Surface tension, represented by the symbol ? is defined as the force along a line of unit length, where the force is parallel to the surface but perpendicular to the line.
The cohesive forces among the liquid molecules are responsible for this phenomenon of surface tension. In the bulk of the liquid, each molecule is pulled equally in every direction by neighboring liquid molecules, resulting in a net force of zero. The molecules at the surface do not have other molecules on all sides of them and therefore are pulled inwards. This creates some internal pressure and forces liquid surfaces to contract to the minimal area.
The imbalance of forces near the upper surface of a liquid has the effect of an elastic film stretched across the surface, thus, it acts like a membrane.