(463h) Altering Shear Thickening in Kaolin Clay + Silica Nanoparticle Mixtures

Aqueous dispersions of kaolin clay demonstrate distinctive flow behaviors influenced by factors including particle morphology, surface chemistry, and concentration. Understanding the rheology of these dispersions is important across various applications from cosmetics to ceramics. We investigated the effects of concentration, pH, and particle size on the shear thickening in concentrated (60 to 70 wt.%) kaolin dispersions and kaolin mixtures with silica nanoparticles.

The rheological behavior of aqueous kaolin clay dispersions and mixtures with silica nanoparticles manifests several notable rheological responses. Concentrated kaolin dispersions (60 to 70 wt.%) exhibit two types of shear thickening: continuous and discontinuous. The onset shear rate of continuous shear thickening varies with concentration, occurring at lower rates for higher kaolin concentrations. In addition, pH influences the rheological response of kaolin dispersions. For example, raising the pH from 6 to 10 increases the discontinuous thickening shear rate from 61,000 to 85,000 s^-1 for 63 wt.% kaolin dispersion. So, higher pH reduced the tendency for shear thickening, indicating the role of electrostatic repulsion in tuning colloidal interactions.

Binary mixtures of kaolin clay and spherical silica were prepared to assess the impact of adding spherical particles on shear thickening. The addition of 22 nm spherical silica particles alters the rheological response of kaolin dispersions, affecting both continuous and discontinuous shear thickening behaviors. At pH 10, the onset of continuous thickening initially increases with the addition of 22 nm spheres up to 3 wt.%, followed by a decrease from 4 to 6 wt.%. However, the onset of discontinuous thickening increases with the addition of more silica particles. Discontinuous thickening was eliminated at 4 wt.% and above when adding 22 nm silica spheres (i.e., was not measurable up to 180,000 s^-1). Larger spherical silica particles (112 nm diameter) exhibited minimal effects on the shear thickening of clay dispersions and did not eliminate discontinuous shear thickening. Also, the addition of spherical silica nanoparticles exhibited pH-dependent effects on shear thickening, with spherical silicas more effective in increasing the onset of shear thickening at higher pH levels.

This study addresses gaps in the literature regarding shear thickening in concentrated kaolin dispersions and offers opportunities for tailoring rheological properties through pH adjustment and mixing particles with different shapes and sizes to meet specific application requirements. Employing higher solids concentrations to mitigate or shift shear thickening to higher shear rates may seem counterintuitive but holds potential for diverse industrial applications