Adhesion and cohesion are two types of attraction between atoms and molecules. The two words sound similar, yet they mean different things.
- Adhesion is the attraction between different types of molecules.
- Cohesion is attraction between the same type of molecules.
- Cohesion makes liquids contract into the smallest possible surface area. This is surface tension.
- The meniscus and capillary action depend on adhesion and cohesion.
Both adhesion and cohesion occur because of intermolecular forces. Water has high cohesion largely because water molecules form hydrogen bonds with other water molecules. Water has high adhesion to polar surfaces because of partial electrical charges on molecules. On the other hand, water has low adhesion to nonpolar surfaces because there is nothing there that attracts the water molecules. For example, water has high adhesion to glass, but low adhesion to waxed paper.
Examples of Adhesion and Cohesion
Adhesion occurs when molecules are attracted to different molecules. Mostly, this results from intermolecular forces, like hydrogen bonding, London dispersion forces, and van der Waals forces. However, sometimes surfaces stick together because of their physical structure.
Here are examples of adhesion:
- Tape sticking to paper
- Honey sticking to your fingers
- Dewdrops clinging to a leaf tip
- Water drops sticking to a window without falling
- Water wetting a plastic cutting board
- Mercury wetting a metal surface
Cohesion occurs when atoms or molecules experience attraction to themselves. This occurs within compounds capable of forming hydrogen bonds and in metals.
Here are examples of cohesion:
- Water forming beads on a window
- Mercury forming beads in a glass container
- Mercury sticking to itself as it flows
- Water striders floating on water (from the high surface tension resulting from cohesion)
Part of the reason people have trouble keeping adhesion and cohesion straight is because both processes occur at once. For example, a water drop sticking to a window illustrates both concepts. That the water forms a rounded shape is an example of cohesion. That the droplet sticks to the glass instead of falling right away demonstrates adhesion.
Adhesion, Cohesion, and the Meniscus
A meniscus is the curved line that forms at the upper surface of a liquid in a container. The shape of the meniscus depends on the difference between the adhesion between the liquid and the container surface and the cohesion between the liquid molecules.
- A concave meniscus forms when the liquid is more attracted to its container than it is to itself. In other words, adhesion is greater than cohesion. Water in a glass tube has a concave meniscus, even though it has high cohesion.
- A convex meniscus forms when the liquid is more attracted to itself than to its container. Cohesion is greater than adhesion. Mercury in a glass container has a convex meniscus.
- A flat meniscus occurs when adhesion and cohesion are in balance. Water in a plastic container has a nearly flat meniscus.
Note, the curve of the meniscus also depends on how narrow the walls of the container are. The curve is more pronounced in thin tubes.
Relating Cohesion and Adhesion to Surface Tension
Surface tension is the tension on the outer film of a liquid that minimizes its surface area. It describes the relationship between cohesion and adhesion. A great example of surface tension is slightly overfilling a glass of water. The water forms a rounded or convex surface past the rim of the glass because of its high surface tension. Water overfills the glass both because it has high cohesion and because it has high adhesion.
Surface tension is mainly due to the cohesion between like molecules. So, a highly cohesive liquid, like water, also has high surface tension. Similarly, mercury has high surface tension.
Whether or not a liquid wets a surface depends on its surface tension. Surfactants and other wetting agents reduce surface tension. For examples, ducks and geese are heavier than water, yet they float on it because of water’s high surface tension. Water does not wet their feathers because the cohesion of water exceeds adhesion to oily feathers. Adding a surfactant, like soap, reduces the surface tension. Ducks and geese sink in soapy water. Some surface treatments alter the nature of the surface, increasing the adhesion between the liquid and the surface.
Capillary Action
Capillary action is another phenomenon based on the interplay between adhesion and cohesion. In capillary action, a liquid travels through a narrow tube, sometimes against the force of gravity. Capillary action occurs when adhesion sticks the liquid to the container wall and cohesion pulls other liquid molecules along for the ride.
If you place a capillary tube upright in a container of water, the water rises in the tube via capillary action to a level higher than the surface. On the other hand, if you place a capillary tube in a container of mercury, the level of mercury within the tube is lower than the surface. The high cohesion between mercury atoms draws liquid from the tube.
References
- Kelly Reese, J. B.; Taylor, M. R.; Simon, E. J.; et al. (2020) Campbell Biology (12th ed.). Pearson. ISBN: 978-0135188743.
- Kendall, K. (1994). “Adhesion: Molecules and Mechanics”. Science. 263 (5154): 1720–5. doi: 10.1126/science.263.5154.1720
- Maeda, N.; Chen, N.; Tirrell, M.; Israelachvili, J.N. (2002). “Adhesion and Friction Mechanisms of Polymer-on-Polymer Surfaces”. Science. 297 (5580): 379–82. doi: 10.1126/science.1072378
- Margenau, H.; Kestner, N.R. (1969). Theory of Intermolecular Forces. International Series of Monographs in Natural Philosophy. Vol. 18 (1st ed.). Oxford: Pergamon Press. ISBN 978-0-08-016502-8.