interfacial tension

Pour oil onto water and the two refuse to mix, meeting at a sharp boundary. That boundary is itself under tension, just as a liquid surface in air is. Interfacial tension is the contracting force acting at the dividing line between two immiscible liquids, such as oil and water.

Physically it is the same phenomenon as surface tension: molecules at the boundary are attracted more strongly by their own kind than by the foreign liquid on the other side, leaving an unbalanced inward pull. Because the two liquids do attract each other somewhat, interfacial tension is usually lower than either liquid's surface tension against air. It is also measured in mN/m; a typical clean oil-water interface might sit around 30-50 mN/m before any additive is introduced.

Interfacial tension is the central quantity in emulsion science. A high interfacial tension means the system strongly resists creating new oil-water area, so droplets tend to coalesce back together. Emulsifying agents work precisely by adsorbing at the interface and slashing this tension, sometimes to near zero, which is why a little surfactant lets oil and water be whipped into a stable cream or lotion. The honest limitation is that low interfacial tension alone does not guarantee stability; a mechanically strong interfacial film often matters just as much.