Rheology and microstructure of dense suspensions of plate-shaped colloidalparticles

The flow properties and microstructure of dense kaolin clay suspensions are explored for volume fractions, phi, as large as 0.39. To avoid flocculatio n these particles were suspended in pH 10 buffer solutions where edge-face interactions are negligible. These plate-shaped particles have an aspect ra tio of 10-12 and, thus, are likely to show alignment above a critical volum e 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 regio n 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 beha vior as being a consequence of changes in alignment of domains in the suspe nsion. Conductivity and x-ray scattering measurements confirm that particle alignment increases with increasing shear rate. The normalized modulus val ue, G'(norm)= [G'((gamma) over dot) - G'(infinity)]/[G'(0)- G'(infinity)], is independent of phi indicating that the characteristic alignment shear ra te is independent of volume fraction. Conductivities and x-ray scattering i ntensities normalized in the same manner are also independent of the volume fraction, and superimpose on the modulus data. Surprisingly, these data in dicate that fractional particle alignment at a given shear rate is independ ent of phi for 0.05 < phi < 0.39. (C) 1999 The Society of Rheology. [S0148- 6055(99)00503-9].