superlubricity

{{short description|Regime of motion with little or no friction}}

File:Incommensurabilité 2.jpg which simulates the atomic surface structure of graphite, commensurable due to alignment in this photo]]

File:Incommensurabilité 4.jpg

Superlubricity is a regime of relative motion in which friction vanishes or very nearly vanishes. However, the definition of "vanishing" friction level is not clear, which makes the term vague. As an ad hoc definition, a kinetic coefficient of friction less than 0.01 can be adopted.{{Cite book|title=Fundamentals of Friction and Wear on the Nanoscale|last=Müser|first=Martin H.|date=2015-01-01|publisher=Springer International Publishing|isbn=9783319105598|editor-last=Gnecco|editor-first=Enrico|series=NanoScience and Technology|pages=209–232|language=en|doi=10.1007/978-3-319-10560-4_11|editor-last2=Meyer|editor-first2=Ernst|chapter = Theoretical Studies of Superlubricity}} This definition also requires further discussion and clarification.

Superlubricity may occur when two crystalline surfaces slide over each other in dry incommensurate contact. This was first described in the early 1980s{{Cite journal |last=Aubry |first=S |date=1983-05-10 |title=Exact models with a complete Devil's staircase |url=https://iopscience.iop.org/article/10.1088/0022-3719/16/13/012 |journal=Journal of Physics C: Solid State Physics |volume=16 |issue=13 |pages=2497–2508 |doi=10.1088/0022-3719/16/13/012 |bibcode=1983JPhC...16.2497A |issn=0022-3719}} for Frenkel–Kontorova models and is called the Aubry transition. It has been extensively studied as a mathematical model,{{Cite journal |last1=Sharma |first1=S. R. |last2=Bergersen |first2=B. |last3=Joos |first3=B. |date=1984-06-01 |title=Aubry transition in a finite modulated chain |url=https://link.aps.org/doi/10.1103/PhysRevB.29.6335 |journal=Physical Review B |language=en |volume=29 |issue=11 |pages=6335–6340 |doi=10.1103/PhysRevB.29.6335 |bibcode=1984PhRvB..29.6335S |issn=0163-1829}} in atomistic simulations{{Cite journal |last=Lançon |first=F |date=2002 |title=Aubry transition in a real material: Prediction for its existence in an incommensurate gold/gold interface |url=https://iopscience.iop.org/article/10.1209/epl/i2002-00543-x |journal=Europhysics Letters (EPL) |volume=57 |issue=1 |pages=74–79 |doi=10.1209/epl/i2002-00543-x |bibcode=2002EL.....57...74L |issn=0295-5075}} and in a range of experimental systems.{{Cite journal |last1=Bylinskii |first1=Alexei |last2=Gangloff |first2=Dorian |last3=Counts |first3=Ian |last4=Vuletić |first4=Vladan |date=July 2016 |title=Observation of Aubry-type transition in finite atom chains via friction |url=https://www.nature.com/articles/nmat4601 |journal=Nature Materials |language=en |volume=15 |issue=7 |pages=717–721 |doi=10.1038/nmat4601 |pmid=26998915 |issn=1476-1122|arxiv=1510.07585 |bibcode=2016NatMa..15..717B }}{{Cite journal |last1=Brazda |first1=T. |last2=Silva |first2=A. |last3=Manini |first3=N. |last4=Vanossi |first4=A. |last5=Guerra |first5=R. |last6=Tosatti |first6=E. |last7=Bechinger |first7=C. |date=2018-03-28 |title=Experimental Observation of the Aubry Transition in Two-Dimensional Colloidal Monolayers |url=https://link.aps.org/doi/10.1103/PhysRevX.8.011050 |journal=Physical Review X |language=en |volume=8 |issue=1 |page=011050 |doi=10.1103/PhysRevX.8.011050 |issn=2160-3308|arxiv=1802.09075 |bibcode=2018PhRvX...8a1050B }}

