Yielding mechanisms in binary suspensions of colloidal Mg(OH)₂ and spherical SiO₂ particles

The yield stress of magnesium hydroxide (Mg(OH)₂) suspensions can be significantly reduced by adding a low vol% of spherical nano silica to the suspension. The nano silica acts to lubricate the contacts between the irregularly shaped Mg(OH)₂ by interrupting the aggregated structure and acting akin to a ball-bearing, which allows particles to rotate past one another with little resistance. The dependence on nanoparticle size (100, 250, 500 and 800 nm) and concentration (blending ratios Mg(OH)₂:SiO₂– 27:3, 28:2, 29:1 (vol%:vol%)) was found to be directly influenced by the dispersion (number of dispersed particles) and distribution of particles throughout the Mg(OH)₂ network. Creep rheology identified a multi-step yielding process to transition from an abrupt, brittle yield in Mg(OH)₂ suspensions to a more gradual, softer yield with SiO₂. Large amplitude oscillatory shear (LAOS) revealed variations in intracycle strain stiffening and energy dissipation, with 500 nm SiO₂ promoting the most effective disruption of the Mg(OH)₂ network. Microstructural analysis via SEM and X-ray computed tomography confirmed that 500 nm SiO₂ achieved optimal dispersion and distribution due to minimal clustering. These results highlight the critical relationship between nanoparticle size, dispersion, and number ratio between large and small particles in tuning the rheology of concentrated suspensions, offering new insights for the mechanisms of yield stress modification in binary particle systems.