Editing Triboelectric effect (section)

== 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" />