Mechanics of aggregated alumina suspensions: Behavior under shear and compression

The mechanical properties of aggregated suspensions significantly impact the processing and handling of solids in a wide range of industries. Despite the importance of these properties, however, the ability to control and manipulate such mechanical responses remains in a poorly developed state. The response of aqueous alumina suspensions to shear and compressive loads are investigated, with emphasis on the role of the strength of the interparticle attractions and on the effects of microstructural variations.;The constitutive relationship used to describe the compressive response of an aggregated system embodies the concept of a compressive yield stress, Py, which is a strong function of both the concentration of solids and of the particle attractions. The dependence on microstructure is probed, and a semi-quantitative model is used to clearly show that the presence of large heterogeneities in the microstructure significantly reduce the network's compressive strength. The dependence of both the shear and the compressive properties of the aggregated alumina suspensions studied is shown to be well described by scaling the data on the gelation volume fraction, &phis; g.;A series of experiments performed in a novel shear cell clearly demonstrate that the presence of a combined load alters the mechanical response for an aggregated suspension. Imposing a shear field on the particle network reduces the compressive strength of the network. The magnitude of this reduction in compressive strength is controlled by the relationship of the applied shear rate to a critical shear rate, g&d2;, which is a material property of the suspension. Similarly the presence of an applied normal load increases the measured dynamic yield stress. However, the magnitude of this increase is very small, suggesting that Coulombic friction plays a very minor role in the mechanical response of such aggregated systems. This study has wide-ranging implications for the design and control of solids handling processes which are ubiquitous in modern chemical and ceramics industries.