This effect, also called structural lubricity, was verified between two graphite surfaces in 2004.{{cite journal | last1=Dienwiebel | first1=Martin | last2=Verhoeven | first2=Gertjan S. | last3=Pradeep | first3=Namboodiri | last4=Frenken | first4=Joost W. M. | last5=Heimberg | first5=Jennifer A. | last6=Zandbergen | first6=Henny W. | title=Superlubricity of Graphite | journal=Physical Review Letters | publisher=American Physical Society (APS) | volume=92 | issue=12 | date=2004-03-24 | issn=0031-9007 | doi=10.1103/physrevlett.92.126101 | page=126101| pmid=15089689 | bibcode=2004PhRvL..92l6101D |url=http://www.physics.leidenuniv.nl/sections/cm/ip/group/PDF/Phys.rev.lett/2004/92(2004)12601.pdf}}

The atoms in graphite are oriented in a hexagonal manner and form an atomic hill-and-valley landscape, which looks like an egg-crate. When the two graphite surfaces are in registry (every 60 degrees), the friction force is high. When the two surfaces are rotated out of registry, the friction is greatly reduced. This is like two egg-crates which can slide over each other more easily when they are "twisted" with respect to each other.

Observation of superlubricity in microscale graphite structures was reported in 2012,{{cite journal | last1=Liu | first1=Ze | last2=Yang | first2=Jiarui | last3=Grey | first3=Francois | last4=Liu | first4=Jefferson Zhe | last5=Liu | first5=Yilun | last6=Wang | first6=Yibing | last7=Yang | first7=Yanlian | last8=Cheng | first8=Yao | last9=Zheng | first9=Quanshui | title=Observation of Microscale Superlubricity in Graphite | journal=Physical Review Letters | publisher=American Physical Society (APS) | volume=108 | issue=20 | date=2012-05-15 | issn=0031-9007 | doi=10.1103/physrevlett.108.205503 | page=205503| pmid=23003154 | arxiv=1104.3320 | bibcode=2012PhRvL.108t5503L | s2cid=119192523 }} by shearing a square graphite mesa a few micrometers across, and observing the self-retraction of the sheared layer. Such effects were also theoretically described{{Cite journal|url=https://link.aps.org/doi/10.1103/PhysRevB.87.205428|title=Superlubricity through graphene multilayers between Ni(111) surfaces|first1=S.|last1=Cahangirov|first2=S.|last2=Ciraci|first3=V. Ongun|last3=Özçelik|date=May 21, 2013|journal=Physical Review B|volume=87|issue=20|pages=205428|via=APS|doi=10.1103/PhysRevB.87.205428|arxiv=1305.3136|bibcode=2013PhRvB..87t5428C |hdl=11693/20960 }} for a model of graphene and nickel layers. This observation, which is reproducible even under ambient conditions, shifts interest in superlubricity from a primarily academic topic, accessible only under highly idealized conditions, to one with practical implications for micro and nanomechanical devices.Graphite super lube works at micron scale Philip Robinson, Chemistry World, 28 May 2012 [http://www.rsc.org/chemistryworld/2012/05/graphite-super-lube-works-micron-scale]

A state of ultralow friction can also be achieved when a sharp tip slides over a flat surface and the applied load is below a certain threshold. Such a "superlubric" threshold depends on the tip-surface interaction and the stiffness of the materials in contact, as described by the Tomlinson model.{{cite journal | last1=Socoliuc | first1=A. | last2=Bennewitz | first2=R. | last3=Gnecco | first3=E. | last4=Meyer | first4=E. | title=Transition from Stick-Slip to Continuous Sliding in Atomic Friction: Entering a New Regime of Ultralow Friction | journal=Physical Review Letters | publisher=American Physical Society (APS) | volume=92 | issue=13 | date=2004-04-01 | issn=0031-9007 | doi=10.1103/physrevlett.92.134301 | page=134301| pmid=15089616 | bibcode=2004PhRvL..92m4301S }}

