Understanding the rheology of yield stress materials

This thesis presents the PhD research on the flow behavior of yield stress materials, using rheological measurements and confocal laser scanning microscopy. Experiments are performed in dispersed systems, such as emulsions, gels and foams; for these, when the amount of the dispersed phase is high enough, they exhibit a yield stress and are called yield stress materials.
The two main problems when dealing with yield stress materials are: (i) the yield stress of a given material is very difficult to determine and (ii) when these materials are made to flow, most of the time shear banding appears. These two problems provide the motivation for performing research focused on understanding the rheology of yield stress materials.
It is shown that yield stress materials can be classified into ‘simple’ and thixotropic. This distinction is extremely important when considering shear banding and the determination of the yield stress.
We show that gap-dependent viscosities can arise due to either the formation of a lubricating layer close to the shearing walls or cooperative effects when the flow is strongly confined. These cooperative effects can also be translated into an effective slip velocity.
In addition, we demonstrate that the entire mechanical behavior in the vicinity of the jamming point can be understood if the mechanical transition is assumed to be analogous to a phase transition.
We also compare different methods used for determining the yield stress, the yield strain and the storage modulus.
Finally, we give recommendations for further work.