=A. F. |last2=Fenzel-Alexander |first2=D. |date=1993 |title=An ion transfer model for contact charging |url= https://pubs.acs.org/doi/abs/10.1021/la00028a021 |journal=Langmuir |volume=9 |issue=4 |pages=1009–1015 |doi=10.1021/la00028a021 |issn=0743-7463}}</ref> as well as [[Laurence D. Marks]] support both,<ref name=":5" /><ref name=":26" /> and others just electrons.<ref>{{Cite journal |last1=Liu |first1=Chongyang |last2=Bard |first2=Allen J. |date=2008 |title=Electrostatic electrochemistry at insulators |url= https://www.nature.com/articles/nmat2160 |journal=Nature Materials |volume=7 |issue=6 |pages=505–509 |doi=10.1038/nmat2160 |pmid=18362908 |bibcode=2008NatMa...7..505L |issn=1476-4660}}</ref>

=== Thermodynamic irreversibility ===

In the latter half of the 20th century the [[Soviet Union|Soviet]] school led by chemist [[Boris Derjaguin]] argued that triboelectricity and the associated phenomenon of [[triboluminescence]] are fundamentally [[irreversible process|irreversible]].<ref name="Deryagin">{{cite book |last1=Deryagin |first1=B. V. |last2=Krotova |first2=N. A. |last3=Smilga |first3=V. P. | title=Adhesion of Solids |chapter=II |publisher=Consultants Bureau |date=1978 |translator-last=Johnston |translator-first=R. K. |isbn=978-1-4615-8191-8 |url=https://link.springer.com/book/9781461581918}}</ref>  A similar point of view to Derjaguin's has been more recently advocated by [[Seth Putterman]] and his collaborators at the [[University of California, Los Angeles]] (UCLA).<ref name="Putterman08">{{Cite journal |last1=Camara |first1=Carlos G. |last2=Escobar |first2=Juan V. |last3=Hird |first3=Jonathan R. |last4=Putterman |first4=Seth J. |date=2008 |title=Correlation between nanosecond X-ray flashes and stick–slip friction in peeling tape |url=https://www.nature.com/articles/nature07378 |journal=Nature |language=en |volume=455 |issue=7216 |pages=1089–1092 |doi=10.1038/nature07378 |bibcode=2008Natur.455.1089C |s2cid=4372536 |issn=0028-0836}}</ref><ref name="Putterman18"}>{{Cite journal |last1=Collins |first1=Adam L. |last2=Camara |first2=Carlos G. |last3=Van Cleve |first3=Eli |last4=Putterman |first4= Seth J. |date=2018 |title=Simultaneous measurement of triboelectrification and triboluminescence of crystalline materials |journal=Rev. Sci. Instrum. |language=en |volume=89 |issue=1 |pages=013901 |doi=10.1063/1.5006811|pmid=29390647 |bibcode=2018RScI...89a3901C }}</ref>

A proposed theory of triboelectricity as a fundamentally irreversible process was published in 2020 by theoretical physicists Robert Alicki and [[Alejandro Jenkins]].<ref name=":11" /> They argued that the electrons in the two materials that slide against each other have different velocities, giving a [[Non-equilibrium thermodynamics|non-equilibrium state]].  [[Quantum mechanics|Quantum effects]] cause this imbalance to pump electrons from one material to the other.<ref name=":11" />  This is a [[fermion]]ic analog of the mechanism of [[Superradiance#Rotational_superradiance|rotational superradiance]] originally described by [[Yakov Zeldovich]] for [[boson]]s.<ref name=":11" />  Electrons are pumped in both directions, but small differences in the electronic potential landscapes for the two surfaces can cause net charging.<ref name=":11" /> Alicki and Jenkins argue that such an irreversible pumping is needed to understand how the triboelectric effect can generate an [[electromotive force]].<ref name=":11" /><ref>{{cite news |last=Demming |first=Anna |date= 6 Oct 2020 |title=Quantum treatment sheds fresh light on triboelectricity |url=https://physicsworld.com/a/quantum-treatment-sheds-fresh-light-on-triboelectricity/ |work=Physics World |location= Bristol, UK |access-date=18 Jan 2021}}</ref>

