View source for Physics:Van der Waals force

{{DISPLAYTITLE:van der Waals force}}
{{short description|Interactions between groups of atoms that do not arise from chemical bonds}}
[[File:RainDrops1.jpg|thumb|upright=1.25|Rainwater flux from a canopy. Among the forces that govern drop formation: Van der Waals force, [[Physics:Surface tension|surface tension]], cohesion, Plateau–Rayleigh instability.]]
[[File:Brillenputztücher-trocken.jpg|thumb|[[Engineering:Microfiber|Microfiber]] cloth makes use of van der Waals force to remove dirt without scratches.<ref>{{Cite web|date=2008-07-02|author-first=Chris|author-last=Woodford|title=How do microfiber cloths work? {{!}} The science of cleaning|url=http://www.explainthatstuff.com/microfibercloths.html|access-date=2022-02-11|website=Explain that Stuff}}</ref>]]

In [[Physics:Molecular physics|molecular physics]] and [[Chemistry:Chemistry|chemistry]], the '''van der Waals force''' is a distance-dependent interaction between [[Physics:Atom|atom]]s or [[Physics:Molecule|molecule]]s. Unlike ionic or [[Chemistry:Covalent bond|covalent bond]]s, these attractions do not result from a [[Chemistry:Chemical bond|chemical electronic bond]];<ref>{{GoldBookRef|file=V06597|title=van der Waals forces}}</ref><!-- also verifies lower-case "v" --> they are comparatively weak and therefore more susceptible to disturbance. The van der Waals force quickly vanishes at longer distances between interacting molecules.

Named after Dutch physicist [[Biography:Johannes Diderik van der Waals|Johannes Diderik van der Waals]], the van der Waals force plays a fundamental role in fields as diverse as [[Chemistry:Supramolecular chemistry|supramolecular chemistry]], [[Biology:Structural biology|structural biology]], [[Physics:Polymer science|polymer science]], [[Engineering:Nanotechnology|nanotechnology]], [[Physics:Surface science|surface science]], and [[Physics:Condensed matter physics|condensed matter physics]]. It also underlies many properties of [[Chemistry:Organic chemistry|organic compounds]] and [[Chemistry:Molecular solid|molecular solid]]s, including their [[Chemistry:Solubility|solubility]] in [[Chemistry:Chemical polarity|polar and non-polar]] media.

If no other [[Physics:Force|force]] is present, the distance between atoms at which the force becomes repulsive rather than attractive as the atoms approach one another is called the '''van der Waals contact distance'''; this phenomenon results from the mutual repulsion between the atoms' [[Physics:Atomic orbital|electron cloud]]s.<ref>{{Cite book |title=Biochemistry |last1=Garrett |first1=Reginald H. |last2=Grisham |first2=Charles M. |date = 2016|edition=6th | publisher = University of Virginia |pages=12–13}}</ref>

The van der Waals forces<ref>{{Cite book |title=Reviews in Computational Chemistry |last=Tschumper |first=Gregory S. |date = October 20, 2008 |publisher = John Wiley & Sons |isbn=9780470399545 |editor1-last=Lipkowitz |editor1-first=Kenny B. |editor-last2=Cundari |editor-first2=Thomas R. | volume = 26 |pages=39–90 |doi=10.1002/9780470399545.ch2 |chapter = Reliable Electronic Structure Computations for Weak Noncovalent Interactions in Clusters}}</ref> are usually described as a combination of the [[Physics:London dispersion force|London dispersion force]]s between "instantaneously induced [[Physics:Dipole|dipole]]s",<ref>{{Cite book|last=Mahan, Gerald D.|url=https://www.worldcat.org/oclc/226037727|title=Quantum mechanics in a nutshell|date=2009|publisher=Princeton University Press|isbn=978-0-691-13713-1|location=Princeton|oclc=226037727}}</ref> Debye forces between permanent dipoles and induced dipoles, and the Keesom force between permanent molecular dipoles whose rotational orientations are dynamically averaged over time.

==Definition==
Van der Waals forces include attraction and repulsions between [[Physics:Atom|atoms]], [[Physics:Molecule|molecules]], as well as other [[Physics:Intermolecular force|intermolecular forces]]. They differ from [[Chemistry:Covalent bond|covalent]] and ionic bonding in that they are caused by correlations in the fluctuating [[Physics:Polarizability|polarizations]] of nearby particles (a consequence of [[Physics:Quantum dynamics|quantum dynamics]]<ref name=Abrikosov>
{{Cite book|author1=A.A. Abrikosov |author2=L.P. Gorkov |author3=I.E. Dzyaloshinsky |title=Methods of Quantum Field Theory in Statistical Physics
|year= 1963–1975 |publisher=[[Social:Dover Publications|Dover Publications]] |isbn=978-0-486-63228-5 }}
Chapter 6 Electromagnetic Radiation in an Absorbing Medium</ref>).

