Sommaire:

In this thesis, we investigate two kinds of jammed colloidal systems confined in millimetric spheres: beads of colloidal gels and drops of colloidal suspensions. We investigate these systems in three sets of experiments: beads of colloidal gel under uniaxial deformation, drying of beads of colloidal gel and drying drops of colloidal suspensions. Each of these experiments addresses a specific question. First, beads under uniaxial compression are investigated to obtain new insight on the yielding behaviour of colloidal gels. Indeed, the yielding transition of colloidal gels is commonly studied by shear rheology; thanks to an innovative experimental approach, we unveil new facets of yielding. Second, we study the drying of beads of gels to investigate the network restructuring due to the compression occurring during drying. We are interested in the spatial distribution of the network rearrangements, and their relevant lengths scales. Third, it is known that drying drops of colloidal suspensions undergo mechanical instabilities that change their shape, due to the evaporation-induced formation of a dense colloidal shell. Previous works proposed that this instability occurs when the particles in the shell aggregate forming a porous solid. However, no measurements could directly test this scenario and assess its generality with respect to changes in the drying rate and initial volume fraction of the colloidal particles.
To address these questions, we tackled several experimental challenges. To produce spherical beads of colloidal gels, we developed a new experimental protocol whose soundness has been demonstrated by checking that the structural and mechanical properties of the gel beads do not differ from those of bulk gels prepared according to conventional protocols. The stiffness and structure of the beads have been tuned by varying the colloidal volume fraction (VF). Moreover, for studying the drying of beads and drops, we developed a new Dynamic Light Scattering (DLS) setup suited to probe the microscopic dynamics with space- and time-resolution of spherical samples.
To impose a uniaxial compression the beads, we used a rheometer coupled with a camera, allowing for imaging the system while measuring its mechanical response. Beads prepared at high VF abruptly yielded in the linear regime breaking into pieces, while those prepared at low VF dissipated energy by deforming plastically, leading to a smooth yielding. These observations indicated a brittle-to-ductile transition (BDT), predicted theoretically but elusive in shear rheology experiments. Thus, our experimental approach opens a promising experimental avenue for further investigations of the BDT.
Beads of colloidal gel were dried on top of hydrophobic surfaces, minimizing the interactions between the substrate and the gel, leading to an isotropic drying. We investigated isotropic drying through three experimental techniques: DLS, X-ray scattering and uniaxial compression tests. DLS revealed that the stresses imposed by the drying on the surface of the bead are homogeneously propagated, thus inducing rearrangements in the whole volume. X-ray scattering indicated that drying gels rearrange in a hierarchical manner, restructuring first at large length scales and then at smaller length scales, down to few particles size.
The drying of drops of colloidal suspension have been investigated with our new DLS setup. We measured the evolution of the thickness of the shell and the spatial dependence of the volume fraction of the colloids. We found that above a critical evaporation rate the drop undergoes successively two distinct shape instabilities: invagination and cracking. Interestingly, the permanent aggregation of nanoparticles accompanies only the second instability, while the first one results from a reversible glass transition within the shell, unreported so far. Based on our discovery, we proposed a unified state diagram that rationalizes the drying of colloidal suspensions.

Pour plus d'informations, merci de contacter Milani M.