=== Humidity ===
Generally, increased [[humidity]] (water in the air) leads to a decrease in the magnitude of triboelectric charging.<ref>{{Cite journal |last1=Matsusaka |first1=S. |last2=Maruyama |first2=H. |last3=Matsuyama |first3=T. |last4=Ghadiri |first4=M. |date=2010 |title=Triboelectric charging of powders: A review |url=https://www.sciencedirect.com/science/article/pii/S0009250910004239 |journal=Chemical Engineering Science |language=en |volume=65 |issue=22 |pages=5781–5807 |doi=10.1016/j.ces.2010.07.005 |bibcode=2010ChEnS..65.5781M |hdl=2433/130693 |issn=0009-2509|hdl-access=free }}</ref> The size of this effect varies greatly depending on the contacting materials; the decrease in charging ranges from up to a factor of 10 or more to very little humidity dependence.<ref>{{cite journal |last1=Németh |first1=Ernő |last2=Albrecht |first2=Victoria |last3=Schubert |first3=Gert |last4=Simon |first4=Frank |date=2003 |title=Polymer tribo-electric charging: dependence on thermodynamic surface properties and relative humidity |url=https://www.sciencedirect.com/science/article/pii/S0304388602001377 |journal=Journal of Electrostatics |volume=58 |issue=1–2 |pages=3–16 |doi=10.1016/S0304-3886(02)00137-7}}</ref> Some experiments find increased charging at moderate humidity compared to extremely dry conditions before a subsequent decrease at higher humidity.<ref name=":31">{{Cite journal |last1=Pence |first1=S. |last2=Novotny |first2=V. J. |last3=Diaz |first3=A. F. |date=1994 |title=Effect of Surface Moisture on Contact Charge of Polymers Containing Ions |url=https://pubs.acs.org/doi/abs/10.1021/la00014a042 |journal=Langmuir |volume=10 |issue=2 |pages=592–596 |doi=10.1021/la00014a042}}</ref> The most widespread explanation is that higher humidity leads to more water [[Adsorption|adsorbed]] at the surface of contacting materials, leading to a higher [[Surface conductivity#Surface Science|surface conductivity]].<ref name=":30">{{Cite journal |last1=Németh |first1=Ernő |last2=Albrecht |first2=Victoria |last3=Schubert |first3=Gert |last4=Simon |first4=Frank |date=2003 |title=Polymer tribo-electric charging: dependence on thermodynamic surface properties and relative humidity |url=https://www.sciencedirect.com/science/article/pii/S0304388602001377 |journal=Journal of Electrostatics |language=en |volume=58 |issue=1 |pages=3–16 |doi=10.1016/S0304-3886(02)00137-7 |issn=0304-3886}}</ref><ref>{{Cite journal |last1=Awakuni |first1=Y |last2=Calderwood |first2=J H |date=1972 |title=Water vapour adsorption and surface conductivity in solids |url=https://iopscience.iop.org/article/10.1088/0022-3727/5/5/323 |journal=Journal of Physics D: Applied Physics |volume=5 |issue=5 |pages=1038–1045 |doi=10.1088/0022-3727/5/5/323|bibcode=1972JPhD....5.1038A |s2cid=250802832 }}</ref> The higher conductivity allows for greater [[Carrier generation and recombination|charge recombination]] as contacts separate, resulting in a smaller transfer of charge.<ref name=":30" /><ref>{{Cite journal |last1=Lesprit |first1=Ugo |last2=Paillat |first2=Thierry |last3=Zouzou |first3=Noureddine |last4=Paquier |first4=Anna |last5=Yonger |first5=Marc |date=2021 |title=Triboelectric charging of a glass bead impacting against polymers: Antistatic effects in glass/PU electrification in a humidity-controlled environment |journal=Journal of Electrostatics |language=en |volume=113 |page=103605 |doi=10.1016/j.elstat.2021.103605 |issn=0304-3886|doi-access=free }}</ref><ref>{{Cite journal |last1=Toth |first1=Joseph R. |last2=Phillips |first2=Amber K. |last3=Rajupet |first3=Siddharth |last4=Sankaran |first4=R. Mohan |last5=Lacks |first5=Daniel J. |date=2017 |title=Particle-Size-Dependent Triboelectric Charging in Single-Component Granular Materials: Role of Humidity |url=https://pubs.acs.org/doi/10.1021/acs.iecr.7b02328 |journal=Industrial & Engineering Chemistry Research |language=en |volume=56 |issue=35 |pages=9839–9845 |doi=10.1021/acs.iecr.7b02328 |issn=0888-5885}}</ref> Another proposed explanation for humidity effects considers the case when charge transfer is observed to increase with humidity in dry conditions. Increasing humidity may lead to the formation of water bridges between contacting materials that promote the transfer of ions.<ref name=":31" />

== Examples ==
=== Friction and adhesion from tribocharging ===
[[Friction]]<ref>{{Cite journal |last1=Popova |first1=Elena |last2=Popov |first2=Valentin L. |date=2015 |title=The research works of Coulomb and Amontons and generalized laws of friction |journal=Friction |language=en |volume=3 |issue=2 |pages=183–190 |doi=10.1007/s40544-015-0074-6 |s2cid=256405946 |issn=2223-7704|doi-access=free }}</ref> is a retarding force due to different [[Friction#The origin of friction|energy dissipation]] process such as elastic and plastic deformation, phonon and electron excitation, and also [[Tribology#The cohesion theory|adhesion]].<ref>{{Cite book |last1=Stachowiak |first1=Gwidon |url=https://books.google.com/books?id=SQ8p0x2S1dwC |title=Engineering Tribology |last2=Batchelor |first2=Andrew W. |date=2011 |publisher=Elsevier |isbn=978-0-08-053103-8 |language=en}}</ref>  As an example, in a car or any other vehicle the wheels elastically deform as they roll. Part of the energy needed for this deformation is recovered ([[Elastic Deformation|elastic deformation]]), some is not and goes into heating the tires. The energy which is not recovered contributes to the back force, a process called [[rolling friction]].

