Intermolecular forces govern the physical properties of all substances, particularly liquids. Adhesion and cohesion are two specific manifestations of these forces, describing how molecules interact with themselves and with other materials. The interplay between these two forces dictates observable phenomena, such as how water moves through soil or how a liquid behaves in a container.
Cohesion: Attraction Between Like Molecules
Cohesion describes the attraction between molecules of the same substance, causing a liquid to stick to itself. This force is particularly strong in water due to its ability to form extensive hydrogen bonds between individual water molecules. The positively charged hydrogen atom of one water molecule is drawn to the negatively charged oxygen atom of a neighboring molecule, creating a strong internal network of attraction.
The most observable result of this strong cohesive force is a phenomenon known as surface tension. Molecules on the surface of a liquid are pulled inward and sideways by their neighbors, as there are no molecules above them to provide an upward pull. This unbalanced attraction causes the surface to act like a thin, stretched elastic film, giving the liquid the capacity to resist external stress or rupture.
This high surface tension allows small objects, like a paperclip or a water strider insect, to rest on the surface without sinking. Cohesion also causes liquids to minimize their surface area, which is why water naturally forms spherical droplets.
Adhesion: Attraction Between Unlike Molecules
Adhesion is the attractive force that occurs between molecules of different substances, causing one material to stick to another. This force is what allows water to cling to surfaces like glass, or what enables paint to bond to a wall. The attraction is often due to electrostatic interactions between the liquid molecules and the molecules of the solid surface.
When a liquid comes into contact with a solid, the strength of the adhesive force relative to the cohesive force determines whether the liquid will “wet” the surface. Wetting occurs when the adhesive forces between the liquid and the surface are stronger than the cohesive forces within the liquid itself. In this situation, the liquid spreads out to form a thin film, maximizing its contact with the solid.
A clear example is water on a clean glass surface, where the water’s molecules are strongly attracted to the polar molecules in the glass, leading to good wetting. Conversely, when water is placed on a waxed or oily surface, the cohesive forces within the water are greater than the adhesive forces to the non-polar surface. This imbalance causes the water to minimize contact and “bead up” into droplets.
Observable Phenomena Driven by Both Forces
Many natural and technological processes are the result of cohesion and adhesion working in concert. Capillary action is a prime example, describing the ability of a liquid to flow in narrow spaces against the force of gravity. In this process, the adhesive forces cause the liquid molecules to climb up the sides of a narrow tube, such as a glass capillary.
As the liquid adheres to the walls, the cohesive forces simultaneously pull the rest of the liquid column up along with the adhering molecules. The liquid continues to rise until the upward pull from the combined adhesive and cohesive forces is balanced by the downward force of gravity acting on the liquid’s mass. This combined action is how trees transport water from their roots to their highest leaves through tiny vessels.
The forces also create the curved surface of a liquid when it is held in a container, a phenomenon known as the meniscus. A concave meniscus, where the liquid curves downward, forms when the adhesive forces to the container walls are stronger than the internal cohesive forces. This is typical for water in a glass cylinder, where the water creeps up the sides of the glass.
In contrast, a convex meniscus, where the liquid surface bulges upward, forms when the cohesive forces within the liquid are stronger than the adhesive forces to the container. Liquid mercury in a glass container demonstrates this, as its atoms are much more strongly attracted to each other than to the glass, causing the liquid to pull away from the walls and minimize contact.