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Water droplets on bluebell blossom

Adhesion ( Latin adhaerere "to stick"), also called adhesion or attachment force , is the physical state of an interface layer that forms between two contacting condensed phases , ie solids and liquids with negligible vapor pressure . The main property of this state is the mechanical cohesion of the phases involved, caused by molecular interactions in the interface layer. The forces causing this mechanical cohesion have not all been fully researched, which is why there are different theories of adhesion .

Adhesion theories

A general distinction is made between mechanical adhesion based on physical-mechanical forces and specific adhesion based on chemical, physical and thermodynamic forces, for each of which there are different theories of adhesion. These theories were developed individually, but as far as we know today, mechanical and specific adhesion form a unit.

Mechanical adhesion

The theory of mechanical adhesion refers to the form-fitting interlocking of a phase in the microscopic pores and depressions on the surface of a solid. In the past, this was the only way to explain adhesion; the question of the cohesion of two phases with smooth and pore-free contact surfaces cannot be answered with this.

Specific adhesion

Theories of specific adhesion were developed because the theory of mechanical adhesion was insufficient to explain the cohesion of solids with smooth surfaces. The various theories of specific adhesion and the theory of mechanical adhesion are not mutually exclusive, but complement one another.

The polarization theory ( De Bruyne 1935) According based adhesion to the dipole character of the molecules . However, this explanation is limited to polar substances .

The electrostatic theory ( Derjagin 1950) assumes an electrical double layer (a space charge zone several molecular or atomic layers thick , caused by charge shifts) as the cause of the adhesive force. However, suitable charge carriers , such as electrons or ions , must be present for this.

The diffusion theory ( Voyutzkij 1960) is based on Brownian molecular motion - i.e. the proper motion of the molecules caused by temperature - which causes particles of the two substances involved to diffuse into one another . However, the substances must have a chemical affinity to one another, which is usually only the case with plastics . In the case of metals, for example, the metal bond prevents diffusion.

Wetting behavior of a drop on a solid surface. According to the theory of adsorption and wetting, in case A little or no, in case C and S (spreading) very high adhesion forces act at the interface.

The basis of the adsorption and wetting theory ( Zismann , Fowkes , Good and Wu , 1963) is the surface and interface theory . According to this thermodynamic consideration of the adhesion, such liquids wet solid surfaces particularly well which are in an only slightly less favorable energetic state at the interface with the solid phase than inside. If the energetic state at the phase boundary is more favorable than inside, complete wetting ( spreading ) occurs, in which all particles of the liquid adhere to the solid surface. No wetting - the other extreme case - occurs when the energetic state for the particles inside the liquid is so favorable that it forms a sphere, which reduces the contact surface with the solid to a minimum. Taking into account the structure of the interface layer ( roughness and foreign matter particles), the temperature and other factors beyond the thermodynamic consideration, conclusions can be drawn about the adhesion.


Adhesion in vehicles

Under adhesion is understood in the road , the road holding (grip tightly.) - rubber ( tires ) to ground ( road ) - or in the rail , the rail adhesion - iron (train wheels) iron ( rail ). An adhesion track is used when a track can cope with steep gradients without aids (e.g. gearwheel or rope ) and only the adhesion of the wheels is sufficient for movement. The adhesion also works when flying boats take off and keeps their hulls in the water. The designers solved this problem by means of a tear-off edge halfway along the keel (also known as a step) so that the hull of the machine can detach from the water surface.

Adhesion in adhesives

Adhesion encompasses the adhesive forces on the contact surfaces of two different or identical substances due to molecular forces. The substances can be in solid or liquid state. In the field of adhesives , adhesion is understood as the adhesion of adhesive layers to the surfaces of the parts to be joined. The processes involved in adhesion are not yet fully understood. They are particularly difficult because the dependencies between the adhesive systems and the various surfaces of the parts to be joined are very complex.

Adhesion in foils

Adhesive foils adhere to smooth / shiny surfaces without glue due to the attraction of the molecules between the two materials. The prerequisite is that the molecules come as close as possible in order to achieve adhesion. This is why this only works on smooth surfaces, for example as protective films on displays or tint films on the glass of motor vehicles .

Adhesion to organic tissues


  • Claus Bischof, Wulff Possart: Adhesion - Theoretical and experimental foundations. Akademie-Verlag, Berlin 1983.
  • Valentin L. Popov: Contact Mechanics and Friction. A text and application book from nanotribology to numerical simulation. Springer, Berlin et al. 2009, ISBN 978-3-540-88836-9 .

Web links

Wiktionary: Adhesion  - explanations of meanings, word origins, synonyms, translations
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