Similar to rolling friction there are energy terms in charge transfer, which contribute to friction. In [[static friction]] there is coupling between elastic strains, polarization and surface charge which contributes to the frictional force.<ref name=":29" /> In sliding friction,<ref>{{Cite book |last=Persson |first=Bo |url=https://books.google.com/books?id=1jb-nZMnRGYC&q=kinetic+friction |title=Sliding Friction: Physical Principles and Applications |date=2000 |publisher=Springer Science & Business Media |isbn=978-3-540-67192-3 |language=en}}</ref> when asperities contact<ref name=":18" /> and there is charge transfer, some of the charge returns as the contacts are released, some does not<ref>{{Cite journal |last1=Ko |first1=Hyunseok |last2=Lim |first2=Yeong-won |last3=Han |first3=Seungwu |last4=Jeong |first4=Chang Kyu |last5=Cho |first5=Sung Beom |date=2021 |title=Triboelectrification: Backflow and Stuck Charges Are Key |url=https://pubs.acs.org/doi/10.1021/acsenergylett.1c01019 |journal=ACS Energy Letters |language=en |volume=6 |issue=8 |pages=2792–2799 |doi=10.1021/acsenergylett.1c01019 |s2cid=237720731 |issn=2380-8195}}</ref> and will contribute to the macroscopically observed friction. There is evidence for a retarding [[Coulomb Force|Coulomb force]] between asperities of different charges,<ref name="Burgo1">{{Cite journal |last1=Burgo |first1=Thiago A. L. |last2=Silva |first2=Cristiane A. |last3=Balestrin |first3=Lia B. S. |last4=Galembeck |first4=Fernando |date=2013 |title=Friction coefficient dependence on electrostatic tribocharging |journal=Scientific Reports |language=en |volume=3 |issue=1 |page=2384 |doi=10.1038/srep02384 |issn=2045-2322 |pmc=3740278 |pmid=23934227|bibcode=2013NatSR...3E2384B }}</ref> and an increase in the adhesion from contact electrification when geckos walk on water.<ref>{{Cite journal |last1=Izadi |first1=Hadi |last2=Stewart |first2=Katherine M. E. |last3=Penlidis |first3=Alexander |date=2014 |title=Role of contact electrification and electrostatic interactions in gecko adhesion |journal=Journal of the Royal Society Interface |language=en |volume=11 |issue=98 |doi=10.1098/rsif.2014.0371 |issn=1742-5689 |pmc=4233685 |pmid=25008078}}</ref> There is also evidence of connections between jerky (stick–slip) processes during sliding with charge transfer,<ref name=":12" /> electrical discharge<ref>{{Cite journal |last1=Schnurmann |first1=Robert |last2=Warlow-Davies |first2=Eric |date=1942 |title=The electrostatic component of the force of sliding friction |url=https://iopscience.iop.org/article/10.1088/0959-5309/54/1/303 |journal=Proceedings of the Physical Society |volume=54 |issue=1 |pages=14–27 |doi=10.1088/0959-5309/54/1/303 |bibcode=1942PPS....54...14S |issn=0959-5309}}</ref> and x-ray emission.<ref name="Putterman08" /> How large the triboelectric contribution is to friction has been debated. It has been suggested by some<ref name="Burgo1" /> that it may dominate for polymers, whereas Harper<ref>{{Cite journal |last=Harper |first=W. R. |date=1955 |title=Adhesion and charging of quartz surfaces |url=https://royalsocietypublishing.org/doi/10.1098/rspa.1955.0182 |journal=Proceedings of the Royal Society of London. Series A. Mathematical and Physical Sciences |language=en |volume=231 |issue=1186 |pages=388–403 |doi=10.1098/rspa.1955.0182 |bibcode=1955RSPSA.231..388H |s2cid=137276822 |issn=0080-4630}}</ref>  has argued that it is small.

