Understanding Surface Tension in Liquids

The question asks about the specific name given to the tensile force that acts on the surface separating two immiscible liquids. This force causes the contact surface to behave as if it were a stretched membrane under tension.

Let's look at the provided options and see which one accurately describes this phenomenon:

• Vapour pressure: This refers to the pressure exerted by a vapour in equilibrium with its liquid or solid phase in a closed system. It is related to the phase transition between liquid/solid and gas, not the tensile force between two liquid surfaces.
• Capillarity: This is the ability of a liquid to flow in narrow spaces against external forces like gravity. It is a consequence of surface tension and adhesive forces, but it is not the tensile force itself.
• Surface tension: This is a property of liquids that describes the tensile force per unit length existing in the surface film of a liquid, caused by the attraction between molecules. This force makes the surface contract to the smallest possible area. When applied to the interface between two immiscible liquids, it is often referred to as interfacial tension, but the underlying principle is the same tensile force causing the surface to act like a stretched membrane.
• Cavitation: This is the formation of vapour bubbles in a liquid, typically when the pressure falls below the liquid's vapour pressure. It is not related to the tensile force on a liquid surface.

Based on the definitions, the phenomenon where the surface between two immiscible liquids acts like a membrane under tension due to a tensile force is known as surface tension (or more specifically, interfacial tension when between two liquids, but surface tension is the general term for this kind of tensile force on a surface).

Therefore, the correct term for the tensile force acting on the surface between two immiscible liquids, causing the surface to behave like a membrane under tension, is surface tension.