Rheology and microstructure of dense suspensions of plate-shaped colloidal particles

The flow properties and microstructure of dense kaolin clay suspensions are explored for volume fractions, phi, as large as 0.39. To avoid flocculation these particles were suspended in pH 10 buffer solutions where edge-face interactions are negligible. These plate-shaped particles have an aspect ratio of 10-12 and, thus, are likely to show alignment above a critical volume fraction phi* > 0.10. At low phi, the suspensions are Newtonian but show substantial orientation with increased shear rate. As phi is increased, the suspensions develop a yielding type of behavior. In the concentrated region phi > phi*, elastic moduli are a function of the previously applied shear rate, (gamma) over dot, decreasing from a low shear rate plateau of G'(0) to a high shear rate plateau of G'(infinity). We interpret the modulus behavior as being a consequence of changes in alignment of domains in the suspension. Conductivity and x-ray scattering measurements confirm that particle alignment increases with increasing shear rate. The normalized modulus value, G'(norm)= [G'((gamma) over dot) - G'(infinity)]/[G'(0)- G'(infinity)], is independent of phi indicating that the characteristic alignment shear rate is independent of volume fraction. Conductivities and x-ray scattering intensities normalized in the same manner are also independent of the volume fraction, and superimpose on the modulus data. Surprisingly, these data indicate that fractional particle alignment at a given shear rate is independent of phi for 0.05 < phi < 0.39. (C) 1999 The Society of Rheology. [S0148-6055(99)00503-9].