=== Liquids and gases ===
[[File:Tribocharge generated from a sliding drop.tif|thumb|Illustration of tribocharge generated from a sliding drop]]
The generation of static electricity from the relative motion of liquids or gases is well established, with one of the first analyses in 1886 by [[Lord Kelvin]] in his [[Kelvin water dropper|water dropper]] which used falling drops to create an electric generator.<ref>{{Cite journal |last=Thomson |first=W. |date=1868 |title=XVI. On a self-acting apparatus for multiplying and maintaining electric charges, with applications to illustrate the voltaic theory |journal=Proceedings of the Royal Society of London |volume=16 |pages=67–72 |doi=10.1098/rspl.1867.0019 |s2cid=110760051 |issn=0370-1662|doi-access=free }}</ref> Liquid mercury is a special case as it typically acts as a simple metal, so has been used as a reference electrode.<ref name=":34">{{Cite journal |last=Freund |first=Thomas |date=1979 |title=Tribo-electricity |url= https://linkinghub.elsevier.com/retrieve/pii/0001868679800032 |journal=Advances in Colloid and Interface Science |volume=11 |issue=1 |pages=43–66 |doi=10.1016/0001-8686(79)80003-2}}</ref> More common is water, and electricity due to water droplets hitting surfaces has been documented since the discovery by [[Philipp Lenard]] in 1892 of the ''spray electrification'' or ''waterfall effect''.<ref>{{Cite journal |last=Lenard |first=Philipp |date=1892 |title=Ueber die Electricität der Wasserfälle |journal=Annalen der Physik und Chemie |volume=282 |issue=8 |pages=584–636 |doi=10.1002/andp.18922820805 |bibcode=1892AnP...282..584L |issn=0003-3804|url=https://zenodo.org/record/1587780 }}</ref><ref>{{Cite book |last=Loeb |first=Leonard B. |url= http://link.springer.com/10.1007/978-3-642-88243-2 |title=Static Electrification |date=1958 |publisher=Springer |isbn=978-3-642-88245-6 |location=Berlin / Heidelberg |doi=10.1007/978-3-642-88243-2}}</ref> This is when falling water generates static electricity either by collisions between water drops or with the ground, leading to the finer mist in updrafts being mainly negatively charged, with positive near the lower surface. It can also occur for sliding drops.<ref>{{Cite journal |last1=Helseth |first1=L. E. |last2=Wen |first2=H Z |date=2017 |title=Visualisation of charge dynamics when water droplets move off a hydrophobic surface |url= https://iopscience.iop.org/article/10.1088/1361-6404/aa82f7 |journal=European Journal of Physics |volume=38 |issue=5 |page=055804 |doi=10.1088/1361-6404/aa82f7 |bibcode=2017EJPh...38e5804H |s2cid=125757544 |issn=0143-0807}}</ref>

Another type of charge can be produced during rapid solidification of water containing ions, which is called the ''Workman–Reynolds effect''.<ref>{{Cite journal |last=Gross |first=Gerardo Wolfgang |date=1965 |title=The Workman–Reynolds effect and ionic transfer processes at the ice-solution interface |journal=Journal of Geophysical Research |volume=70 |issue=10 |pages=2291–2300 |doi=10.1029/jz070i010p02291 |bibcode=1965JGR....70.2291G |issn=0148-0227}}</ref> During the solidification the positive and negative ions may not be equally distributed between the liquid and solid.<ref>{{Cite journal |last=Aziz |first=M. J. |date=1982 |title=Model for solute redistribution during rapid solidification |url= https://pubs.aip.org/jap/article/53/2/1158/11000/Model-for-solute-redistribution-during-rapid |journal=Journal of Applied Physics |volume=53 |issue=2 |pages=1158–1168 |doi=10.1063/1.329867 |bibcode=1982JAP....53.1158A |issn=0021-8979}}</ref> For instance, in thunderstorms this can contribute (together with the waterfall effect) to separation of positive hydrogen ions and negative hydroxide ions, leading to static charge and [[Lightning#Electrification|lightning]].<ref>{{Cite journal |last=Illingworth |first=A. J. |date=1985 |title=Charge separation in thunderstorms: Small scale processes |url=http://doi.wiley.com/10.1029/JD090iD04p06026 |journal=Journal of Geophysical Research |language=en |volume=90 |issue=D4 |page=6026 |doi=10.1029/JD090iD04p06026 |bibcode=1985JGR....90.6026I |issn=0148-0227}}</ref>

