Adhesion 835157 221521776 2008-06-24T21:38:23Z 131.107.0.73 /* Electrostatic adhesion */ {{expert-subject|Physics}} [[Image:Spider web Luc Viatour.jpg|thumb|right|250px|[[Dew]] drops adhering to a [[spider web]]]] {{otheruses}} '''Adhesion''' is the tendency of certain dissimilar molecules to cling together due to [[attractive forces]]. ==Mechanisms of adhesion== [[Image:Drops I.jpg|thumb|right|200px|[[Cohesion]] causes water to form [[Drop (liquid)|drops]], [[surface tension]] causes them to be nearly spherical, and adhesion keeps the drops in place.]] [[Image:Hibiscus pink.jpg|thumb|right|200px|Water droplets are flatter on a [[Hibiscus]] flower which shows better adhesion.]] Five mechanisms have been proposed to explain why one material sticks to another: ===Mechanical adhesion=== Adhesive materials fill the voids or pores of the surfaces and hold surfaces together by interlocking. Sewing forms a large scale mechanical bond, [[velcro]] forms one on a medium scale, and some textile adhesives form one at a small scale. This is similar to surface tension ===Chemical adhesion=== Two materials may form a [[Chemical compound|compound]] at the join. The strongest joins are where atoms of the two materials swap ([[ionic bonding]]) or share ([[covalent bonding]]) outer electrons. A weaker bond is formed if [[oxygen]], [[nitrogen]] or [[fluorine]] atoms of the two materials share a [[hydrogen]] nucleus ([[hydrogen bonding]]). ===[[Dispersive adhesion]]=== In [[dispersive adhesion]], also known as adsorption, two materials are held together by [[van der Waals force]]s: the attraction between two molecules, each of which has a regions of positive and negative charge. In the simple case, such molecules are therefore polar with respect to average charge density, although in larger or more complex molecules, there may be multiple "poles" or regions of greater positive or negative charge. These positive and negative poles may be a permanent property of a molecule ([[Keesom force]]s) or a transient effect which can occur in any molecule, as the random movement of electrons within the molecules may result in a temporary concentration of electrons in one region ([[London forces]]). ===Electrostatic adhesion=== Some conducting materials may pass [[electrons]] to form a difference in [[electrical charge]] at the join. This results in a structure similar to a [[capacitor]] and creates an attractive [[electrostatic]] force between the materials. ===Diffusive adhesion=== Some materials may merge at the joint by [[Atomic diffusion|diffusion]]. This may occur when the molecules of both materials are mobile and [[soluble]] in each other. This would be particularly effective with [[polymer]] chains where one end of the molecule diffuses into the other material. It is also the mechanism involved in [[sintering]]. When [[metal]] or [[ceramic]] powders are pressed together and heated, atoms diffuse from one particle to the next. This joins the particles into one. ==What makes an adhesive bond strong?== The strength of the adhesion between two materials depends on which of the above mechanisms occur between the two materials, and the surface area over which the two materials contact. Materials that [[wetting|wet]] against each other tend to have a larger contact area than those that don't. Wetting depends on the [[surface energy]] of the materials. Well-known examples of adhesion are tape, glue, stickers, and rubbing dirt on golf balls. == See also == * [[Adhesion (medicine)]] * [[Cell adhesion]] * [[Cohesion (chemistry)|Cohesion]] * [[Rail adhesion]] * [[Scratch tester]] Method of testing the adhesion of coatings to substrates ==References== * John Comyn, ''Adhesion Science'', Royal Society of Chemistry Paperbacks, 1997 * A.J. Kinloch, ''Adhesion and Adhesives: Science and Technology'', Chapman and Hall, 1987 [[Category:Materials science]] [[Category:Chemical properties]] [[Category:Intermolecular forces]] [[cs:Adheze]] [[da:Adhæsion]] [[de:Adhäsion]] [[es:Adhesión]] [[eo:Adherado]] [[fr:Adhésion]] [[gl:Adhesión]] [[id:Adhesi]] [[it:Adesione]] [[he:אדהזיה]] [[nl:Adhesie]] [[ja:接着]] [[no:Adhesjon]] [[pl:Adhezja]] [[pt:Adesão]] [[ru:Адгезия]] [[fi:Adheesio]] [[sv:Adhesion]] [[tr:Adezyon aşınması]] [[uk:Адгезія]]