Rate dependence in adhesive particle–particle contacts affect ceramic suspension bulk flow behavior

Particle–fluid mixtures are known for displaying a wide variety of interesting flow mechanisms, such as thickening and thinning under shear. It is now becoming increasingly evident that even molecular scale effects are relevant for this macroscopic flow behavior of suspensions. Here we show that even 50 μm ceramic beads in a non-density matched solvent are affected by the pH of the solvent. We trace the origin of the pH dependence to the contact mechanics using colloidal probe atomic force microscopy (CP-AFM). The gel-like structure of the pH 3 samples suggests that adhesion effects play a major role, but which adhesion timescale competes with shear rate to affect the rheology is not obvious. We test here two time scales in the contact mechanics: total contact time t_c and contact retraction speed v_r. We observe that contact time is the most important variable to correlate with rheology. The ceramic particle suspension so serves as a model system to help understand which adhesive mechanism affects flow behavior in the suspension.