On the use of shear rheology to formulate stable foams. Example of a lyotropic lamellar phase

In order to formulate stable foams with gels, it is important to know their shear rheology properties. We illustrate this fact using a weak gel made of a lyotropic lamellar phase (SDS/hexanol/brine system). The effect of shear is investigated using rheology experiments coupled with Small Angle X-ray Scattering (rheo-SAXS) using synchrotron radiation. According to oscillatory tests, the samples exhibit a weak gel behavior. Steady state experiments evidence a shear-induced transition from lamellar to onion (multilamellar vesicle – MLV) textures at moderate shear rates. After the shear is stopped, there is a recovery from the onion texture to the initial lamellar texture within 30 min, as directly demonstrated by the rheo-SAXS data. The lamellar to onion transition is therefore reversible. This excludes the possibility for the foams to be stabilized by onions adsorbed at the air–water interface as in foams made with other types of lamellar phases. In the example presented, the foam stability originates from the bulk rheological properties of the lamellar phase.