Rheology of dense suspensions of platelike particles

The flow properties of dense kaolin suspensions are explored for volume fractions as large as 0.43. The particles were suspended in phosphate buffers at pHs of 7 and 10 where edge-face interactions are negligible. These platey particles have an aspect ratio of approximately 12 and, thus, are likely to show alignment at volume fractions above a critical volume fraction phi* = 0.10. As the concentration is increased from the dilute region of Newtonian behavior, the suspensions develop a yielding type of response near this critical volume fraction. The time dependence of the recoverable strain in some dense suspensions is found to scale on gamma(0)t where t is the time after the stress is released and gamma(0) is the steady shear rate prior to release of the stress. Over the same time period the elastic modulus remains constant. The magnitude of the modulus is, however, dependent on gamma(0) decreasing from a low shear rate plateau value of G'(max) to a high shear rate plateau value of G'(min). The shear rate halfway between G'(max) and G'(min) is independent of volume fraction and lies near 0.39 D-Orot where D-Orot is the free solution rotational diffusion constant of the plates. This behavior is interpreted as indicating that the plates experience a tumbling/shear aligning transition as the shear rate is raised. Further evidence for alignment is found in shear induced changes in suspension conductivity. (C) 1996 Society of Rheology.