Concentration-Dependent Nonlinear Rheology of Agar Hydrogels

Despite decades of research, the nonlinear mechanics of agar remains poorly understood. In this work, we analyze the mechanical response of soft, hard, and very hard agar hydrogels under nonlinear shear deformation. Low-shear viscoelastic behavior across concentrations was characterized using storage and loss moduli, yield strain, flow point, loss factor, and ductility index. The transition to nonlinear response was examined using Fourier analysis of shear stress signals. To describe the high-shear regime, we employed large-amplitude oscillatory shear (LAOS) rheology. The mechanical response was further analyzed using Lissajous-Bowditch plots (stress versus strain and stress versus shear rate), linking agar network structure to intracycle deformation behavior and energy dissipation. By analyzing strain stiffening, shear thickening, yielding, and intracycle structural dynamics, we quantified dissipation rates across concentrations and constructed fingerprint maps of shear stiffening and thickening at different frequencies. Microstructural insights from rheology were compared with macroscopic characterization using phase-contrast microscopy. The nonlinear rheological analysis revealed that structural reorganization shifts systematically toward lower shear strain values with increasing agar concentration.