A third class is associated with contact potential differences between liquids or gases and other materials, similar to the work function differences for solids. It has been suggested that a triboelectric series for liquids is useful.<ref>{{Cite journal |last1=Yoo |first1=Donghyeon |last2=Jang |first2=Sunmin |last3=Cho |first3=Sumin |last4=Choi |first4=Dongwhi |last5=Kim |first5=Dong Sung |date=2023 |title=A Liquid Triboelectric Series |journal=Advanced Materials |volume=35 |issue=26 |pages=e2300699 |doi=10.1002/adma.202300699 |pmid=36947827 |bibcode=2023AdM....3500699Y |s2cid=257695984 |issn=0935-9648|doi-access=free }}</ref> One difference from solids is that often liquids have charged [[Double layer (surface science)|double layers]], and most of the work to date supports that ion transfer (rather than electron) dominates for liquids<ref>{{Cite journal |last1=Wong |first1=William S. Y. |last2=Bista |first2=Pravash |last3=Li |first3=Xiaomei |last4=Veith |first4=Lothar |last5=Sharifi-Aghili |first5=Azadeh |last6=Weber |first6=Stefan A. L. |last7=Butt |first7=Hans-Jürgen |date=2022 |title=Tuning the Charge of Sliding Water Drops |journal=Langmuir |volume=38 |issue=19 |pages=6224–6230 |doi=10.1021/acs.langmuir.2c00941 |issn=0743-7463 |pmc=9118544 |pmid=35500291}}</ref> as first suggested by [[Irving Langmuir]] in 1938.<ref>{{Cite journal |last=Langmuir |first=Irving |date=1938 |title=Surface Electrification Due to the Recession of Aqueous Solutions from Hydrophobic Surfaces |url= https://pubs.acs.org/doi/abs/10.1021/ja01272a054 |journal=Journal of the American Chemical Society |volume=60 |issue=5 |pages=1190–1194 |doi=10.1021/ja01272a054 |issn=0002-7863}}</ref>

Finally, with liquids there can be flow-rate gradients at interfaces, and also viscosity gradients. These can produce electric fields and also polarization of the liquid, a field called [[electrohydrodynamics]].<ref>{{Cite journal |last=Papageorgiou |first=Demetrios T. |date=2019 |title=Film Flows in the Presence of Electric Fields |journal=Annual Review of Fluid Mechanics |volume=51 |issue=1 |pages=155–187 |doi=10.1146/annurev-fluid-122316-044531 |bibcode=2019AnRFM..51..155P |s2cid=125898175 |issn=0066-4189|doi-access=free }}</ref> These are analogous to the electromechanical terms for solids where electric fields can occur due to elastic strains as described [[#Electromechanical contributions|earlier]].

