Is the yield stress of aqueous foam a well-defined quantity?

Dimensional arguments suggest that the yield stress of aqueous foams and emulsions, for a given volume fraction of the dispersed phase, should scale as the ratio of surface tension and a length scale, such as the mean bubble radius. We show that yield stress data from the literature and from our experiments follow this scaling only in a very rough approximation. We present new experimental evidence that clarifies the role of shear banding in this context and we discuss the robustness of the yield stress with respect to changes of flow geometry and experimental technique. Finally, we show that scaling oscillatory yield stress data by the elastic shear modulus at small amplitude provides a master curve when plotted versus the volume fraction of the dispersed phase.