Orientational order in concentrated dispersions of plate-like kaolinite particles under shear

The orientational order in concentrated dispersions of plate-like, kaolinite particles subjected to shear has been studied using neutron diffraction. The relationship between the direction of orientation and the extent of alignment, calculated as an order parameter, is similar to that predicted for dilute dispersions. The plate-like particles align with their normals in the compressional direction. As the shear rate is increased, the degree of order increases and the particles align with normals closer to the gradient direction. However, the degree of order observed at a particular shear rate was found to vary with concentration in a complex manner. At intermediate concentrations, the extent of order is enhanced by the particle interactions. In contrast, at high concentrations the extent of order is reduced. An explanation for this effect is suggested in terms of an effective temperature arising from particle interactions and collisions. (C) 2000 The Society of Rheology. [S0148-6055(00)01302-X].