Editing Triboelectric effect (section)

=== 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>