  A finite simple shear deformation of an elastic solid induces unequal normal
stresses. This nonlinear phenomenon, known as the Poynting effect, is governed
by a universal relation between shear strain and first normal stresses
difference, valid for non-dissipative elastic materials. We provide the first
experimental evidence that an analog of the Poynting effect exists in aqueous
foams where besides the elastic stress, there are significant viscous or
plastic stresses. These results are interpreted in the framework of a
constitutive model, derived from a physical description of foam rheology.