The force results from a transient shift in [[Physics:Electron density|electron density]]. Specifically, the electron density may temporarily shift to be greater on one side of the nucleus. This shift generates a transient charge which a nearby atom can be attracted to or repelled by. The force is repulsive at very short distances, reaches zero at an equilibrium distance characteristic for each atom, or molecule, and becomes attractive for distances larger than the equilibrium distance. For individual atoms, the equilibrium distance is between 0.3&nbsp;[[Nanometre|nm]] and 0.5&nbsp;nm, depending on the atomic-specific diameter.<ref>{{Cite journal |last=Batsanov |first=S. S. |date=2001 |title=Van der Waals Radii of Elements |url=http://link.springer.com/10.1023/A:1011625728803 |journal=Inorganic Materials |volume=37 |issue=9 |pages=871–885 |doi=10.1023/A:1011625728803|s2cid=52088903 }}</ref> When the interatomic distance is greater than 1.0&nbsp;nm the force is not strong enough to be easily observed as it decreases as a function of distance ''r'' approximately with the 7th power (~''r''<sup>−7</sup>).<ref>{{Cite book |last=Hirschfelder |first=Joseph O. |url=https://www.worldcat.org/oclc/534717 |title=Molecular theory of gases and liquids |date=1954 |publisher=Wiley |others=Charles F. Curtiss, R. Byron Bird, University of Wisconsin. Naval Research Laboratory |isbn=0-471-40065-3 |location=New York |oclc=534717}}</ref>

Van der Waals forces are often among the weakest chemical forces. For example, the pairwise attractive van der Waals interaction energy between H ([[Chemistry:Hydrogen|Hydrogen]]) atoms in different H<sub>2</sub> molecules equals 0.06 kJ/mol (0.6 meV) and the pairwise attractive interaction energy between O ([[Chemistry:Oxygen|Oxygen]]) atoms in different O<sub>2</sub> molecules equals 0.44 kJ/mol (4.6 meV).<ref>{{Cite journal |last1=Wang |first1=Shiyi |last2=Hou |first2=Kaiyi |last3=Heinz |first3=Hendrik |date=2021-08-10 |title=Accurate and Compatible Force Fields for Molecular Oxygen, Nitrogen, and Hydrogen to Simulate Gases, Electrolytes, and Heterogeneous Interfaces |url=https://pubs.acs.org/doi/10.1021/acs.jctc.0c01132 |journal=Journal of Chemical Theory and Computation |language=en |volume=17 |issue=8 |pages=5198–5213 |doi=10.1021/acs.jctc.0c01132 |pmid=34255965 |s2cid=235823673 |issn=1549-9618}}</ref> The corresponding vaporization energies of H<sub>2</sub> and O<sub>2</sub> molecular liquids, which result as a sum of all van der Waals interactions per molecule in the molecular liquids, amount to 0.90 kJ/mol (9.3 meV) and 6.82 kJ/mol (70.7 meV), respectively, and thus approximately ~15 times the value of the individual pairwise interatomic interactions (excluding [[Chemistry:Covalent bond|covalent bonds]]).

The strength of van-der-Waals bonds increases with higher [[Physics:Polarizability|polarizability]] of the participating atoms.<ref>{{Cite journal |last1=Heinz |first1=Hendrik |last2=Lin |first2=Tzu-Jen |last3=Kishore Mishra |first3=Ratan |last4=Emami |first4=Fateme S. |date=2013-02-12 |title=Thermodynamically Consistent Force Fields for the Assembly of Inorganic, Organic, and Biological Nanostructures: The INTERFACE Force Field |url=https://pubs.acs.org/doi/10.1021/la3038846 |journal=Langmuir |language=en |volume=29 |issue=6 |pages=1754–1765 |doi=10.1021/la3038846 |pmid=23276161 |issn=0743-7463}}</ref> For example, the pairwise van der Waals interaction energy for more polarizable atoms such as S ([[Chemistry:Sulfur|Sulfur]]) atoms in H<sub>2</sub>S and [[Chemistry:Sulfide|sulfides]] exceeds 1 kJ/mol (10 meV), and the pairwise interaction energy between even larger, more polarizable Xe ([[Chemistry:Xenon|Xenon]]) atoms is 2.35 kJ/mol (24.3 meV).<ref>{{Cite journal |last=Halgren |first=Thomas A. |date=September 1992 |title=The representation of van der Waals (vdW) interactions in molecular mechanics force fields: potential form, combination rules, and vdW parameters |url=https://pubs.acs.org/doi/abs/10.1021/ja00046a032 |journal=Journal of the American Chemical Society |language=en |volume=114 |issue=20 |pages=7827–7843 |doi=10.1021/ja00046a032 |issn=0002-7863}}</ref> These van der Waals interactions are up to 40 times stronger than in H<sub>2</sub>, which has only one valence electron, and they are still not strong enough to achieve an aggregate state other than gas for Xe under standard conditions. The interactions between atoms in metals can also be effectively described as van-der-Waals interactions and account for the observed solid aggregate state with bonding strengths comparable to covalent and ionic interactions. The strength of pairwise van-der-Waals type interactions is on the order of 12 kJ/mol (120 meV) for low-melting Pb ([[Chemistry:Lead|Lead]]) and on the order of 32 kJ/mol (330 meV) for high-melting Pt ([[Chemistry:Platinum|Platinum]]), which is about one order of magnitude stronger than in Xe due to the presence of a highly polarizable [[Physics:Fermi gas|free electron gas]].<ref>{{Cite journal |last1=Heinz |first1=Hendrik |last2=Vaia |first2=R. A. |last3=Farmer |first3=B. L. |last4=Naik |first4=R. R. |date=2008-11-06 |title=Accurate Simulation of Surfaces and Interfaces of Face-Centered Cubic Metals Using 12−6 and 9−6 Lennard-Jones Potentials |url=https://pubs.acs.org/doi/10.1021/jp801931d |journal=The Journal of Physical Chemistry C |language=en |volume=112 |issue=44 |pages=17281–17290 |doi=10.1021/jp801931d |issn=1932-7447}}</ref> Accordingly, van der Waals forces can range from weak to strong interactions, and support integral structural loads when multitudes of such interactions are present.