=== Powders ===
During commercial powder processing<ref name=":10" /><ref>{{Cite journal |last=Castellanos |first=A. |date=2005 |title=The relationship between attractive interparticle forces and bulk behaviour in dry and uncharged fine powders |journal=Advances in Physics |volume=54 |issue=4 |pages=263–376 |doi=10.1080/17461390500402657 |bibcode=2005AdPhy..54..263C |s2cid=122683411 |issn=0001-8732}}</ref><ref>{{Cite journal |last1=Grosshans |first1=Holger |last2=Jantač |first2=Simon |date=2023 |title=Recent progress in CFD modeling of powder flow charging during pneumatic conveying |url= https://linkinghub.elsevier.com/retrieve/pii/S1385894722063987 |journal=Chemical Engineering Journal |volume=455 |page=140918 |doi=10.1016/j.cej.2022.140918|arxiv=2212.04915 |bibcode=2023ChEnJ.45540918G |s2cid=254535685 }}</ref> or in natural processes such as dust storms,<ref>{{Cite journal |last=Rudge |first=W. A. Douglas |date=1912 |title=LXXXI. A note on the electrification of the atmosphere and surface of the earth |url= https://www.tandfonline.com/doi/full/10.1080/14786440508637281 |journal=The London, Edinburgh, and Dublin Philosophical Magazine and Journal of Science |volume=23 |issue=137 |pages=852–855 |doi=10.1080/14786440508637281 |issn=1941-5982}}</ref><ref>{{Cite journal |last=Kunkel |first=W. B. |date=1950 |title=The Static Electrification of Dust Particles on Dispersion into a Cloud |url= https://pubs.aip.org/aip/jap/article/21/8/820-832/508449 |journal=Journal of Applied Physics |volume=21 |issue=8 |pages=820–832 |doi=10.1063/1.1699765 |bibcode=1950JAP....21..820K |issn=0021-8979}}</ref><ref name=":24" /> triboelectric charge transfer can occur. There can be electric fields of up to 160kV/m with moderate wind conditions, which leads to Coulomb forces of about the same magnitude as gravity.<ref>{{Cite journal |last1=Schmidt |first1=D. S. |last2=Schmidt |first2=R. A. |last3=Dent |first3=J. D. |date=1998 |title=Electrostatic force on saltating sand |journal=Journal of Geophysical Research: Atmospheres |volume=103 |issue=D8 |pages=8997–9001 |doi=10.1029/98jd00278 |bibcode=1998JGR...103.8997S |issn=0148-0227|doi-access=free }}</ref> There does not need to be air present, significant charging can occur, for instance, on airless planetary bodies.<ref>{{Cite journal |last1=Wang |first1=X. |last2=Schwan |first2=J. |last3=Hsu |first3=H.-W. |last4=Grün |first4=E. |last5=Horányi |first5=M. |date=2016 |title=Dust charging and transport on airless planetary bodies: Electrostatic Dust Transport |journal=Geophysical Research Letters |volume=43 |issue=12 |pages=6103–6110 |doi=10.1002/2016GL069491|s2cid=132181033 |doi-access=free }}</ref> With pharmaceutic powders and other commercial powders the tribocharging needs to be controlled for quality control of the materials and doses. Static discharge is also a particular hazard in [[grain elevator]]s owing to the danger of a [[dust explosion]],<ref>{{Cite journal |last=Glor |first=Martin |date=2009 |title=Ignition source static electricity: Incident investigation |journal=Journal of Electrostatics |volume=67 |issue=2–3 |pages=242–246 |doi=10.1016/j.elstat.2009.01.016 |issn=0304-3886}}</ref> in places that store explosive powders,<ref>{{Cite journal |last1=Lotfzadeh |first1=Habibeh |last2=Khorasanloo |first2=Fatemeh Hemmati |last3=Fathollahi |first3=Manoochehr |date=2020 |title=Reduction of electrostatic charging PETN and HMX explosives by PVP and ionic liquid |url= https://www.sciencedirect.com/science/article/pii/S0304388620300978 |journal=Journal of Electrostatics |volume=108 |page=103513 |doi=10.1016/j.elstat.2020.103513 |s2cid=224879902 |issn=0304-3886}}</ref> and in many other cases.<ref>{{Cite book |last1=Sandu |first1=Ioana |url= https://www.nfpa.org/-/media/Files/News-and-Research/Fire-statistics-and-reports/Electrical/RFStaticElectricityIncidentReview.pdf |title=Static Electricity Incident Review |last2=Resticcia |first2=Francesco |publisher=Fire Protection Research Foundation |date=2021 |location=Quincy, Massachusetts}}</ref> Triboelectric powder separation has been discussed as a method of separating powders, for instance different biopolymers.<ref>{{Cite journal |last1=Żenkiewicz |first1=Marian |last2=Żuk |first2=Tomasz |last3=Markiewicz |first3=Ewa |date=2015 |title=Triboelectric series and electrostatic separation of some biopolymers |journal=Polymer Testing |volume=42 |pages=192–198 |doi=10.1016/j.polymertesting.2015.01.009 |issn=0142-9418}}</ref> The principle here is that different degrees of charging can be exploited for electrostatic separation, a general concept for powders.<ref>{{Cite journal |last1=El-Mouloud Zelmat |first1=Mohamed |last2=Rizouga |first2=Mohamed |last3=Tilmatine |first3=Amar |last4=Medles |first4=Karim |last5=Miloudi |first5=Mohamed |last6=Dascalescu |first6=Lucien |date=2013 |title=Experimental Comparative Study of Different Tribocharging Devices for Triboelectric Separation of Insulating Particles |url= https://ieeexplore.ieee.org/document/6477116 |journal=IEEE Transactions on Industry Applications |volume=49 |issue=3 |pages=1113–1118 |doi=10.1109/tia.2013.2251991 |s2cid=16419622 |issn=0093-9994}}</ref>

=== In industry ===
[[File:Danger_static_electricity.png|thumb|Static electricity hazard sign (ISO 7010)]]
There are many areas in industry where triboelectricity is known to be an issue. some examples are:

