Plasticity and the Transition from Physical Gels to Yielding Liquids.

This study examines the possibility of plastic deformation in low-modulus gels and yielding liquids. The model systems were a gelatin-sodium alginate hydrogel and the same gel filled with nanoscale zinc oxide particles. The experiments involved short- and long-term observations of deformation development under a prescribed shear stress, followed by stress removal and monitoring of deformation recovery. The initial hydrogel is a typical soft-matter system with an elastic modulus of 63 Pa. At low stresses, residual deformations were observed in addition to elastic deformations; these residual deformations reached up to approximately one half of the total deformation. They appeared instantaneously, depended on the applied stress, and did not change during long-term observation. This behavior is characteristic of plastic deformation. The incorporation of 5% dispersed ZnO nanoparticles converted the gel into a yielding liquid. This transition is attributed to partial disruption of the physical network, as evidenced by a sharp decrease in the elastic modulus to 18 Pa and by comparison of the FTIR spectra of the unfilled gel and the nanoparticle-modified gel. The yield stress of the yielding liquid was 7 Pa. However, at stresses below this value, while the material remained in a gel-like state, steady-state flow with a very high viscosity, at the order of 10