More broadly, intermolecular forces have several possible contributions:

# A repulsive component resulting from the [[Physics:Pauli exclusion principle|Pauli exclusion principle]] that prevents close contact of atoms, or the collapse of molecules.
# Attractive or repulsive [[Physics:Electrostatic|electrostatic]] interactions between permanent charges (in the case of molecular ions), dipoles (in the case of molecules without inversion centre), [[Physics:Quadrupole|quadrupole]]s (all molecules with symmetry lower than cubic), and in general between permanent multipoles. These interactions also include [[Chemistry:Hydrogen bond|hydrogen bond]]s, [[Chemistry:Cation–π interaction|cation-pi]], and [[Chemistry:Pi-Stacking|pi-stacking]] interactions. Orientation-averaged contributions from  electrostatic interactions are sometimes called the Keesom interaction or Keesom force after Willem Hendrik Keesom.
# Induction (also known as [[Physics:Polarizability|polarization]]), which is the attractive interaction between a permanent multipole on one molecule with an induced multipole on another. This interaction is sometimes called [[Debye]] force after Peter J.W. Debye.
# Dispersion (usually named [[Physics:London dispersion force|London dispersion interactions]] after [[Biography:Fritz London|Fritz London]]), which is the attractive interaction between any pair of molecules, including non-polar atoms, arising from the interactions of instantaneous multipoles.

Hereby, different texts may refer to a different spectrum of interactions using the term "van der Waals force". Typically, contributions (1) and (4) are considered as van-der-Waals forces, excluding effects from permanent multipoles as described in (2) and from permanent polarization in (3). However, some texts describe the van der Waals force as the totality of forces, including repulsion; others mean all the attractive forces (and then sometimes distinguish van der Waals–Keesom, van der Waals–Debye, and van der Waals–London).

All intermolecular/van der Waals forces are anisotropic (except those between two [[Chemistry:Noble gas|noble gas]] atoms), which means that they depend on the relative orientation of the molecules. The induction and dispersion interactions are always attractive, irrespective of orientation, but the electrostatic interaction changes sign upon rotation of the molecules. That is, the electrostatic force can be attractive or repulsive, depending on the mutual orientation of the molecules. When molecules are in thermal motion, as they are in the gas and liquid phase, the electrostatic force is averaged out to a large extent because the molecules thermally rotate and thus probe both repulsive and attractive parts of the electrostatic force. Random thermal motion can disrupt or overcome the electrostatic component of the van der Waals force but the averaging effect is much less pronounced for the attractive induction and dispersion forces.

The [[Physics:Lennard-Jones potential|Lennard-Jones potential]] is often used as an approximate model for the isotropic part of a total (repulsion plus attraction) van der Waals force as a function of distance.

