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 φ*=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 γ̇0t where t is the time after the stress is released and γ̇0 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 γ0 decreasing from a low shear rate plateau value of Gmax′ to a high shear rate plateau value of Gmin′. The shear rate halfway between Gmax′ and Gmin′ is independent of volume fraction and lies near 0.39 D0rot where D0rot 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.