What Does Cohesion Mean?
Cohesion is the mutual attraction and sticking property of molecules. It is an intrinsic property in certain substances that is commonly caused by the structure and shape of its molecules, resulting in electrical attraction. Cohesion paves the way for surface tension, establishing a solid-like condition where, in low density or light, materials can be set. Cohesion is also known as cohesive force or cohesive attraction.
Parameters should be properly monitored in order to manage cohesive failure in certain materials. This is why such parameters have to be considered in corrosion prevention measures. Cohesive failure is a phenomenon whereby a fracture allows a layer of adhesive to remain on both surfaces.
The intermolecular force, when broken down within a substance, reduces the attraction between molecules which results in a substance that separates under external forces. The point where the adherend fails before the adhesive, is what is known as a cohesive failure in the substrate. Lower rates of adhesive effectiveness and durability is the result of high cohesive failure rates.
Some causes of cohesive failure include improper joint movement, insufficient curing and improper selection of an adhesive. When applying corrosion prevention practices, it is important to include cohesive features as a parameter to be considered. This is necessary to determine the rate of molecular consistency within adhesives that are applied to a metallic surface in order to aid protection with the use of a coating.
Corrosionpedia Explains Cohesion
Cohesion promotes attraction between dissimilar molecules. This concept has many applications, the most common being its role in promoting coating strength. The strength in coatings refers to its cohesion measure to a substrate or surface. Without good cohesion, the coating is useless. Depending on the type of coating, as long as coatings remain sticking to the surface, the material is protected against:
- Weather and environment.
- Impact forces.
- Stress and scratches.
The performance and durability of coatings greatly relies on cohesion. With this example, cohesion can be defined as the material's inner strength and is identified by the strength of the molecular structure.
Therefore, industries perform cohesion testing to evaluate the adhesion and cohesion properties of paints. This may include tests like the scrape adhesion test, that gauges adhesion tenacity when coated on flat and smooth panel surfaces. This test along with others ensures that coatings adhere appropriately to the coated surface.
Adhesion and cohesion are attractive forces that act between material bodies. The difference between both these forces, however, is that an adhesive force acts to stick two separate bodies together while a cohesive force is what holds like or unlike atoms, ions, or molecules of a single body together. Both these forces, however, are similar to one another in the sense that they result from the same basic properties of matter.
Some of the phenomena that can be explained with the use of adhesion and cohesion include surface tension in liquids that results from cohesion. Capillarity results from a combination of adhesion and cohesion as well. Another place where cohesion can be seen in action is in the hardness of a diamond which is due to the strong cohesive forces between the carbon atoms of which it is made up.
Adhesion is the force that causes friction between two solid bodies.
Cohesion failure is generally considered to be a good thing to have but in cases where interface strength is the desired measurement, it is of little value. Once cohesive failure occurs in such a case, it can be said that the stress in the joint region of such an interface was less than the strength of the interface at the initial time of testing.
When two molecules are close together, they repel each other and when farther apart, they are attracted; and when they are at an intermediate distance, their potential energy is at a minimum,which requires use of work to either bring them close or separate them. Thus, work is required for pulling apart two objects in intimate contact, whether they be of the same or different material.
The attractive forces of cohesion and adhesion act over a short range and change in magnitude, depending on the substances concerned. If a piece of glass is submerged in water and then withdrawn, it will be wet—i.e., water will adhere to it, which shows that the force of adhesion between water and glass molecules is greater than the force of cohesion between water molecules.