Protein and surfactant foams: linear rheology and dilatancy effect

We report results on the dependence of foam rheology on the nature of its components (gas, liquid and surfactant). We have varied the liquid bulk properties, the gas–liquid interfacial properties, which implies also different thin liquid films properties, and the gas. Microscopic characterizations are also performed to find correlations between the different length scales, and to interpret the macroscopic rheological behavior (oscillations and creep). We have found that the elastic modulus G′, once normalized by the Laplace pressure, does not depend on the foam components, whereas the loss modulus G″ shows some dependence with the interfacial properties. We show that the gas is an important factor for these moduli because of its role in the foam aging, via coarsening. We have also evidenced a dilatancy effect taking place under continuous shear, by using a special foam sample shape, in which only a foam band is subjected to shear. We have observed and measured an increase of the liquid volume fraction ɛ in that band.