The large amplitude oscillatory strain response of aqueous foam: Strain localization and full stress Fourier spectrum

We study the low frequency stress response of aqueous foams, subjected to oscillatory strain. As the strain amplitude is progressively increased starting from zero, the initially linear viscoelastic response becomes nonlinear as yielding sets in. To characterize this crossover from solid-like to liquid-like behaviour quantitatively, the full harmonic spectrum of the stress is measured. These results are compared to the soft glassy rheology model as well as to elastoplastic models. Moreover, to check for strain localization, we monitor the displacement profile of the bubbles at the free surface of the foam sample in a Couette cell using video microscopy. These observations indicate that strain localisation occurs close to the middle of the gap, but only at strain amplitudes well above the yield strain.