Surface tension is a property of liquids that causes the surface to behave almost like a thin, invisible sheet. It is the tendency of a liquid’s surface at rest to shrink into the smallest area possible. This phenomenon is strong enough to support small, light objects that would otherwise sink. This “skin” on the water’s surface can resist an external force, which seems surprising because water appears so soft.
The Science Behind the Skin on Water
The science behind surface tension lies in how water molecules stick to one another, a property called cohesion. Water molecules are attracted to each other through strong bonds. A molecule deep inside the water is pulled equally in all directions by its neighbors, resulting in a balanced, net-zero pull. Molecules right at the surface, however, have no water molecules above them. This imbalance means they are pulled mostly inward and sideways by the molecules below and beside them. This inward pull causes the surface to tighten up, creating the effect of a stretched elastic membrane.
Amazing Examples in Nature and Home
This elastic-like surface allows small insects, such as the water strider, to walk across the water without falling through. Surface tension is also responsible for why water forms spherical droplets, like beads of dew on a leaf or on a waxed car. The molecules pull inward to minimize the surface area, and a sphere is the shape with the smallest surface area for its volume. At home, you can observe this by gently placing a paperclip or a needle on the water’s surface, where the tension will hold the denser object up. Even when a glass is slightly overfilled, a dome of water forms above the rim due to the cohesive forces.
Breaking the Water’s Skin
The strong bonds that create surface tension can be easily broken by adding substances called surfactants, such as dish soap. Soap contains molecules that are partly attracted to water and partly repelled by it. When soap is added, these molecules rush to the surface, wedging themselves between the water molecules. This action interferes with the water molecules’ ability to hold hands tightly, instantly weakening the surface tension. A classic experiment demonstrating this is the “pepper trick.” Pepper flakes floating on the water are held there by the tension. As soon as a drop of dish soap touches the water, the surface tension is lowered, and the water quickly spreads out, carrying the pepper flakes to the edges of the dish.