Reversible Control of Rheological Properties in Microgel Composites via Nanoparticle Aggregation

Microgels are soft materials with tunable rheological properties, making them useful for applications such as 3D printing, drug delivery, and coatings. However, balancing printability and structural stability remains a key challenge. In this study, to overcome this issue, we investigate nanoparticle aggregation as a reversible physical cross-linking mechanism in silica-Carbopol microgel composites. The surface charge of silica nanoparticles is pH-sensitive, resulting in aggregation at low pH and dispersion at high pH. This aggregation enhances rheological properties by increasing elasticity and yield stress, while dispersion reduces the rheological properties by allowing for easy flow. Using SAXS, NMR, and recovery rheology, we characterize these structural transitions and demonstrate that aggregation kinetics can be accelerated by tuning the nanoparticle size, temperature, and concentrations. This tunable cross-linking mechanism allows precise control over microgel behavior, enabling their use as recyclable direct-ink-writing printing inks. By using pH and temperature control, our approach provides a pathway to create microgel composites with reversible mechanical properties, opening new possibilities for advanced ink formulations and sustainable material applications.