Surface Tension

Surface tension arises because molecules deep within the bulk of a liquid are pulled equally in every direction by their neighbours, so the net force on them is zero, whereas molecules right at the liquid's surface have neighbours only on the sides and below them, never above -- so they experience a net inward pull toward the interior of the liquid. Because of this imbalance, a liquid's surface behaves rather like a stretched elastic membrane and the liquid naturally tries to minimise its own surface area, which is why free liquid drops pull themselves into spheres, and why liquids rise up narrow capillary tubes. Formally, surface tension is defined as the force acting per unit length, along a line drawn on the liquid's surface, perpendicular to that line; its SI unit is N m$^{-1}$, denoted by the Greek letter $\gamma$ (gamma). Surface tension is a temperature-dependent property: the stronger the intermolecular attractive forces, the larger the surface tension, and as temperature rises, molecules gain kinetic energy, attractive forces are relatively weakened, and surface tension falls. Lowering a liquid's surface tension has practical uses -- it is why soaps and detergents clean effectively (a lowered water-oi …