Steady shear rheology of dilute polystyrene particle gels

This paper describes an experimental study on dispersions of monodisperse polystyrene (PS) spheres with a typical radius of 1 mu m, dispersed in an electrolyte at high ionic strength, screening the electrostatic repulsion. These suspensions gelate at rest even at low volume fractions of PS particles. The density of the particles is matched with the solvent by using deuterium oxide for volume fractions phi less than or equal to 0.117. Steady-state flow curves, viscosity as a function of shear rate, are measured and reported for 0.014 < phi < 0.322. The measured flow curves are analyzed on the basis of two models: 1. In the giant flee model (van Diemen and Stein 1983, 1984; Schreuder et al. 1986, 1987; Laven et al. 1988), at low shear rates, the shear is not distributed homogeneously but is limited to certain shear planes; the energy dissipation during steady flow is due primarily to overcoming the viscous drag on the suspended particles during motion caused by encounters of particles in the shear planes. Though this model was developed for higher solid volume fractions (0.35-0.425), we found that it also describes the the rheology of dilute particle gels for 0.15 less than or equal to phi less than or equal to 0.3, using the same values for the parameters in the model as in the high solid volume fraction region. For phi < 0.15, the model also describes the data if the fraction of distance by which a moving particle entrains its neighbors, is assumed to increase in this phi region. 2. The model of de Rooij (de Rooij et al. 1993, 1994) considers aggregates in shear flow to be monodisperse impermeable spheres with a fractal structure. The permeability is taken into account by considering a hydrodynamic radius smaller than the gyration radius in the Krieger-Dougherty expression for the hydrodynamic contribution to the viscosity. Through the use of a yield criterion the aggregate radius is modeled as a function of shear rate. We found that the model describes our experimental results, with a combination of parameter values used already by de Rooij, but only for phi < 0.15.