Understanding Fumed Silica Suspension Rheology: The Effect of Shear History on Particle Structure and Rheology

Dennis Kalman1, Tarig Hassan2, Donald Kessler III1, Vijaya K Rangari2, and Norman J. Wagner1. (1) Department of Chemical Engineering, University of Delaware, 150 Academy Street, Newark, DE 19711, (2) Tuskegee University Center for Advanced Materials (T-CAM), Tuskegee, AL 36088

Concentrated colloidal suspensions undergo many types of complicated flow behavior; including shear thinning, shear thickening, yielding, and thixotropy. Fumed silica is a commonly used colloidal additive in paints and coatings as a thickening agent; its open structure of fused primary particles provides significant structuring of the suspension at relatively low particle concentrations. The resulting suspension viscosity is often sensitive to the shear history. Many studies investigating the rheology of these fumed silica suspensions exist; however, they typically only attempt to measure or characterize the rheology based on a specific method of incorporating the dry particles into a particular solvent yielding a specific shear history. In fact, a wide range of suspension properties are available based on the method in which these suspensions are prepared. Here, we attempt to show the wide variation in rheological properties of these suspensions based on the method of preparation, and compare that to measures of the particle size, shape, and structure within the suspension. We investigate suspensions from a minimum contact method of preparation done by using a non-contact centrifugal mixer. These suspensions are very thixotropic gels that break down under any shear. Hand mixing provides a higher amount of shear that allows these suspensions to shear-thicken at extraordinarily low volume fractions, below 12%, but typically are fluids at rest. High shear from a blender allows much higher quantities of the particles to be incorporated into the suspending media while eliminating most of the thixotropy. When small quantities of the unblended material are added back to this suspension, large changes in the rheology are seen and the thixotropy returns. These rheological measurements are understood in terms of processing and flow-induced microstructural changes as detected by measurements of particle size and microstructure, both at rest and under flow. This understanding can lead to better formulation of dispersions with desired rheological behavior.