Shear banding in thixotropic and normal emulsions

When made to flow, yield stress materials rarely flow homogeneously. This is mostly attributed to the fact that such materials show a transition from a solid- to a liquid-like state when the stress exceeds some critical value: the yield stress. Thus, if the stress is heterogeneous, so is the flow. Here we consider emulsion flows in a cone-plate geometry that, for Newtonian fluids, correspond to a homogeneous stress situation and show that shear banding can also be observed either due to wall slip or to the existence of a critical shear rate. By means of velocity profiles obtained using a confocal laser scanning microscope combined with a rheometer we conclude that the last type of shear banding occurs only in thixotropic yield stress materials.