Synergistic Stabilization and Rheological Regulation of Fluorine-Free Foams Stabilized by Silica Nanoparticles and Siloxane-Hydrocarbon Surfactant Mixtures.

Nanoparticle (NP)-stabilized fluorine-free foams, as a sustainable alternative to traditional aqueous film-forming foams (AFFF), rely on foam stability and rheological properties for their practical effectiveness. NP concentration governs microscopic structural evolution and flow behavior, thereby determining foam stability as well as the spreading and coverage performance on fuel surfaces. This study systematically investigates the rheological properties and stabilization mechanisms of fluorine-free foams costabilized by hydrophilic silica NPs and a 648/BS-12 binary surfactant system. The NP-stabilized foams exhibit distinct non-Newtonian shear-thinning behavior. With increasing NP concentration, the system is proposed to undergo three characteristic stages: adsorption mode transition, rheological percolation, and stable jamming. The adsorption mode transition at 1.5 wt % NP is suggested by simultaneous anomalies in surface tension and electrical conductivity. At 1.75 wt %, the system is interpreted as undergoing a fundamental liquid-to-solid transition, marked by a pronounced stress overshoot, and the solid-like features become more pronounced at higher concentrations. At 2.0 wt %, a three-dimensional network constructed by NP-surfactant aggregates is proposed to effectively suppress foam drainage and bubble coarsening, as independently supported by time-resolved optical microscopy. These findings provide a theoretical basis and quantitative design criteria for high-performance fluorine-free firefighting foams based on nanoparticle-binary surfactant systems.