Difference between Adhesion and Cohesion
Adhesion and cohesion are the forces of attraction that exist between different molecules and same molecules respectively. Though they sound to be similar, they are different from each other. Let us see how cohesion differs from adhesion.
Cohesion refers to the intermolecular attraction between the molecules of a same substance. It is the tendency of like molecules to stick together. For example, the cohesive forces between the water molecules allow water to travel with consistency, to form water drops, and are responsible for the surface tension, and capillary action. It is the intrinsic property of a substance which is due to the shape and structure of the molecules of the substance. Thus, we can say that the mutual attraction between like molecules that causes them to stick together is cohesion.
Adhesion refers to the attraction between the molecules of the different substances. It is the tendency of different molecules to attract one another. Adhesion is responsible for proper functioning of glue, cement, paint, ink etc. It also makes possible the water transportation in plants through xylem vessels. Here, the adhesion forces between water molecules and the cell wall help water travel through xylem vessels.
The Relationship between cohesion and adhesion:
The cohesion and adhesion forces generally exist together; you can find these forces in various processes and activities. For example, the meniscus, which is a curvature of a liquid surface stored in a container or tube, is caused by both adhesion and cohesion. The force of attraction between the edges of liquid and wall of the container is adhesion and the force of attraction between water molecules, which makes the liquid surface curved at the middle, is cohesion.
The shape of the meniscus is decided by these forces. If cohesion force, which exists between liquid molecules, is more than adhesion force, which exists between the liquid and an inner surface of the tube, the shape of the meniscus will be convex, e.g. mercury in a glass tube. Similarly, if adhesion is more than cohesion, the meniscus will be concave, e.g. water in a glass tube. If the cohesion is equal to adhesion, the surface will be horizontal.
Suppose you spill water on a surface. If the adhesive force is strong the water will get absorbed soon by the surface and it will get wet. If the cohesive force is strong, there will be more attraction between water molecules than between the water molecules and the surface, so less water will be absorbed by the surface.
Based on the above information, some of the key differences between cohesion and adhesion are as follows: