Modeling the rheology of gelation using recovery rheology

Gels are materials that continue to be heavily used in the food industry, biological applications, numerous commercial products, as well as other applications. The process of forming polymer gels often involves crosslinking polymers so that the bulk undergoes a phase transition from liquid to solid at a critical extent of reaction. Typical mechanisms that initiate gelation include chemical reactions induced by exposure of UV or elevated temperature, physical reactions, ionic complexation, and addition of particles. Given the wide usage of gelation for product design, there are limited studies in developing rheological models to understand the material responses during gelation under different external stimuli. Moreover, the current understanding of gelation fails to explain interesting experimental observations such as the existence of an overshoot in the loss modulus. In the present work, we construct a phenomenological model of the rheology of gelation based on recovery rheology and demonstrate its generality on systems that gel via a variety of mechanisms. The model accounts for many qualitative features and is quantitatively accurate for some materials. We show valuable insights in understanding gelation from decomposing traditional dynamic moduli into elastic (recoverable) and viscous (unrecoverable) contributions. We believe the new understanding can be further used to design and understand manufacturing processes and engineer novel formulations.