Van der Waals forces are responsible for certain cases of pressure broadening (van der Waals broadening) of spectral lines and the formation of [[Physics:Van der Waals molecule|van der Waals molecule]]s.<!-- most use "Van der Waals molecules" ([http://pubs.acs.org/doi/abs/10.1021/cr00088a008], [http://www.wesleyan.edu/chem/faculty/novick/vdw.html], [http://doi.wiley.com/10.1002/anie.197204861], [https://books.google.com/books?id=PP6vtMJlWowC&pg=PA220]) few use "van" ([https://archive.today/20130202065320/http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6TFN-4CNJCXP-2&_user=10&_rdoc=1&_fmt=&_orig=search&_sort=d&_docanchor=&view=c&_searchStrId=1113973534&_rerunOrigin=google&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=31e5934c994d877beafb7c335e513529], [http://www.infography.com/content/718688290900.html] (just a search aggregator)) --> The London–van der Waals forces<!-- see [http://www.clarkson.edu/projects/crcd/me437/downloads/5_vanderWaals.pdf] and [http://linkinghub.elsevier.com/retrieve/pii/S1672251507601560] --> are related to the [[Physics:Casimir effect|Casimir effect]] for dielectric media, the former being the microscopic description of the latter bulk property. The first detailed calculations of this were done in 1955 by E. M. Lifshitz.<ref>For further investigation, one may consult the University of St. Andrews' levitation work in a popular article: [https://www.sciencedaily.com/releases/2007/08/070806091137.htm Science Journal: ''New way to levitate objects discovered''], and in a more scholarly version: {{cite journal |doi=10.1088/1367-2630/9/8/254|title=Quantum levitation by left-handed metamaterials|year=2007|last1=Leonhardt|first1=Ulf|last2=Philbin|first2=Thomas G.|s2cid=463815|journal=New Journal of Physics|volume=9|issue=8|page=254|arxiv=quant-ph/0608115|bibcode=2007NJPh....9..254L}}, which relate the Casimir effect to the gecko and how the reversal of the Casimir effect can result in physical levitation of tiny objects.</ref> A more general theory of van der Waals forces has also been developed.<ref>{{cite journal|last1= Dzyaloshinskii|first1= I E|last2= Lifshitz|first2= E M|last3= Pitaevskii|first3= Lev P|title= General theory of van der waals' forces|journal= Soviet Physics Uspekhi|volume= 4|page= 153|year= 1961|doi= 10.1070/PU1961v004n02ABEH003330|bibcode=1961SvPhU...4..153D|issue= 2}}</ref><ref>{{cite journal|last1= Zheng|first1= Y.|last2= Narayanaswamy|first2= A.|s2cid= 64619547|title= Lifshitz Theory of van der Waals Pressure in Dissipative Media|journal= Phys. Rev. A|volume= 83|issue= 4|page= 042504 |year= 2011|doi=10.1103/PhysRevA.83.042504|arxiv = 1011.5433 |bibcode = 2011PhRvA..83d2504Z }}</ref>

The main characteristics of van der Waals forces are:<ref name="Sethi 1992">{{cite book | last1=Sethi | first1=M. S. | last2=Satake | first2=M. | title=Chemical bonding | publisher=Discovery Publishing House | publication-place=New Delhi | year=1992 | isbn=978-81-7141-163-4 | oclc=912437861 | page=}}</ref>

* They are weaker than normal covalent and ionic bonds.
* The Van der Waals forces are additive in nature, consisting of several individual interactions, and cannot be saturated. 
* They have no directional characteristic.
* They are all short-range forces and hence only interactions between the nearest particles need to be considered (instead of all the particles). Van der Waals attraction is greater if the molecules are closer.
* Van der Waals forces are independent of temperature except for dipole-dipole interactions.
In low molecular weight alcohols, the hydrogen-bonding properties of their polar [[Chemistry:Hydroxyl group|hydroxyl group]] dominate other weaker van der Waals interactions. In higher molecular weight alcohols, the properties of the nonpolar hydrocarbon chain(s) dominate and determine their solubility.

Van der Waals forces are also responsible for the weak [[Chemistry:Hydrogen bond|hydrogen bond]] interactions between unpolarized dipoles particularly in acid-base aqueous solution and between [[Physics:Biomolecule|biological molecules]].

==London dispersion force==
{{Main|Physics:London dispersion force}}

[[Physics:London dispersion force|London dispersion force]]s, named after the German-American physicist [[Biography:Fritz London|Fritz London]], are weak [[Physics:Intermolecular force|intermolecular force]]s that arise from the interactive forces between instantaneous multipoles in molecules without permanent [[Multipole expansion|multipole moments]]. In and between organic molecules the multitude of contacts can lead to larger contribution of dispersive attraction, particularly in the presence of heteroatoms. London dispersion forces are also known as '[[Physics:London dispersion force|dispersion]] forces', 'London forces', or 'instantaneous dipole–induced dipole forces'. The strength of London dispersion forces is proportional to the polarizability of the molecule, which in turn depends on the total number of electrons and the area over which they are spread. Hydrocarbons display small dispersive contributions, the presence of heteroatoms lead to increased LD forces as function of their polarizability, e.g. in the sequence RI>RBr>RCl>RF.<ref>{{Cite journal | doi=10.1021/acs.accounts.5b00111| title=Dispersive Interactions in Solution Complexes| year=2015| last1=Schneider| first1=Hans-Jörg| journal=Accounts of Chemical Research| volume=48| issue=7| pages=1815–1822| pmid=26083908}}</ref> In absence of solvents weakly polarizable hydrocarbons form crystals due to dispersive forces; their sublimation heat is a measure of the dispersive interaction.

