surface tension

Picture a water strider skating on a pond, or a droplet of water beading up on a freshly waxed car. Both are possible because a liquid surface behaves like a stretched, invisible elastic skin that resists being pulled apart. That contracting pull is surface tension.

It arises because molecules deep inside the liquid are attracted equally in all directions by their neighbours, but molecules at the surface have no liquid above them, so the net inward pull leaves the surface under tension. The liquid responds by minimising its surface area, which is why free droplets are spherical. Surface tension is measured as force per unit length (typically millinewtons per metre, mN/m); pure water is about 72 mN/m at room temperature, while many oils are near 20-30 mN/m.

In pharmaceutics, surface tension governs droplet formation in sprays and aerosols, the rise of liquids in capillaries, and how easily a formulation wets a powder or a tablet. Adding a surfactant lowers it sharply, which is the lever formulators pull to improve wetting, foaming, or emulsification. One caveat: surface tension is temperature-dependent and falls as temperature rises, so values must always be quoted with the measurement temperature.