Abstract
Dry two-dimensional soap foams are very well described at the level of a single cell or of a vertex by the the laws of Laplace and Plateau. However there is a lack of understanding of their behavior at amacroscopic scale. Continuum mechanics, which relates stress and strain seems to fail for soap foams (as well as for other macroscopic systems such as granular media). After a brief discussion of the commonly used definition for the stress tensor, we introduce a discrete local stress field at the vertices of the foam. We show that an equilibrium relation is directly induced by the definition, and that the whole stress field can be determined for given boundary conditions, without any supplementary stress-strain relation. Macroscopic properties such as the shape of the foam and its free energy are directly given by the stress field.