==Van der Waals forces between macroscopic objects==
For [[Physics:Macroscopic scale|macroscopic]] bodies with known volumes and numbers of atoms or molecules per unit volume, the total van der Waals force is often computed based on the "microscopic theory" as the sum over all interacting pairs. It is necessary to integrate over the total volume of the object, which makes the calculation dependent on the objects' shapes. For example, the van der Waals interaction energy between spherical bodies of radii R<sub>1</sub> and R<sub>2</sub> and with smooth surfaces was approximated in 1937 by Hamaker<ref>H. C. Hamaker, Physica, 4(10), 1058–1072 (1937)</ref> (using London's famous 1937 equation for the dispersion interaction energy between atoms/molecules<ref>F. London, Transactions of the Faraday Society, 33, 8–26 (1937)</ref> as the starting point) by:

{{NumBlk|:|<math>\begin{align}
     &U(z;R_{1},R_{2}) = -\frac{A}{6}\left(\frac{2R_{1}R_{2}}{z^2 - (R_{1} + R_{2})^2} + \frac{2R_{1}R_{2}}{z^2 - (R_{1} - R_{2})^2} + \ln\left[\frac{z^2-(R_{1}+ R_{2})^2}{z^2-(R_{1}- R_{2})^2}\right]\right)
\end{align}</math>|{{EquationRef|1}}}}

where A is the [[Physics:Hamaker constant|Hamaker coefficient]], which is a constant (~10<sup>−19</sup> − 10<sup>−20</sup> J) that depends on the material properties (it can be positive or negative in sign depending on the intervening medium), and ''z'' is the center-to-center distance; i.e., the sum of ''R''<sub>1</sub>, ''R''<sub>2</sub>, and ''r'' (the distance between the surfaces): <math>\ z = R_{1} + R_{2} + r</math>.

The van der Waals ''[[Physics:Force|force]]'' between two spheres of constant radii (''R''<sub>1</sub> and ''R''<sub>2</sub> are treated as parameters) is then a function of separation since the force on an object is the negative of the derivative of the potential energy function,<math>\ F_{\rm VdW}(z) = -\frac{d}{dz}U(z)</math>. This yields:

{{NumBlk|:|<math>\ F_{\rm VdW}(z)= -\frac{A}{6}\frac{64R_{1}^3R_{2}^3z}{[z^2-(R_{1}+R_{2})^2]^2[z^2-(R_{1}-R_{2})^2]^2}</math>|{{EquationRef|2}}}}

In the limit of close-approach, the spheres are sufficiently large compared to the distance between them; i.e., <math>\ r \ll R_{1}</math> or <math>R_{2}</math>, so that equation (1) for the potential energy function simplifies to:

{{NumBlk|:|<math>\ U(r;R_{1},R_{2})= -\frac{AR_{1}R_{2}}{(R_{1}+R_{2})6r}</math>|{{EquationRef|3}}}}

with the force:

{{NumBlk|:|<math>\ F_{\rm VdW}(r)= -\frac{AR_{1}R_{2}}{(R_{1}+R_{2})6r^2}</math>|{{EquationRef|4}}}}

The van der Waals forces between objects with other geometries using the Hamaker model have been published in the literature.<ref>{{cite journal |author=R. Tadmor|title=The London–Van der Waals interaction energy between objects of various geometries |journal=[[Physics:Journal of Physics: Condensed Matter|Journal of Physics]]|volume=13|issue=9 |date=March 2001|pages=L195–L202|doi=10.1088/0953-8984/13/9/101|bibcode=2001JPCM...13L.195T|s2cid=250790137 }}</ref><ref>{{cite book|author=Israelachvili J.|title=Intermolecular and Surface Forces|publisher=[[Company:Academic Press|Academic Press]] |date=1985–2004|isbn=978-0-12-375181-2}}</ref><ref>{{cite book|author=V. A. Parsegian|title=Van der Waals Forces: A Handbook for Biologists, Chemists, Engineers, and Physicists |publisher={{wipe|Cambridge University Press}} |date=2006|isbn=978-0-521-83906-8}}</ref>

From the expression above, it is seen that the van der Waals force decreases with decreasing size of bodies (R). Nevertheless, the strength of inertial forces, such as gravity and drag/lift, decrease to a greater extent. Consequently, the van der Waals forces become dominant for collections of very small particles such as very fine-grained dry powders (where there are no capillary forces present) even though the force of attraction is smaller in magnitude than it is for larger particles of the same substance. Such powders are said to be cohesive, meaning they are not as easily fluidized or pneumatically conveyed as their more coarse-grained counterparts. Generally, free-flow occurs with particles greater than about 250 μm.

The van der Waals force of adhesion is also dependent on the surface topography. If there are surface asperities, or protuberances, that result in a greater total area of contact between two particles or between a particle and a wall, this increases the van der Waals force of attraction as well as the tendency for mechanical interlocking.

