Mechanical response and yielding transition of silk-fibroin and silk-fibroin/cellulose nanocrystals composite gels

We investigated the mechanical properties of gels formed by silk fibroin and silk fibroin/cellulose nanocrystal mixtures. Viscosity and linear viscoelasticity measurements showed that the addition of cellulose nanocrystals induces a speed-up of the gelation kinetics and leads to moderately stiffer gels. Combining continuous shear tests and large amplitude oscillatory shear (LAOS) measurements we additionally characterized the shear-induced solid-fluid transition. Continuous shear tests indicated a larger yield stress for the silk fibroin/cellulose nanocrystal gel, confirming thus the stiffening effect. They also revealed that the yield strain of the gels becomes smaller at higher shear rates, possibly as a result of shear-induced compaction. The detailed analysis of the full anharmonic response in LAOS measurements additionally evidenced that the fluidization of the gels is a broad and gradual transition, which is characterized by two different yield strains: one corresponding to the onset of solid melting and one to the complete fluidization of the system.