The threshold can be significantly increased by exciting the sliding system at its resonance frequency, which suggests a practical way to limit wear in nanoelectromechanical systems.{{cite journal | last1=Socoliuc | first1=Anisoara | last2=Gnecco | first2=Enrico | last3=Maier | first3=Sabine | last4=Pfeiffer | first4=Oliver | last5=Baratoff | first5=Alexis | last6=Bennewitz | first6=Roland | last7=Meyer | first7=Ernst | title=Atomic-Scale Control of Friction by Actuation of Nanometer-Sized Contacts | journal=Science | publisher=American Association for the Advancement of Science (AAAS) | volume=313 | issue=5784 | date=2006-07-14 | issn=0036-8075 | doi=10.1126/science.1125874 | pages=207–210| pmid=16840695 | bibcode=2006Sci...313..207S | s2cid=43269213 }}

Superlubricity was also observed between a gold AFM tip and Teflon substrate due to repulsive Van der Waals forces and a hydrogen-bonded layer formed by glycerol on the steel surfaces.{{cite web |last1=Akchurin |first1=Aydar |title=Superlubricity by means of repulsive van der Waals forces - About Tribology |url=http://www.tribonet.org/superlubricity-by-means-of-repulsive-van-der-waals-forces/ |website=Tribonet |access-date=11 April 2025 |date=6 April 2016}}{{unreliable source|date=April 2025}}{{cite web |last1=Akchurin |first1=Aydar |title=Superlubricity between steel surfaces with glycerol/water mixture lubricant - About Tribology |url=http://www.tribonet.org/superlubricity-between-steel-surfaces-using-glycerolwater-mixture/ |website=Tribonet |access-date=11 April 2025 |date=18 March 2016}}{{unreliable source|date=April 2025}} Formation of the hydrogen-bonded layer was also shown to lead to superlubricity between quartz glass surfaces lubricated by biological liquid obtained from mucilage of Brasenia schreberi.{{cite web |last1=Akchurin |first1=Aydar |title=Brasenia schreberi mucilage - superlubric biological liquid - About Tribology |url=http://www.tribonet.org/brasenia-schreberi-mucilage-superlubric-biological-liquid/ |website=Tribonet |access-date=11 April 2025 |date=21 March 2016}}{{unreliable source|date=April 2025}} Other mechanisms of superlubricity may include:{{Cite journal|last=Popov|first=Valentin L.|date=2020|title=Contacts With Negative Work of "Adhesion" and Superlubricity|journal=Frontiers in Mechanical Engineering|language=en|volume=5|doi=10.3389/fmech.2019.00073|doi-access=free}} (a) thermodynamic repulsion due to a layer of free or grafted macromolecules between the bodies so that the entropy of the intermediate layer decreases at small distances due to stronger confinement; (b) electrical repulsion due to external electrical voltage; (c) repulsion due to electrical double layer; (d) repulsion due to thermal fluctuations.{{Cite journal|last1=Zhou|first1=Yunong|last2=Wang|first2=Anle|last3=Müser|first3=Martin H.|date=2019|title=How Thermal Fluctuations Affect Hard-Wall Repulsion and Thereby Hertzian Contact Mechanics|journal=Frontiers in Mechanical Engineering|language=en|volume=5|doi=10.3389/fmech.2019.00067|doi-access=free|url=https://publikationen.sulb.uni-saarland.de/bitstream/20.500.11880/34415/1/fmech-05-00067.pdf}}

The similarity of the term superlubricity with terms such as superconductivity and superfluidity is misleading; other energy dissipation mechanisms can lead to a finite (normally small) friction force. Superlubricity is more analogous to phenomena such as superelasticity, in which substances such as Nitinol have very low, but nonzero, elastic moduli; supercooling, in which substances remain liquid until a lower-than-normal temperature; super black, which reflects very little light; giant magnetoresistance, in which very large but finite magnetoresistance effects are observed in alternating nonmagnetic and ferromagnetic layers; superhard materials, which are diamond or nearly as hard as diamond; and superlensing, which have a resolution which, while finer than the diffraction limit, is still finite.