Alginate biopolymer viscoelasticity from sol to gel: Linear and nonlinear rheology, and Brownian motion of tracer particles embedded in the polymer network

The linear and nonlinear rheological behavior of alginate/Ca2+ gels made through slow solubilization of CaCO3 using gluconic acid-δ-lactone was studied. Rheological modulus follows a power law at the critical point; exponents (n = 0.60–0.65) decrease slightly, increasing Ca2+ ions. These values agree with those reported in the literature with similar mannuronic/guluronic residue ratios and total polymer concentration. The strain-hardening behavior of matured gels was investigated using large amplitude oscillatory shear. The Blatz–Sharda–Tschoegl scaling model was used to estimate their fractal dimension, whose values were 1.23–1.31; the fractal dimension is not sensible to Ca2+ concentration. The Lissajous–Bowditch curves show a higher nonlinearity and constitute a rheological fingerprint of these gels. The geometrical decomposition of intracycle strain in terms of strain thickening and stiffening ratios shows a weak formation of temporary network junctions during the breaking process due to the convexity of their curves. Diffusion wave spectroscopy was used to determine the mean square displacement of microspheres embedded in polymer solution exhibiting a subdiffusive process, with two slopes: m ∼ 0.3 at short times and m ∼ 1 at long times. The diffusive region section decreases as the gelation progresses due to Ca2+ and disappears when gels are formed. Gelation time was estimated to identify a terminal relaxation time, whose evolution follows almost the same exponential curve for gels with high Ca2+ concentration due to the formation of many multiple egg-box structures. After 24 h of gelation, mean square displacement curves show an apparent plateau, indicating important particle confinement.