Complex fluids, Colloidal sciences

This paper adopts a previously developed activation model of shear thickening, published by the authors to sterically stabilized colloidal suspensions. When particles arranged along the compression axis of a sheared suspension, they may overcome the repulsive interaction and form hydroclusters associated with shear thickening. Taking advantage of the total interaction potential of polymeric brush coating and van der Waals attraction, the applicability of the activation model is shown within the validity range of a continuum theory. For the comparison with an extensive experimental investigation, where some parameters are not available, the onset of shear thickening can be predicted with realistic assumptions of the model parameters.

We have studied the Saffman-Taylor instability for Laponite and mud. Our experiments showed several kinds of finger patterns which don't obey the classical theory of Saffman-Taylor instability. Our observations, on both finger patterns and rheology analyses results showed shear thinning behavior in high shear rates and shear thickening behavior in low shear rates for Laponite. This showed that the shear viscosity diverges at a point in which the shear viscosity has a bifurcation, this point has been identified. This result is in good agreement with the MRI rheology analysis results by Coussot et.al . This is one of the important reasons that make Laponite such a complex fluid. We tested Darcy's Law for Laponite, we verified that the generalized Darcy's Law, suggested by Bonn et.al is in good agreement with rheology analyses results in the shear thinning region of Laponite. Calculating the shear viscosity as a function of shear rate using generalized Darcy's Law on the fingers movements showed good agreement with rheology analyses results. We also tested Mud; the finger patterns and Darcy's Law showed Newtonian behavior for this suspension. We also tested Darcy's Law for three Newtonian Fluids, detergent, oil and clothes conditioner, All three fluids resulted in a linear relationship between the fluids velocity and the pressure gradient in different rates of shear rates and pressure gradient. This finding supports the applicability of Darcy's Law in distinguishing Newtonian and non-Newtonian behavior in Fluids.

A theory is presented for the onset of shear thickening in colloidal suspensions of particles, stabilized by an electrostatic repulsion. Based on an activation model a critical shear stress can be derived for the onset of shear thickening in dense suspensions for a constant potential and a constant charge approach of the spheres. Unlike previous models the total interaction potential is taken into account (sum of attraction and repulsion). The critical shear stress is related to the maximum of the total interaction potential scaled by the free volume per particle. A comparison with experimental investigations shows the applicability of the theory.

The authors previously introduced an activation model for the onset of shear thickening in electrically stabilized colloidal suspensions. It predicts that shear thickening occurs, when particles arranged along the compression axis in a sheared suspension do overcome the electrostatic repulsion at a critical shear stress, and are captured in the primary minimum of the DLVO interaction potential. A comparison with an experimental investigation on non-aqueous silica suspensions, carried out by Maranzano and Wagner, is performed. For particle systems that fall into the applicability range of the theory, a good coincidence between the experimental data and the model predictions can be found.