Rheology modification in mixed shape colloidal dispersions. Part I: pure components

The flow behaviour and rheology of colloidal dispersions are of considerable interest in many applications, for example colloidal clay particles find applications in oilfield and construction-drilling fluids. The rheological properties of such fluids can be enhanced significantly by adding colloidal particles of different size and shape. To gain insight into the mechanism of this phenomenon, we have studied model mineral-colloid systems whose shape changes systematically from a plate-like aluminasol (gibbsite), through a lath-like smectite clay (hectorite), to a rod-like aluminasol (boehmite). The paper presents the results of a systematic and comprehensive multi-technique study (oscillatory, transient and steady shear) of the rheology of dispersions of these model systems. This gives a detailed account of the ` yield space' that characterises the complex transition of these soft materials from elastoviscous gels to viscoelastic liquids, and of the effect of particle size and shape on this behaviour. The observed phenomena are underpinned by two competing flow-mediated microstructural rearrangements that have significantly different timescales. A physical model invoking flow-mediated building and disruption of fluid structure is described to rationalise the observed behaviour. The study also forms the baseline to a companion study (part II), which investigates the rheological behaviour of mixed anisometric colloid systems based on these pure components.