Surface Tension

Surface tension is defined as the force per unit length (γ) acting along the surface of a liquid. It arises due to strong intermolecular forces of attraction. A molecule located inside the bulk of the liquid is surrounded by other molecules and experiences equal forces in all directions. However, a molecule at the surface experiences unbalanced forces because there are more neighboring molecules below than above. This creates a net inward force that pulls surface molecules toward the interior, causing the liquid to minimize its surface area. As a result, liquids tend to form shapes with minimum surface area, such as spherical droplets.

Surface molecules have higher energy compared to bulk molecules because they are less stabilized. This excess energy is called surface energy, which is the work required to move molecules from the bulk to the surface. Surface tension acts to reduce this energy. When temperature increases, molecular motion increases and intermolecular forces weaken, leading to a decrease in surface tension. At the critical temperature, surface tension becomes zero because the distinction between liquid and vapor phases disappears.

Interfacial tension is similar to surface tension but occurs between two immiscible liquids. It is generally lower because attractive forces between the two liquids partially balance the forces at the interface. Additionally, surface tension causes excess pressure inside droplets, which increases with curvature and contributes to the instability of small droplets.