Shear thickening-shear thinning and shear thinning-shear thickening transitions in polymeric systems

This review explores the complex rheological behaviors of hydrogels, specifically focusing on shear thinning-shear thickening (PD) and the less common shear thickening-shear thinning (DP) transitions. While shear thinning (viscosity decrease with increasing shear rate) is widely utilized in applications like 3D printing and injectables, and shear thickening (viscosity increase) is observed in concentrated suspensions, the transitions between these states are critical for advanced material design. The PD transition, often occurring at higher shear rates, is attributed to mechanisms such as hydrodynamic clustering, frictional contacts, and network rearrangement. In contrast, the DP transition, particularly noted in chitosan hydrogels, manifests at very low shear rates. This low-shear DP behavior, characterized by an initial viscosity increase followed by a decrease, is proposed to stem from a competition between temporary shear-induced associations and subsequent structure breakdown or chain alignment. The precise molecular mechanisms for DP transitions remain largely unelucidated and require further experimental validation. Understanding and controlling these distinct rheological transitions are crucial for rationally designing tunable hydrogels with tailored flow properties for diverse applications in biomedical engineering, drug delivery, tissue regeneration, food products, and cosmetics.