Oscillatory rheology of aqueous foams: surfactant, liquid fraction, experimental protocol and aging effects

Abstract

We report a new set of rheological data on well controlled aqueous foams. We investigate and analyze how the linear viscoelastic regime, the foam yielding and the non-linear regimes above yielding actually depends on the interfacial properties, bubble size, liquid fraction and foam age. Results are compared to previous works on foams and emulsions, and to models. The viscoelastic linear properties and yield stress are strongly dependent on the liquid fraction, and for a low molecular weight surfactant, providing “fluid-like” interfaces, a universal behavior is recovered. However, discrepancies are observed for protein foams, and are discussed in relation to the interface and thin film properties. We also discuss the features of the non linear regimes above the yield stress, which cannot be fully explained by recent models. As the foam ages, the evolution of the viscoelastic properties can be interpreted in terms of foam drainage and coarsening; nevertheless, some of the aging effects remain unexplained. We also present the results of a new mode of oscillatory experiments, at constant shear rate the macroscopic results obtained with this new protocol turn out to be strikingly well correlated to microscopic measurements at the bubble scale. We then show that a same solid-liquid transition is obtained either by applying a deformation, or by the foam coarsening; we propose that the transition is controlled by a Deborah number De, which can be seen either as a frequency ratio or a deformation ratio. For De < 1, the foam is fluid-like and the bubbles are unjammed (and the opposite is true when De > 1).