Two-dimensional shear modulus of a Langmuir foam

We deform a two-dimensional (2D) foam, created in a Langmuir monolayer, by applying a mechanical perturbation, and simultaneously image it by Brewster-angle microscopy. We determine the foam stress tensor (through a determination of the 2D gas-liquid line tension, 2.35 +/- 0.4 pJ . m(-1)) and the statistical strain tensor, by analyzing the images of the deformed structure. We deduce the 2D shear modulus of the foam, mu = 38 +/- 3 nN . m(-1). The foam effective rigidity is predicted to be 35 +/- 3 nN . m(-1), which agrees with the value 37.5 +/- 0.8 nN . m(-1) obtained in an independent mechanical measurement.