Due to their complex disordered structure, colloidal gels are exquisite models to capture the behavior of amorphous solids under mechanical stresses and are relevant for various applications: from food industry to the manufacturing of ceramics.
We produce millimetric beads of colloidal gels made by controlled aggregation of nanoparticles into disordered network-like structure with tunable characteristic length scale and elastic modulus, which typically varies from 1 kPa to 1000 kPa, when the nanoparticles volume fraction varies between 1 and 10%. We couple mechanical measurements, imaging and scattering techniques to investigate the behavior of the beads in uniaxial and isotropic compression in boundary free conditions.
The isotropic compression is imposed by depositing the gel bead on a super hydrophobic substrate and letting the solvent slowly evaporate. In addition to conventional video imaging, we use small-angle X-ray scattering (SAXS) and space-resolved dynamic light scattering to probe the evolution of the microscopic structure and dynamics under compression. We find that the gel volume may be reduced by up to ~70% with no macroscopic failure, in sharp contrast to failure under a shear strain, which typically occurs at much lower strains. Surprisingly, for all volume fraction, the microscopic dynamics are close to the affine deformation field predicted for an elastic, homogeneous sphere, in spite of the irreversible nature of the rearrangements leading to compaction. SAXS data on the other hand suggest that the network structure compresses in a hierarchical manner.
Very different responses are measured during uniaxial compression. The mechanical response displays features akin to brittle-like or ductile-like behavior depending on the network volume fraction and on the compression rate, which we vary over more than three orders of magnitude. Beads of hard materials compressed at fast rates exhibit a brittle-like response and are observed to fracture abruptly producing distinct macroscopic pieces of seemingly intact gel. By contrast soft beads exhibit a ductile-like behavior and deform smoothly keeping their integrity during the all compression process. We will discuss the yielding of beads of colloidal gels as resulting from an interplay between material elasticity, plasticity and poroelasticity.