* Non-conducting pipes carrying combustible liquids or fuels such as petrol can result in tribocharge accumulation on the walls of the pipes, which can lead to potentials as large as 90 kV.<ref>{{Cite web |title=Static Electricity Basics {{!}} OPW Retail Fueling EMEA |url= https://www.opwglobal.com/emea/resources/retail-fueling/static-electricity-prevention/static-electricity-basics |access-date=12 July 2023 |website=www.opwglobal.com}}</ref> Pneumatic transport systems in industry can lead to fires due to the tribocharge generated during use.<ref>{{Cite journal |last=Pratt |first=Thomas H. |date=1994 |title=Static electricity in pneumatic transport systems: Three case histories |url= https://onlinelibrary.wiley.com/doi/10.1002/prs.680130302 |journal=Process Safety Progress |volume=13 |issue=3 |pages=109–113 |doi=10.1002/prs.680130302 |s2cid=109719864 |issn=1066-8527}}</ref>
* On ships, contact between cargo and pipelines during loading and unloading, as well as flow in steam pipes and water jets in cleaning machines can lead to dangerous charging.<ref>{{Cite journal |last1=Elidolu |first1=Gizem |last2=Akyuz |first2=Emre |last3=Arslan |first3=Ozcan |last4=Arslanoğlu |first4=Yasin |date=2022 |title=Quantitative failure analysis for static electricity-related explosion and fire accidents on tanker vessels under fuzzy bow-tie CREAM approach |url=https://www.sciencedirect.com/science/article/pii/S1350630721007780 |journal=Engineering Failure Analysis |volume=131 |page=105917 |doi=10.1016/j.engfailanal.2021.105917 |s2cid=244408454 |issn=1350-6307}}</ref> Courses exist to teach mariners the dangers.<ref>{{Cite web |title=Static Electricity Onboard {{!}} Seably |url=https://www.seably.com/courses/static-electricity-onboard |access-date=2023-09-07 |website=www.seably.com |language=en}}</ref>
* US authorities require nearly all industrial facilities to measure particulate dust emissions. Various sensors based on triboelectricity are used, and in 1997 the [[United States Environmental Protection Agency]] issued guidelines for triboelectric fabric-filter bag leak-detection systems.<ref>{{Cite book |last=Midwest Research Institute |url= https://www3.epa.gov/ttnemc01/cem/tribo.pdf |title=Fabric Filter Bag Leak Detection Guidance |publisher=Office Of Air Quality, Environmental Protection Planning And Standards |date=1997}}</ref> Commercial sensors are available for triboelectric dust detection.<ref>{{Cite web |last=Parker |first=Earl |title=Triboelectric Dust Detection Vs Opacity Meters – Big Difference? |url= https://www.auburnsys.com/blog/triboelectric-dust-detection-vs.-opacity-meters-is-there-a-difference |access-date=15 July 2023 |website=www.auburnsys.com}}</ref>
* Wiping a rail near a chemical tank while it is being filled with a flammable chemical can lead to sparks which ignite the chemical. This was the cause of a 2017 explosion that killed one and injured many.<ref>{{Citation |title=WATCH: Dramatic Video Shows Deadly Explosion Inside New Windsor Cosmetics Plant |date=29 November 2017 |url=https://www.cbsnews.com/newyork/news/cosmetics-plant-explosion-video/ |access-date=14 August 2023 |language=en-US}}</ref>

=== Other examples ===
[[File:Winglet_Airbus_A319-132.jpg|thumb|Static wicks on a Winglet Airbus A319-132]]
While the simple case of stroking a cat is familiar to many, there are other areas in modern technological civilization where triboelectricity is exploited or is a concern: 
* Air moving past an aircraft can lead to a buildup of charge called "precipitation static" or "P-static"; aircraft typically have one or more [[static wick]]s to remove it.<ref>{{Cite journal |last1=Pettit |first1=Duane |last2=Turnbull |first2=Andrew |last3=Roelant |first3=Henk A. |date=2001-02-01 |title=General Aviation Aircraft Reliability Study |url=https://ntrs.nasa.gov/citations/20010027423 |journal=National Aeronautics and Space Administration |language=en}}</ref> Checking the status of these is a standard task for pilots.<ref>{{Cite web |last=Tallman |first=Jill |date=11 January 2019 |title=How It Works: Static Wick |url= https://www.aopa.org/news-and-media/all-news/2019/november/flight-training-magazine/how-it-works-static-wick#:~:text=Static%20wick&text=Friction%20generated%20as%20an%20aircraft,with%20navigation%20and%20communication%20radios. |access-date=12 July 2023 |website=www.aopa.org}}</ref> Similarly, helicopter blades move fast, and tribocharging can generate voltages up to 200 kV.<ref>{{Cite journal |last=Siebert |first=Jame M. |date=1 June 1962 |title=Helicopter Static-Electricity Measurements |url= https://apps.dtic.mil/sti/citations/AD0282087 |journal=Defence Technical Information Center |via=Army Transportation Research Command, Fort Eustis, VA}}</ref>
* During planetary formation, a key step is aggregation of dust or smaller particles.<ref name=":36" /> There is evidence that triboelectric charging during collisions of granular material plays a key role in overcoming barriers to aggregation.<ref>{{Cite journal |last1=Steinpilz |first1=Tobias |last2=Joeris |first2=Kolja |last3=Jungmann |first3=Felix |last4=Wolf |first4=Dietrich |last5=Brendel |first5=Lothar |last6=Teiser |first6=Jens |last7=Shinbrot |first7=Troy |last8=Wurm |first8=Gerhard |date=2020 |title=Electrical charging overcomes the bouncing barrier in planet formation |url=https://www.nature.com/articles/s41567-019-0728-9 |journal=Nature Physics |language=en |volume=16 |issue=2 |pages=225–229 |doi=10.1038/s41567-019-0728-9 |bibcode=2020NatPh..16..225S |s2cid=256713457 |issn=1745-2473}}</ref>
* Single-use medical protective clothing must fulfill certain triboelectric charging regulations in China.<ref>{{Cite web |editor-last=Zheng |editor-first=Wayne |title=National Standard of the People's Republic of China |url=https://www.chinesestandard.net/PDF.aspx/GB19082-2009 |access-date=17 July 2023 |website=www.chinesestandard.net}}</ref>
* Space vehicles can accumulate significant tribocharge which can interfere with communications such as the sending of self-destruct signals. Some launches have been delayed by weather conditions where tribocharging could occur.<ref>{{Cite web |last=Shiga |first=David |date=27 October 2009 |title=Static electricity worry halts NASA rocket test flight |url= https://www.newscientist.com/article/dn18053-static-electricity-worry-halts-nasa-rocket-test-flight/ |access-date=12 July 2023 |website=New Scientist}}</ref>
* Triboelectric [[nanogenerator]]s are [[energy harvesting]] devices which convert mechanical energy into electricity.<ref>{{Cite journal |last1=Cheng |first1=Tinghai |last2=Shao |first2=Jiajia |last3=Wang |first3=Zhong Lin |date=2023 |title=Triboelectric nanogenerators |url=https://www.nature.com/articles/s43586-023-00220-3 |journal=Nature Reviews Methods Primers |volume=3 |issue=1 |doi=10.1038/s43586-023-00220-3 |s2cid=258745825 |issn=2662-8449}}</ref>
* Triboelectric noise within medical cable assemblies and lead wires is generated when the conductors, insulation, and fillers rub against each other as the cables are flexed during movement. Keeping triboelectric noise at acceptable levels requires careful material selection, design, and processing.<ref>{{cite web |url=https://experience.molex.com/triboelectric-noise-in-medical-cables-and-wires/ |title=Triboelectric Noise in Medical Cables and Wires|last=Molex |date=29 August 2014}}</ref> It is also an issue with underwater electroacoustic transducers if there are flexing motions of the cables; the mechanism is believed to involve relative motion between a dielectric and a conductor in the cable.<ref>{{Cite journal |last=Donovan |first=John E. |date=1970 |title=Triboelectric Noise Generation in Some Cables Commonly Used with Underwater Electroacoustic Transducers |journal=The Journal of the Acoustical Society of America |volume=48 |issue=3B |pages=714–724 |doi=10.1121/1.1912194 |bibcode=1970ASAJ...48..714D |issn=0001-4966}}</ref>
[[File:Antistatremmar.jpg|thumb|Antistatic belts on a car in Russia in 2014]]
* Vehicle tires are normally dark because carbon black is added to help conduct away tribocharge that can shock passengers when they exit.<ref>{{Cite web |last=Ralph |first=Vartabedian |date=29 July 1994 |title=The Goods: Shocking News About Seats, Tires |url= https://www.latimes.com/archives/la-xpm-1994-07-29-ls-21275-story.html |access-date=12 July 2023 |website=Los Angeles Times}}</ref> There are also discharging straps than can be purchased.<ref>{{Cite web |title=Halfords Anti Static Strip {{!}} Halfords UK |url=https://www.halfords.com/motoring/car-accessories/interior-car-accessories/halfords-anti-static-strip-265584.html |access-date=2023-09-05 |website=www.halfords.com |language=en}}</ref>

