Mechanical consequences of fluid transport in gels and suspensions

We investigate the rheological behaviour of complex materials that are a mixture of a solid and liquid phases whose displacements can be decoupled. We emphasize the consequences that a potential decoupling between the motion of the two phases can have on the mechanical response of the material.
We focus in a first part on hydrogels, materials made of water in a cross-linked network. The purpose of the study is to understand why some synthetic gels show positive normal stresses, and why biopolymers exhibit the opposite tendency. We show that the main factor for this is the pore size of the network, which controls the permeability of the solid material.
We then study another two phase material : Dense Non-Brownian suspensions. These are solid spheres floating in a density matched liquid. These materials exhibit non trivial physics, among which normal stresses. We discover that a finite size effect appears at very high confinement. We suspect that, once again, the solvent mobility through the pores, plays a decisive role.
In the last chapter, we consider the suitability of the fluidity model in predicting flow curves of sheared suspensions. In such materials, particle migration happen and inhomogeneities will appear. We probe the sheared suspension with magnetic resonance imaging, which gives the local volume fraction and flow velocity. The model describes accurately the experimental data, but requires the knowledge of a parameter, which depends of the flow geometry. This prevents the model to be used for predictive purposes.