Tension in a Floating film

We describe a simple experiment to extract the in-plane stress in a thin sheet floating at a liquid/vapour interface. This allows us to determine what combination of solid-liquid, solid-vapor and liquid-vapour surface energies affect the in-plane stress. We address this question by pulling one edge of the floating sheet upward, such that part of the sheet is rigidly suspended, while the remainder floats on the liquid. We determine the tensile stress in the sheet from the two-sided meniscus that is thus formed: one side is covered by the sheet and the other side is a liquid-vapor interface. The observed symmetry between the two sides of the meniscus shows that the tensile stress in the sheet is equal to the liquid-vapor surface tension, and is independent of the solid-liquid or solid-vapor surface energy. We confirm this result by repeating the experiment with different sheets and liquids.