as dislocations and grain boundaries are defined precisely and their evolution under stress can be followed.

This representation of the foam as a space-filling cellular complex simplifies greatly the description of its deformation and of its evolution. The foam is in quasi-static, statistical equilibrium [@dubertret; @dubertret2], punctuated by elementary, local topological transformations involving an [*interface*]{}. As shown on the figure \[fg1\], the flip of an interface (usually called $T1$ transformation) produces a switch of neighbor cells. The disappearance of an interface (a coalescence event) leads to the disappearance of one of the two cells that it separates (called $T2$ process, although $T2$ is an elementary transformation for three-sided cells only [@dubertret]). These transformations introduce topological defects, notably dislocations, with specific and dramatic effects on the properties of the material, not least on its mechanical resistance or plasticity [@mott; @miri]. Topological defects can be described for two dimensional foams (a packing of polygonal cells) or three dimensional foams. Dislocations have to be referred to a perfect structure which for 2D foams is generally a hexagonal structure [@miri]. In three dimensional structures the reference order is not so simple to define and $T1$ transformations are not unique. In this paper we are 