The microscopic theory assumes pairwise additivity. It neglects [[Physics:Many-body problem|many-body interactions]] and [[Physics:Retarded potential|retardation]]. A more rigorous approach accounting for these effects, called the "macroscopic theory" was developed by [[Biography:Evgeny Lifshitz|Lifshitz]] in 1956.<ref>E. M. Lifshitz, Soviet Phys. JETP, 2, 73 (1956)</ref> Langbein derived a much more cumbersome "exact" expression in 1970 for spherical bodies within the framework of the Lifshitz theory<ref>D. Langbein, Phys. Rev. B, 2, 3371 (1970)</ref> while a simpler macroscopic model approximation had been made by [[Biography:Boris Derjaguin|Derjaguin]] as early as 1934.<ref>B. V. Derjaguin, Kolloid-Z., 69, 155–64 (1934)</ref> Expressions for the van der Waals forces for many different geometries using the Lifshitz theory have likewise been published.

==Use by geckos and arthropods==
[[File:Gecko Leaftail 1.jpg|thumb|[[Biology:Gecko|Gecko]] climbing a glass surface]]
{{further|Biology:Arthropod adhesion}}

The ability of [[Biology:Gecko|gecko]]s – which can hang on a glass surface using only one toe – to climb on sheer surfaces has been for many years mainly attributed to the van der Waals forces between these surfaces and the spatulae, or microscopic projections, which cover the hair-like [[Biology:Seta|seta]]e found on their footpads.<ref name="RussellHigham2009">{{Cite journal |last1=Russell |first1=Anthony P. |last2=Higham |first2=Timothy. E. |date=2009 |title=A new angle on clinging in geckos: incline, not substrate, triggers the deployment of the adhesive system |journal=Proceedings of the Royal Society B: Biological Sciences |volume=276 |issue=1673 |pages=3705–3709 |doi=10.1098/rspb.2009.0946 |issn=0962-8452 |pmc=2817305 |pmid=19656797}}</ref><ref>{{Cite journal |last1=Autumn |first1=Kellar |last2=Sitti |first2=Metin |last3=Liang |first3=Yiching A. |last4=Peattie |first4=Anne M. |last5=Hansen |first5=Wendy R. |last6=Sponberg |first6=Simon |last7=Kenny |first7=Thomas W. |last8=Fearing |first8=Ronald |last9=Israelachvili |first9=Jacob N. |year=2002 |title=Evidence for van der Waals adhesion in gecko setae |journal=Proceedings of the National Academy of Sciences |volume=99 |issue=19 |pages=12252–6 |bibcode=2002PNAS...9912252A |doi=10.1073/pnas.192252799 |pmc=129431 |pmid=12198184 |first10=Robert J. |last10=Full|doi-access=free }}</ref> 

There were efforts in 2008 to create a [[Chemistry:Dry glue|dry glue]] that exploits the effect,<ref>{{Cite news |last=Steenhuysen |first=Julie |date=8 October 2008 |title=Gecko-like glue is said to be stickiest yet |work={{wipe|Reuters}} |url=https://www.reuters.com/article/us-nano-glue-idUSN0942431020081009 |access-date=5 October 2016}}</ref> and success was achieved in 2011 to create an adhesive tape on similar grounds<ref>{{Cite web |last=Quick |first=Darren |date=6 November 2011 |title=Biologically inspired adhesive tape can be reused thousands of times |url=http://newatlas.com/bioinspired-adhesive-tape-kiel/20406/ |access-date=5 October 2016 |website=New Atlas}}</ref> (i.e. based on van der Waals forces). In 2011, a paper was published relating the effect to both velcro-like hairs and the presence of lipids in gecko footprints.<ref name="HsuGe20112">{{Cite journal |last1=Hsu |first1=Ping Yuan |last2=Ge |first2=Liehui |last3=Li |first3=Xiaopeng |last4=Stark |first4=Alyssa Y. |last5=Wesdemiotis |first5=Chrys |last6=Niewiarowski |first6=Peter H. |last7=Dhinojwala |first7=Ali |date=24 August 2011 |title=Direct evidence of phospholipids in gecko footprints and spatula-substrate contact interface detected using surface-sensitive spectroscopy |journal=Journal of the Royal Society Interface |volume=9 |issue=69 |pages=657–664 |doi=10.1098/rsif.2011.0370 |issn=1742-5689 |pmc=3284128 |pmid=21865250}}</ref>

