Effects of electrostatic interaction on rheological behavior and microstructure of concentrated colloidal suspensions

This research examined the effect of salt concentration on the viscosity behavior of concentrated colloidal suspensions of monodispersed silica particles in aqueous media and ethylene glycol. Silica particles with a lot of negative surface charge form three-dimensional ordered structures ( colloidal crystals) in aqueous media or ethylene glycol due to electrostatic repulsion. These three-dimensional structures are distorted by shear, causing viscosity to decrease and shear thinning to occur. Adding salt increases the ionic strength within the suspension, thereby reducing the Debye length of the particle and weakening the electrostatic repulsion between particles. As a result, the viscosity at low shear rates and the shear thinning gradient decreases. However, if the salt concentration increases further, the ionic strength within the suspension will rise, virtually eliminating the Debye length of the particle. In this event, the electrostatic repulsion between the particles becomes extremely weak, and the microstructures will be formed in the aqueous media by hydrophobic interaction and van der Waals force between the particles. As a result, the viscosity at low shear rates and the shear thinning gradient becomes extremely large. In contrast, the behavior in an ethylene glycol differs from behavior in an aqueous media. In this case, the microstructures are not formed by hydrophobic interaction. Consequently, collisions between particles are promoted, causing shear thickening. This phenomenon provides further support to the hypothesis that shear thickening is caused by collisions between particles and suggests that shear thickening can be avoided by strengthening the electrostatic interaction between particles.