Adhesion (= adhesive force)
The term adhesion is derived from the Latin word adhaerere (in German "adhere"). In physics, he accordingly describes the adherence of two substances or bodies. The exact meaning of adhesion is as follows:
The physical state of an interface layer that arises between two contacting condensed phases (i.e. solids and liquids with negligible vapor pressure).
The special thing about such a state is that the phases involved (unlike magnetism) hold together mechanically. This is due to molecular interactions within the interface layer. So far, researchers have not been able to agree on the forces that cause the mechanical cohesion of the two phases. There are currently several theories of adhesion that suggest different explanations.
Mechanical and specific adhesion
The individual theories can basically be divided into two groups:
- Initial studies on mechanical adhesion assumed that the cohesion is based on physical clinging of an adhesive in the microscopic pores and depressions of a solid body. However, this approach does not explain why adhesive and solid bodies with a smooth contact surface and adhesive also adhere to one another.
- Recent theories on specific adhesion assume that physical and chemical and thermodynamic forces also play a role. Here, considerations regarding mechanical and specific adhesion do not contradict each other, but rather complement each other.
There is now a large number of further studies on the subject of specific adhesion. These primarily include:
- The polarization theory (De Bruyne, 1935)
- The electrostatic theory (Derjagin, 1950)
- The diffusion theory (Voyutzkij, 1960)
- The adsorption and wetting theory (Zismann, Fowkes, Good und Wu, 1963)
Adhesion in everyday life
What may initially sound like an abstract concept is actually encountered everywhere in everyday life. Driving a train, packing gifts or playing with a smartphone - these and similar processes will not work without adhesive forces. The following examples illustrate the specific cases in which adhesion is used.
- Adhesion in traffic means that there is road grip (either between tires and road or between track wheels and rails). One speaks of an adhesion track when a train climbs steep gradients without external aids. The locomotion is based solely on the adhesion between wheels and rails.
- In the case of adhesives, there is adhesion between liquid or solid layers and the surfaces of the parts to be joined. The exact relationships in this area have not yet been fully researched.
- So-called adhesive films adhere to smooth, shiny surfaces without any adhesive. This is due to the attraction of the molecules between the two materials. To achieve adhesion, the molecules must come as close as possible. This is achieved, among other things, with particularly smooth protective films on displays for smartphones or tablets.
- Adhesion is also used in medicine when, for example, injured tissue grows into scars or is glued. Adhesions in the gastrointestinal tract are called peritoneal adhesions. Biologists refer to the contact between cells and an extracellular matrix as cell adhesion.
Cohesion and adhesion
While adhesive forces / attachment forces occur between two substances, cohesive forces move within a substance by definition. Accordingly, cohesion ensures the internal cohesion of liquids or solids. This distinction can be illustrated using the example of adhesive:
- Adhesion refers to the adhesive force between the adhesive and the material to be glued.
- Cohesion refers to the adhesive force between the molecules that together form the adhesive as a whole.
On the outside, cohesion is noticeable through a surface tension. For example, cohesive forces can cause a drop of water to keep its shape on a table top.