A later study suggested that capillary adhesion might play a role,<ref>{{Cite journal |last1=Huber |first1=Gerrit |last2=Mantz |first2=Hubert |last3=Spolenak |first3=Ralph |last4=Mecke |first4=Klaus |last5=Jacobs |first5=Karin |last6=Gorb |first6=Stanislav N. |last7=Arzt |first7=Eduard |date=2005 |title=Evidence for capillarity contributions to gecko adhesion from single spatula nanomechanical measurements |journal=Proceedings of the National Academy of Sciences |volume=102 |issue=45 |pages=16293–6 |bibcode=2005PNAS..10216293H |doi=10.1073/pnas.0506328102 |pmc=1283435 |pmid=16260737 |doi-access=free}}</ref> but that hypothesis has been rejected by more recent studies.<ref>{{Cite journal |last1=Chen |first1=Bin |last2=Gao |first2=Huajian |date=2010 |title=An alternative explanation of the effect of humidity in gecko adhesion: stiffness reduction enhances adhesion on a rough surface |journal=Int J Appl Mech |volume=2 |pages=1–9 |bibcode=2010IJAM...02....1C |doi=10.1142/s1758825110000433}}</ref><ref>{{Cite journal |last1=Puthoff |first1=Jonathan B. |last2=Prowse |first2=Michael S. |last3=Wilkinson |first3=Matt |last4=Autumn |first4=Kellar |year=2010 |title=Changes in materials properties explain the effects of humidity on gecko adhesion |journal=J Exp Biol |volume=213 |issue=21 |pages=3699–3704 |doi=10.1242/jeb.047654 |pmid=20952618 |doi-access=free}}</ref><ref>{{Cite journal |last1=Prowse |first1=Michael S. |last2=Wilkinson |first2=Matt |last3=Puthoff |first3=Michael |last4=Mayer |first4=George |last5=Autumn |first5=Kellar |date=February 2011 |title=Effects of humidity on the mechanical properties of gecko setae |journal=[[Biology:Acta Biomaterialia|Acta Biomaterialia]] |volume=7 |issue=2 |pages=733–738 |doi=10.1016/j.actbio.2010.09.036 |pmid=20920615}}</ref>

A 2014 study has shown that gecko adhesion to smooth Teflon and [[Chemistry:Polydimethylsiloxane|polydimethylsiloxane]]  surfaces is mainly determined by electrostatic interaction (caused by [[Physics:Contact electrification|contact electrification]]), not van der Waals or capillary forces.<ref>{{cite journal |last1=Izadi|first1=H.|last2=Stewart|first2=K. M. E.|last3=Penlidis|first3=A.|title=Role of contact electrification and electrostatic interactions in gecko adhesion |journal=Journal of the Royal Society Interface |date=9 July 2014 |volume=11 |issue=98 |pages=20140371 |doi=10.1098/rsif.2014.0371|pmid=25008078|pmc=4233685|quote=We have demonstrated that it is the CE-driven electrostatic interactions which dictate the strength of gecko adhesion, and not the van der Waals or capillary forces which are conventionally considered as the main source of gecko adhesion.}}</ref>

Among the [[Biology:Arthropod|arthropod]]s, some spiders have similar setae on their scopulae or scopula pads, enabling them to climb or hang upside-down from extremely smooth surfaces such as glass or porcelain.<ref name="KeselMartin2004">{{Cite journal |last1=Kesel |first1=Antonia B. |last2=Martin |first2=Andrew |last3=Seidl |first3=Tobias |date=19 April 2004 |title=Getting a grip on spider attachment: an AFM approach to microstructure adhesion in arthropods |journal=[[Physics:Smart Materials and Structures|Smart Materials and Structures]] |volume=13 |issue=3 |pages=512–518 |doi=10.1088/0964-1726/13/3/009 |issn=0964-1726|bibcode=2004SMaS...13..512K |s2cid=250841250 }}</ref><ref>{{Cite journal |last1=Wolff |first1=Jonas O. |last2=Gorb |first2=Stanislav N. |date=7 January 2012 |title=The influence of humidity on the attachment ability of the spider ''Philodromus dispar'' (Araneae, Philodromidae) |journal=Proceedings of the Royal Society B |volume=279 |issue=1726 |pages=139–143 |doi=10.1098/rspb.2011.0505 |pmid=21593034 |pmc=3223641 }}</ref>

== See also ==
{{Div col}}
* [[Biology:Arthropod adhesion|Arthropod adhesion]]
* [[Engineering:Cold welding|Cold welding]]
* [[Chemistry:Dispersion|Dispersion]]
* [[Biology:Gecko feet|Gecko feet]]
* [[Physics:Lennard-Jones potential|Lennard-Jones potential]]
* Noncovalent interactions
* [[Physics:Synthetic setae|Synthetic setae]]
* [[Physics:Van der Waals molecule|Van der Waals molecule]]
* [[Chemistry:Van der Waals radius|Van der Waals radius]]
* [[Chemistry:Van der Waals strain|Van der Waals strain]]
* [[Physics:Van der Waals surface|Van der Waals surface]]
* Wringing of gauge blocks
{{Div col end}}

