Despite extensive research on the rheology of colloidal gels, the role of interparticle interactions in adjusting the linear and nonlinear viscoelastic properties of colloidal gels comprising a spherical and a non-spherical building block has remained unexplored. Here, we report the stress relaxation and yielding behavior of the colloidal gels based on the charge-driven assembly between
spherical gelatin nanoparticles (GelNPs) and
two-dimensional (2D) clay nanosheets. The stress relaxation behavior of the GelNP/2D clay hybrid colloidal gels at a solid content of 5wt.% and a 2D clay/GelNP weight ratio of 3 was investigated by applying a wide range of strains in the sequential steps. Our results show that compared to the colloidal gels based on oppositely charged GelNPs or GelNP/spherical silica nanoparticles, GelNP/2D clay gels exhibit significantly higher elasticity and yield stress. To determine how the relative size of the spherical and 2D building blocks affect the relaxation and yielding behavior, four different 2D clays with a comparable thickness of about 1 nm and various lateral sizes in the range of 30-3500 nm were used. The average size of GelNPs was kept at about 400 nm. Increasing the applied strain in nonlinear rheology measurements accelerates the stress relaxation of all the samples due to exceeding the critical fluidization strain, which varies between 8 to 21%, by decreasing the size of the clay nanosheets. As a result, the colloidal gels composed of larger 2D clay nanosheets can relax and reduce stress buildup in a shorter time. However, the relative size of the building blocks does not affect the maximum stress tolerated by the gels.
Additionally, based on the calculated stress vs. strain isochrones, GelNP/2D clay colloidal gels exhibit a yielding strain in the range of 0.6 to 3.6%. Our morphological studies reveal that the observed yielding is due to the bond breakage between the clusters while the individual building blocks are still connected. The colloidal gel containing montmorillonite, with a size comparable to GelNPs, showed the highest elasticity, yield strain, and relaxation modulus, confirming a stronger connection and more compact network. These results can provide insight into the nonlinear mechanical properties of colloidal gels comprising anisotropic nanoparticles that facilitate tuning the shear thinning behavior and engineering colloidal inks for 4D bioprinting applications.
