Surface tension and interfacial tension arise because of the differences in attractive intermolecular forces at gas-liquid and liquid-liquid interfaces, respectively. For clarity, the following discussion emphasizes surface tension at gas-liquid interfaces; however, the results are equally applicable to interfacial tension at liquid-liquid interfaces

as a gas-liquid interface is approached from the liquid side, the attractive forces are not felt equally because there are many fewer liquid-phase molecules near the interface. Thus there tends to be an unbalanced attraction toward the interior of the liquid on the molecules near the gas-liquid boundary. This unbalanced attraction leads to surface tension and a pressure increment across a curved gas-liquid interface that must be properly accounted for when conserving fluid momentum