Fluid mechanics (surface phenomena): The free surface of a liquid behaves like a stretched sheet and tends to contract to the smallest possible area. Identify the physical cause responsible for this tendency of liquid surfaces. Choose the correct effect/phenomenon.

What is being asked?Why the free surface of a liquid behaves like a stretched elastic membrane that 'pulls' itself into a minimum-area shape.

Given data

• Phenomenon: Surface behaves like a sheet; contracts to smallest area.

Concept/ApproachMolecular cohesion at the surface creates an energy per unit area called surface tension (symbol γ). For a given volume, minimizing surface energy (E = γ × area) implies minimizing surface area → spherical drops, smooth menisci.

Step-by-step reasoning1) Molecules at the surface experience net inward cohesive forces → surface energy exists.2) System tends to minimize energy → reduce surface area.3) The macroscopic effect is called surface tension, not viscosity/adhesion.

Common pitfallsConfusing viscosity (internal friction against flow) or capillarity (a consequence of surface tension + adhesion) with the root cause.

Final AnswerSurface tension