== References ==
{{Reflist|30em}}

==Further reading==
* {{cite journal | first1 = Iver | last1 = Brevik | first2 = V. N. | last2 = Marachevsky | first3 = Kimball A. | last3 = Milton | title = Identity of the van der Waals Force and the Casimir Effect and the Irrelevance of These Phenomena to Sonoluminescence | journal = Physical Review Letters | arxiv = hep-th/9810062 | year = 1999 | volume = 82 | issue = 20 | pages = 3948–3951 | doi = 10.1103/PhysRevLett.82.3948 | bibcode = 1999PhRvL..82.3948B | s2cid = 14762105 }}
* {{cite journal | first1 = I. D. | last1 = Dzyaloshinskii | first2 = E. M. | last2 = Lifshitz | first3 = Lev P. | last3 = Pitaevskii | title =Общая теория ван-дер-ваальсовых сил | url = http://ufn.ru/ufn61/ufn61_3/Russian/r613b.pdf | trans-title= General theory of van der Waals forces | journal = Uspekhi Fizicheskikh Nauk | language = ru | volume = 73 | issue = 381 | date = 1961 }}
**English translation: {{cite journal | first1 = I. D. | last1 = Dzyaloshinskii | first2 = E. M. | last2 = Lifshitz | first3 = L. P. | last3 = Pitaevskii | title = General theory of van der Waals' forces | journal =Soviet Physics Uspekhi | volume = 4 | issue = 2 | page = 153 | date = 1961 | doi = 10.1070/PU1961v004n02ABEH003330 | bibcode = 1961SvPhU...4..153D }}
* {{cite book | first1 = L. D. | last1 = Landau | first2 = E. M. | last2 = Lifshitz | title = Electrodynamics of Continuous Media | url = https://archive.org/details/electrodynamicso00land | url-access = registration | publisher = Pergamon | location = Oxford | date = 1960 | pages = [https://archive.org/details/electrodynamicso00land/page/368 368–376] }}
*{{cite book |  last = Langbein | first = Dieter | title = Theory of Van der Waals Attraction | publisher = Springer-Verlag | location = New York, Heidelberg | date = 1974 | series = Springer Tracts in Modern Physics | volume = 72 }}
* {{cite web | first = Mark | last = Lefers | url = http://groups.molbiosci.northwestern.edu/holmgren/Glossary/Definitions/Def-V/van_der_Waals_force.html | title = Van der Waals dispersion force | publisher = Holmgren Lab | work = Life Science Glossary | access-date = 2 October 2017 | archive-date = 24 July 2019 | archive-url = https://web.archive.org/web/20190724044455/http://groups.molbiosci.northwestern.edu/holmgren/Glossary/Definitions/Def-V/Van_der_Waals_force.html | url-status = dead }}
*{{cite journal | first = E. M. | last = Lifshitz |title=Russian title is missing | trans-title = The Theory of Molecular Attractive Forces between Solids | journal = Zhurnal Éksperimental'noĭ i Teoreticheskoĭ Fiziki | volume = 29 | issue = 1 | page = 94 | date = 1955 | language = ru }}
**English translation: {{cite journal | first = E. M. | last = Lifshitz | title = The Theory of Molecular Attractive Forces between Solids | url = http://www.jetp.ac.ru/cgi-bin/dn/e_002_01_0073.pdf | journal = Soviet Physics | volume = 2 | issue = 1 | page = 73 | date = January 1956 | access-date = 8 August 2020 | archive-date = 13 July 2019 | archive-url = https://web.archive.org/web/20190713075237/http://www.jetp.ac.ru/cgi-bin/dn/e_002_01_0073.pdf | url-status = dead }}
* {{cite web | publisher = Western Oregon University | url = http://www.wou.edu/las/physci/ch334/lecture/intermol/london.htm | title = London force animation | work = Intermolecular Forces }}
* {{cite book | first = J. | last = Lyklema | title = Fundamentals of Interface and Colloid Science | page = 4.43 }}
* {{cite book | last = Israelachvili | first = Jacob N. | title = Intermolecular and Surface Forces | publisher = [[Company:Academic Press|Academic Press]] | date = 1992 | isbn = 9780123751812 }}

==External links==
* {{cite web |url=http://antoine.frostburg.edu/chem/senese/101/liquids/faq/h-bonding-vs-london-forces.shtml |title=What are van der Waals forces? |first=Fred |last=Senese |publisher=Frostburg State University |year=1999 |access-date=1 March 2010}} An introductory description of the van der Waals force (as a sum of attractive components only)
* {{cite web |url=http://www.ted.com/talks/robert_full_learning_from_the_gecko_s_tail?language=en |title=Robert Full: Learning from the gecko's tail |date=1 February 2009 |publisher=[[Software:TED (conference)|TED]] |access-date=5 October 2016}} TED Talk on biomimicry, including applications of van der Waals force.
* {{cite journal |title=The influence of humidity on the attachment ability of the spider ''Philodromus dispar'' (Araneae, Philodromidae) |journal=Proceedings of the Royal Society B: Biological Sciences |volume=279 |issue=1726 |pages=139–143 |date=18 May 2011 |doi=10.1098/rspb.2011.0505 |pmid=21593034 |pmc=3223641 |last1=Wolff |first1=J. O. |last2=Gorb |first2=S. N. }}
{{Chemical bonds}}


[[Category:Intermolecular forces]]

{{Sourceattribution|Van der Waals force}}