Nonlinear rheological behavior of gelatin gels: In situ gels and individual gel layers filled with hard particles

Hypothesis: Previously, we examined the impact of two preparation procedures on the mechanical properties of native gelatin gels. In this work, we extend our research by considering hard spherical particles as fillers. We expect that the presence of these fillers significantly affects the viscoelastic response.

Experiments: We prepared fresh in situ gels and individual gel layers filled with glass micro-beads up to 1% w/w and aged for 24 and 48 h. The hydrogels were made of 3 and 5% w/w gelatin at 5 °C, and we performed large amplitude oscillatory shear (LAOS) tests. We analyzed the intracycle linear and nonlinear response using normalized Lissajous-Bowditch curves. Utilizing the MITlaos software, we decomposed the total intracycle stress into elastic and viscous contributions and calculated the Chebyshev harmonics coefficients.

Findings: The fresh in situ gels exhibit severe progressive stiffening during the strain sweeps and a subsequent sharp decrease of both moduli. The filled layers show smoother yielding than the in situ gels. The fillers increase the dynamic moduli, affect the terminal LAOS regime, and enhance the intracycle nonlinearities at higher concentrations. The Lissajous-Bowditch curves of the aged layers indicate elastoplastic behavior, which is more pronounced for the 48 h filled gel layer than the native counterpart.