College Park, Maryland      June 6 - 10 , 2004

M4-D5 (5:00 PM): Effect of Attractions on the Microstructure and Rheology of Dense Colloidal Suspensions and Gels

V. Gopalakrishnan, S. Ramakrishnan (Department of Chemical and Biomolecular Engineering), K. Schweizer (Department of Chemical and Biomolecular Engineering; Department of Materiasl Science and Engineering), C. Zukoski (Department of Chemical and Biomolecular Engineering)

Colloidal suspensions and gels are used in a number of technologically important applications inks, paints and ceramics to name a few. Control of the microstructure of the suspension and the resulting flow properties is of utmost importance to achieving the desired properties in these applications. Addition of non-adsorbing polymer to a colloidal suspension (particles of size R) provides one with the ability to tune the microstructure and flow properties of the suspension with two independent variables the molecular weight of the added polymer (radius of gyration Rg) and concentration of the added polymer cp. The non-adsorbing polymer induces attractions between the colloidal particles the range of which is controlled by Rg/R while the strength of attractions is controlled by cp. For Rg/R values less than 0.1, the addition of increasing amounts of polymer causes the suspensions to gel thus drastically changing the flow properties at the gel point (divergent zero shear viscosity and appearance of a nonzero value of the elastic modulus). In this work, we systematically characterize the microstructure (SANS and USAXS) and flow properties of the colloidal silica suspensions (suspended in decalin) with the addition of polystyrene. The size ratio Rg/R used in this work is 0.078 and 0.053. There are two regions which we focus on (1) The equilibrium fluid : the polymer concentration is varied from dilute till the point where the suspensions gel and (2) The gel region: the polymer and colloid concentrations are varied over a wide range to give rise to substantial changes in flow properties. Our goal is to link the mechanical properties to microstructures through the appropriate models.

Here we report on decreases in the shear rate required to produce hydroclusters as the strength of attraction is increased. Hydroclusters are important in the high shear region where shear thickening is observed. With increasing strengths of attraction these clusters form at increasing smaller shear rates but shear thickening is lost. This is understood in terms of the shear rate dependencies of the hydrodynamic and thermodynamic contributions to the stress.

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