Surface tension is a common phenomenon observed in liquids, making surfaces behave like a stretched, elastic film. This property allows light objects to float and water to form beads. Understanding the fundamental forces behind surface tension, specifically whether it is primarily due to cohesion or adhesion, helps clarify this everyday occurrence.
What Surface Tension Is
Surface tension describes the “skin-like” property at a liquid’s surface. It arises from an imbalance of intermolecular forces experienced by molecules at the surface compared to those within the liquid’s bulk. Molecules inside a liquid are surrounded by other molecules, experiencing attractive forces in all directions, which results in a net force of zero. However, molecules at the liquid-air interface lack neighboring molecules above them. This leads to a net inward pull, drawing surface molecules towards the liquid’s interior and causing the surface to contract and minimize its area.
Cohesion’s Contribution
Cohesion refers to the attractive forces between molecules of the same substance. These strong cohesive forces among liquid molecules are the primary reason for surface tension.
Water, for example, exhibits high cohesive forces due to hydrogen bonding. This pulls surface molecules inward and sideways. The inward pull minimizes the surface area, which is why water droplets tend to form spherical shapes. Mercury, with its even stronger cohesive forces, forms very distinct spherical beads.
Adhesion’s Influence
Adhesion describes the attractive forces between molecules of different substances. While adhesion does not directly cause surface tension, it significantly influences how a liquid’s surface tension behaves when interacting with another surface.
The balance between cohesive forces within the liquid and adhesive forces between the liquid and the interacting surface determines phenomena like wetting. For instance, if adhesive forces are stronger than cohesive forces, the liquid will spread out and “wet” the surface.
Capillary action, where a liquid rises or falls in a narrow tube, also depends on this balance. When adhesive forces between the liquid and the tube walls are stronger than the liquid’s cohesive forces, the liquid climbs the tube, forming a concave meniscus. Conversely, if cohesive forces are stronger, the liquid surface will be depressed, forming a convex meniscus.
Everyday Examples
Numerous everyday phenomena illustrate the interplay of surface tension, cohesion, and adhesion. Water striders can glide across the surface of a pond because their light weight and specialized legs do not break through the water’s surface film, which is maintained by water’s strong cohesive forces. Raindrops falling from a faucet or forming on a leaf demonstrate how cohesive forces pull the water into nearly spherical shapes to minimize surface area. Soap reduces water’s surface tension, allowing it to spread more easily and penetrate fabrics for effective cleaning. Capillary action is evident in plants, where water travels from the roots to the leaves through narrow tubes, a process driven by the adhesive forces between water and the plant’s vascular tissue, combined with the cohesive forces among water molecules.