YIELDING PHENOMENA IN MICROGEL SUSPENSIONS ACROSS LENGTH SCALES

Most materials encountered in everyday life, such as food and personal care products, are complex fluids with intricate mechanical properties, blending viscous and elastic behaviors, and exhibiting different responses depending on the timescale or applied stress. These properties significantly impact practical functionalities and user experiences, such as spreadability, sensory perception, active ingredient release, and long-term stability. Yield stress fluids (YSF) specifically behave like solids under low stress but flow like liquids once a critical threshold—the yield stress—is exceeded. Fundamental aspects of this yielding transition, including structural and dynamic precursors, reversibility, local-scale phenomena, and the role of microstructure, remain active areas of investigation. Typically, YSFs consist of network structures, such as fiber gels or colloidal particle gels, or densely packed microgels—microscopic, swollen, cross-linked polymer particles. Understanding the connections among structure, microscopic behaviors, and macroscopic properties is critical for developing advanced and sustainable materials. During my PhD research, I addressed key questions spanning various length scales: • How do molecular-level changes, such as variations in solvation, influence the microscopic behavior of individual microgels and the macroscopic mechanical properties of microgel packings? • How does yield stress, typically measured at the bulk scale, affect microscopic behavior, such as particle trapping or motion? • How do mesoscopic heterogeneities influence the macroscopic response? To address these questions, I primarily investigated Carbopol—a model microgel system—across different length scales, varying its swelling by adjusting pH or solvents. Microgel rheology in aqueous and non-aqueous solvents: By tuning pH, we investigated how microgel swelling affects their effective volume fraction and yield stress. Maximum swelling typically occurs around neutral pH in water-based systems. We extended our analysis to non-aqueous solvents, demonstrating that polar solvents are generally better for microgel dispersion. Adjustments of swelling via strong bases like NaOH enabled us to obtain homogeneous, transparent YSF in multiple non-aqueous solvents. The maximum swelling correlates with peak yield stress at fixed concentrations. Equilibrating samples against pure solvents or osmotic pressures allowed comprehensive characterization of yield stress and viscoelastic properties across solvents, revealing critical concentration thresholds and yield stress values related to swelling and aggregation. Trapping of solid inclusions in yield stress fluids: We studied the stable entrapment of particles in YSF, examining how particle size, density, and the material’s microstructure affect equilibrium between yield and gravitational stresses. Through centrifugation experiments, we assessed particle stability across different Carbopol conditions. Critical gravitational stresses scaled with the bulk yield stress of the matrix, while transient dynamics were explored using optical-access centrifugation to monitor the transition to equilibrium. Large field-of-view heterogeneity maps: The bulk rheological behavior of microgel suspensions is highly sensitive to particle clustering and sample preparation. A novel approach using low-magnification microscopy was developed to quantify structural heterogeneity up to millimeter scales. By tracking tracer particle motion, we identified regions with arrested, diffusive, or flowing dynamics, correlating these results to preparation protocols at laboratory and industrial scales. Motion of yield stress fluid droplets on lubricated surfaces: YSF droplets exhibit limited mobility on standard surfaces but move efficiently on lubricated surfaces. We studied Carbopol droplets on lubricant-infused substrates, identifying regimes of sliding and rolling based on droplet concentration and surface inclination. A simple criterion based on the ratio between yield and gravitational stresses distinguished sliding from rolling behavior, relevant for controlling YSF droplets in microfluidic and biological contexts. This work resulted in a publication in the Journal of Colloid and Interface Science. Complementary research: Rheo-microscopy on viscoelastic surfactant solutions: Parallel studies were conducted on wormlike micellar solutions, examining nonlinear rheological responses and shear banding phenomena under shear flow using microscopy coupled with rheology. This research, performed in collaboration with Università di Napoli "Federico II," contributed insights into surfactant microstructural dynamics under shear conditions, with implications for pharmaceutical and industrial applications.