Surface Tension

Surface tension is the tendency of a liquid surface to contract and minimize its surface area.

Inside the bulk of a liquid, molecules are surrounded by other molecules. They experience attractive forces in all directions.

However, at the surface, molecules have fewer neighbors. As a result, they experience unbalanced attractive forces that pull them toward the interior of the liquid.

Because of these unbalanced forces, surface molecules are less stabilized than molecules in the bulk. So, they possess higher energy, known as surface energy.

To reduce this energy, the liquid tends to minimize its surface area and form spherical droplets.

However, the curved surface exerts pressure on the liquid inside. This pressure increases the vapor pressure at the droplet surface, enabling more molecules to escape into the vapor phase.

This effect is described by the Kelvin equation, which relates the vapor pressure of a droplet p to that of the bulk liquid p₀. As the radius r decreases, p/p₀ increases, causing smaller droplets to evaporate easily.