THE RHEOLOGY OF AQUEOUS DISPERSIONS OF SPINDLE-TYPE COLLOIDAL HEMATITE RODS

The rheology of aqueous dispersions of colloidal hematite (alpha-Fe2O3 ) rods with well-characterized shape and dimension was investigated. T he effects of volume fraction (phi), aspect ratio, and Debye length (k appa(0)(-1)) on the rheology were studied in particular. ?he particles were spindle-type bodies with aspect ratios equal to 8.4 and 4.8. The y were dispersed in aqueous solutions of known ionic strength at a zet a potential, which was characterized in the Smoluchowski limit. Their rheology was compared to that of spherical hematite prepared by the me thod of Matijevic (1985). Dilute solution viscometry indicated that th e effects of kappa(0)(-1) and the aspect ratio on the O(phi(2)) contri bution to the low-shear effective viscosity were of comparable magnitu de, and that the Huggins coefficient was a decreasing function of the aspect ratio. For concentrated suspensions, the dimensionless shear th inning was adequately fit by a correlation involving a critical stress , which was originally developed for spherical suspensions [Krieger an d Dougherty (1959)]. The volume fraction dependence of the zero-shear viscosity eta(0) was found to be a strong function of both the aspect ratio and kappa(0)(-1). The aspect ratio affected both the volume frac tion at which eta(0) diverged (phi(m)), and the strength of the singul ar behavior [characterized by alpha in eta(0) /mu = (1 - phi/phi(m))(- alpha)]. In contrast, kappa(0)(-1) affected phi(m), but not alpha. The effect of the aspect ratio and volume fraction on the linear viscoela sticity of suspensions for phi > phi(m) was quantified. The results hi ghlight the particular ways in which the colloidal suspension rheology of moderate aspect ratio rods differs from that of spheres. (C) 1998 The Society of Rheology.