Adhesion vs Cohesion
There are various phenomena to explain the things we observe in our daily lives. Although sometimes we do not focus on these little details, they are the ones that help to keep the life on earth. Adhesion and cohesion are two such phenomena. Though they sound similar, they are completely different terms.
What is Adhesion?
Adhesion is the attractive forces between two types of molecules, which are different from each other. For example, the attraction between the water molecules with the walls of xylem vessels can be taken. It is because of this force, water travels through the xylem in plants. These are intermolecular forces. There are five mechanisms to explain the mechanism of adhesion. They are mechanical adhesion, chemical adhesion, dispersive adhesion, electrostatic adhesion, and diffusive adhesion. In mechanical adhesion, adhesive material holds the surface by filling into the holes or pores in it. In chemical adhesion, chemical bonds are forming. These could be either ionic or covalent bonds. If the bonds are ionic or covalent, then electrons may donate or attract, or else electron sharing may present. Other than these, inter molecular bonds like hydrogen bonds can participate in holding two materials together. If the two materials are held together due to Van der Waals forces, then that mechanism can be explained by dispersive adhesion. When there is a slight (permanent or temporal) charge separation in a molecule, the molecule is said to be polarized. The opposite charges tend to attract each other; hence, there will be attractive forces between the molecules. Electron passing in conductive material may cause electrical charge differences. The charge differences can cause attractive electrostatic forces between materials. This is known as electrostatic adhesion. When two types of molecules are soluble in each other, they can move into the other surface causing diffusive adhesion. The strength of the adhesion forces depending on the mechanism as to how it is happening. For example, if the contacting surface area is very larger, the strength of Van der Waals forces is higher. Therefore, the strength of dispersive adhesion forces is higher.
What is Cohesion?
Cohesion is the intermolecular forces between two similar molecules. For example, water molecules have intermolecular attraction forces between them. This property of water allows water molecules to travel with consistency. The shape of the rain drops or the existence of water droplets rather than single molecules can be explained by the cohesion. Hydrogen bonding ability of water molecules is the main reason behind the cohesion forces of water molecules. Each water molecule can form four hydrogen bonds with other water molecules; thus, the collection of attraction forces is much stronger. Electrostatic forces and Van der Waals forces between similar molecules also cause adhesion. However, adhesion due to Van der Waals forces is somewhat weaker.
What is the difference between Adhesion and Cohesion?
• Adhesion is the attraction between substances/ molecules which are not similar. On the other hand, cohesion is the attraction between the molecules or substances that are similar.
• For example, cohesion is between two water molecules and adhesion is between water molecules and the walls of xylem vessels.