An alternative route to highly concentrated, freely flowing colloidal dispersions

Dense colloidal dispersions exhibit fluid states due to weak attractive interactions among particles even at particle volume fractions far above the colloidal glass transition. Here we demonstrate that this opens up a new route to manufacture highly concentrated, freely flowing dispersions. We have studied the rheological properties of two model dispersions in the dense, fluid state: PS-microgel particles suspended in an isorefractive organic solvent allowing for light scattering experiments and an aqueous polymer latex dispersion with short range repulsive interactions based on a commercial polymer latex system. Both systems essentially behave like hard spheres, their zero-shear viscosity diverges at a volume fraction phi = 0.58, linear viscoelastic behavior is well-described by the mode coupling theory and the absolute values of the plateau moduli are close to those reported for other hard sphere systems. Fluidization was achieved by introducing weak depletion attraction among particles via addition of non-adsorbing polymers to the continuous phase. Fluid states were observed up to phi approximate to 0.69 for the microgel and phi approximate to 0.644 for the aqueous system. At a given particle loading a minimum viscosity was achieved at polymer concentrations below the overlap concentration c(p)*. For the aqueous dispersion fluidization was observed for a broad range of polymer molecular weights M-w and the respective viscosity minimum did not vary systematically with Mw. The low viscosity values thus achieved for nearly monomodal systems could so far only be obtained for dispersions with broad multimodal particle size distribution, demonstrating the competitive nature of the new concept.