Surface tension is the force acting on the interface when two or more phases are in contact with each other. Assume a gas-liquid interface: say water - air. In the bulk region of stationary water, each water molecule is surrounded by other water molecules in a uniform manner. Therefore, the liquid molecule experiences zero net force. A similar observation can be made for the bulk region in the gas phase. However, the liquid molecules at the interface are surrounded by other liquid molecules at the bottom and gas molecules at the top. The cohesive forces of attraction between the liquid molecules are larger than cohesive forces between the gas molecules. This results in a force imbalance between the liquid and gas molecules at the interface. This force imbalance tends to produce an effect similar to a stretched membrane at the interface and is responsible for minimizing the overall surface area of fluid surfaces. This property of fluid has the dimension of force per unit length.