== See also ==
* [[Electrostatic generator]], machine to produce static electricity
* [[Electrostatic induction]], separation of charges and polarization due to other charges
* [[Electrostriction]], coupling between an electric field and volume of unit cells
* [[Electrohydrodynamics]], coupling in liquids between electric fields and properties
* [[Flexoelectricity]], polarization due to bending and other strain gradients
* [[Mechanoluminescence]], light produced by mechanical action, often involving triboelectric effect
* [[Nanotribology]], science of tribology (friction, lubrication and wear processes) at the nanoscale
* [[Piezoelectricity]], polarization due to linear strains
* [[Polarization density]], general description of the physics of polarization
* [[Static electricity]], electric charge often but not always due to triboelectricity
* [[Tribology]], science of friction, lubrication and wear
* [[Triboluminescence]], light associated with sliding or contacts
* [[Work function]], the energy to remove an electron from a surface

== References ==
{{reflist|25em}}

== External links ==
* [https://www.npr.org/2020/10/02/919711698/the-rise-and-fall-of-static-man-encore The return of Static Man], a podcast for kids about a masked menace who is electrified and goes around zapping people.
* [https://web.archive.org/web/20100630071446/https://ray.crk.umn.edu/physics/1012/demos/mpg/electricity_chargedrod.mpg Video of a charged rod demonstration] at the [[University of Minnesota]] showing repulsion after rods are tribocharged, different cases giving repulsive and attractive forces.
* [https://www.youtube.com/watch?v=rtl9TyMZSP8 Video demonstrating tribocharging] with a plastic comb rubbed by a cotton cloth attracting small pieces of paper.
* [https://www.youtube.com/watch?v=aY3Ju4SdaRA Video on Triboelectric Charging] from the [[Khan Academy]]. It discusses the contact potential difference model, using the term [[electron affinity]] which has the same meaning as [[work function]].

[[Category:Electrical phenomena]]
[[Category:Electrostatics]]
[[Category:Electricity]]
[[Category:Tribology]]