The deformhation and flow of highly concentrated dispersions

Abstract

The definition of what constitutes a highly concentrated solid particulate dispersion in a continuous liquid phase is argued as being more a matter of a distinction of experimental and analytical approach rather than one of an intrinsic compositional range. Highly concentrated dispersions may have solid fractions approaching 60% by volume. They are sometimes termed pastes, and show viscoelastic deformation prior to a yielding which corresponds to the onset of a pronounced ‘plastic’ flow. A number of potentially viable experimental configurations are described and discussed. They involve pipe flow, constriction flows and, in particular, compressive (squeeze film/upsetting) deformation between parallel platens. The resulting velocity fields associated with the deformation and flow are characteristically sensitive to the wall boundary conditions. This fact greatly complicates the analysis of the experimental data which describe the behaviour of these systems. Various approaches to this problem, based on the general theory of plasticity (plastic) and rheological (viscous) treatments, are reviewed. It is demonstrated, by employing experimental data obtained from a model concentrated dispersion (Plasticine) for compression between parallel platens that the two, apparently extreme, approaches can be made mutually compatible. Analytical solutions are considered for lubricated and no-slip wall boundary conditions. However, for slip conditions, in which there is a significant wall shear stress, it is concluded that numerical techniques are required to deconvolute the system response in terms of the wall boundary condition and the constitutive flow relationship.