Liquid-Gel-Liquid Transition and Shear-Thickening in Mixed Suspensions of Silica Colloid and Hyperbranched Polyethyleneimine

The rheological property of mixed suspensions of silica colloid and hyperbranched polylethyleneimine was studied as functions of particle volume fraction, ratio of polymer to particle, and pH value. A mechanism of liquid-gel-liquid transition for this mixed system was proposed based on the amount and the conformation of polyelectrolyte bridges which were able to self-arrange with solution environments. The equilibrium adsorbed amount ($C_{\mathrm{p}}$*) for a given volume fraction of particles is an important concentration ratio of polymer to particle denoting the transition of irreversible and reversible bridging. For mixed suspensions at equilibrium adsorbed state ($C_{\mathrm{p}}\approx C_{\mathrm{p}}$*), the adsorption-desorption of polymer bridges on the particles can reversibly take place, and shear thickening is observed under a steady shear flow as a result of rapid extension of bridges when the relaxation time scale of extension is shorter than that of desorption.