Concentration-dependent rheological behavior in fumed silica suspensions in mineral oil

Fumed silica suspensions in apolar solvents serve as model systems for investigating the influence of solvent polarity and surface chemistry on colloidal interactions and rheological behavior. However, systematic studies in mineral oils remain limited. This study characterizes the rheology of hydrophilic fumed silica suspensions in mineral oil at concentrations from 0.25 to 2.5% v/v using steady shear, oscillatory, creep, and time-dependent protocols, complemented by ex situ Fourier-transform infrared (FTIR) spectroscopy of quiescent samples. The results reveal a concentration-dependent evolution of mechanical behavior. At 0.5 vol. % or lower, suspensions exhibit predominantly viscous responses with rapid relaxation. Between 1.0 and 1.5 vol. %, weakly percolated structures emerge, associated with the onset of yield stress, elastic response, and delayed fluidization under creep. At 2.0 vol. % and above, increasingly structured systems develop, characterized by enhanced elasticity, higher yield stress, and more pronounced time-dependent responses. At high shear rates, a viscosity upturn is observed for the most concentrated suspensions, indicating shear-thickening behavior. Time-dependent protocols show that structural evolution becomes increasingly significant with concentration; however, observed responses cannot be unambiguously classified from individual rheological tests alone. Step-shear and hysteresis experiments reveal that viscoelastic relaxation and structural dynamics may produce similar signatures depending on the imposed timescale. FTIR measurements support this interpretation, revealing progressive changes in surface chemistry—including reduction of silanol groups and increased Si–O–Si connectivity—consistent with enhanced interparticle interactions. Overall, this study establishes a concentration-resolved map of rheological behavior in fumed silica–mineral oil suspensions, demonstrating that time-dependent restructuring is governed by the competition between viscoelastic relaxation and structural dynamics and cannot be unambiguously attributed to thixotropy or antithixotropy based on conventional protocols alone.