Rheological memory effect in aqueous foam

We have measured the complex shear modulus G* of aqueous foam before and after applying a strain of the order of the yield strain for a brief period of time. After such a mechanical perturbation we observe transient "strain softening": the real part of G* is smaller than before and as the foam aging process goes on, it slowly relaxes towards the values obtained for quiescent foam. The characteristic time scale of this relaxation increases with the foam age at which the perturbation is applied, following a scaling law.