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 10⁵-10⁶ Pa s, was detected. After the yield point was exceeded, steady-state flow with a much lower viscosity, at the order of 3 Pa s, occurred, as is characteristic of conventional liquids containing a solid filler. Nevertheless, a small fraction of plastic deformation was still observed. Thus, the experimental results show that physical gels can behave as elastic-plastic media and that yielding liquids may flow below the yield point with very high viscosity. All existing models of the mechanical behavior of gels represent various combinations of viscous and elastic elements, to which, for yielding liquids, a slider is added that begins to slide after overcoming static friction. This element models the yield point. However, no model includes plasticity as an independent mechanical phenomenon. The phenomenon of plasticity should therefore be taken into account when developing rheological models of yielding liquids.