Surface tension

Surface tension is the tendency of a liquid surface at rest to shrink into the smallest possible surface area. This phenomenon occurs at the liquid–air or liquid–vapor interface. It is an inherent property of these boundaries. Surface tension allows objects with higher density than water to float. This includes items like razor blades or insects such as water striders.

These objects can rest on the surface without becoming submerged. This happens because the surface behaves like a stretched elastic membrane.

To understand this mechanism, we must examine the forces between molecules. Molecules of the same type exert cohesive forces on each other. Molecules of different types exert adhesive forces. In the bulk of a liquid, a molecule is pulled equally in every direction by its neighbors. This results in a net force of zero. However, molecules at the surface lack neighbors on all sides. They are pulled inward by the molecules below them. This inward pull creates internal pressure. This pressure forces the liquid surface to contract.

Surface tension can also be explained through the concept of energy. A molecule in contact with a neighbor is in a lower energy state than a lone molecule. Interior molecules have as many neighbors as possible. Boundary molecules are missing neighbors and therefore have higher energy. To minimize its energy state, a liquid must minimize the number of high-energy boundary molecules. This minimization results in a minimal surface area. This process causes the liquid to assume a smooth shape.

There are two primary ways to describe these physical effects. One is an inward force on the surface molecules that causes contraction. The second is a tangential force acting parallel to the liquid surface. This tangential force is what we specifically call surface tension. It resists external forces due to the cohesion between molecules. The balance between cohesion and adhesion determines how a liquid wets a surface. If adhesion energy is less than half of cohesion energy, wetting is low. This creates a convex meniscus, such as mercury in glass. If adhesion energy is more than half of cohesion energy, wetting is high. This creates a concave meniscus, such as water in glass.

Physicists measure surface tension using different units depending on the context. It is represented by the symbol γ, σ, or T. One measurement is force per unit length. In the SI system, the unit is newton per metre. In the older cgs system, the unit is dyne per centimetre. Surface tension can also be viewed as energy per unit area. This is often called surface energy. Surface tension is the term used for liquids. Surface stress and surface energy are the terms used for solids.

Water has a particularly high surface tension compared to most other liquids. At 20 °C, water has a tension of 72.8 millinewtons per meter. This high value is due to the strong attraction between water molecules through hydrogen bonds. Surface tension also dictates the shape of liquid droplets. Droplets tend to be pulled into a spherical shape. This shape minimizes the necessary wall tension of the surface layer. According to Laplace's law, the spherical shape provides the minimum surface area for a given volume.

We can observe how surface tension changes through the use of surfactants. A surfactant is a substance that reduces surface tension. For example, surfactants can reduce the surface tension of water by a factor of three or more. This is why soap bubbles can exist. Bubbles in pure water are unstable because of high surface tension. Surfactants allow for the formation of emulsions. These are colloidal dispersions where tiny droplets of oil are dispersed in water. Without surfactants, these oil droplets would spontaneously coalesce and separate.

Finally, surface tension relates to pressure through the Young–Laplace equation. This equation shows that the pressure difference across a curved interface is related to surface tension. The equation is ΔP = 2γ/R for a spherical drop. This means the internal pressure of a droplet increases as its radius decreases. For a 1 mm drop, the pressure difference is 0.0014 atm. For a 10 nm drop, the pressure difference rises to 143.6 atm. This relationship helps determine the shape of puddles, bubbles, and menisci.