Cohesion (chemistry)

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Water drops on a flower stalk formed by cohesion

Mercury sticks together because of the cohesive forces.

Firestop mortar being installed in a fire test assembly, demonstrating both adhesion and cohesion in an irregular shape

Cohesion (v. lat. cohaerere "stick or stay together") or cohesive attraction or cohesive force is a physical property of a subtsance, caused by the intermolecular attraction between like-molecules within a body or substance that acts to unite them. Water, for example, is strongly cohesive due to strong polarity induced by two hydrogen bonds in a tetrahedral configuration. This results in a relatively strong Coulomb force between molecules. Van der Waals gases such as methane, however, have weak cohesion due only to Van der Waals forces that operate by induced polarity in non-polar molecules.

Cohesion, along with adhesion (attraction between unlike molecules), helps explain phenomena such as surface tension and Capillary action.

[edit] Industry applications

The construction industry uses many materials that undergo a phase transition during installation and use. These include, but are not limited to the following:

Often, these building materials are required not just to be cast and occupy space. They must also stick and maintain a certain thickness or space. Paint typically does not require as much cohesion as caulking, as paint is applied much more thinly. High quality firestop mortars require extreme cohesion with the adhesion because they are often used in large wall openings, where the mortar must not only adhere to adjacent materials but also hold itself up, forming a wall in the plastic state, which must hold long enough to cure and convert to the solid state. The thicker the application, the more cohesion is required.