OSMOTIC-PRESSURE EFFECTS IN DISPERSION RHEOMETRY

Many fluid flow phenomena can be understood and described within the framework of Navier-Stokes hydrodynamics by justifiably neglecting what would otherwise be vitiating effects. The salient flow features of dense particulate suspensions, however, do not conform with these requirements. The fundamental reason is that when a thermalized fluid is perturbed with a shear strain frequency which is of the same order as the relaxation spectra of the degrees of freedom that determine its stress response, interaction occurs and Stokesian hydrodynamics is no longer applicable. In colloidal dispersions shear induced effects arising from the heat capacity and the compressibility of the liquid medium are negligible. Since it is the inertial forces between the macroparticles that determine the effective thermo-rheological state, it is the shear perturbation of the osmotic pressure tensor, and not the hydrostatic pressure, that must be considered. Computer simulation studies are still in early stages but there are sufficient preliminary pressure tensor-shear rate data along a number of isochores of the soft-sphere model to infer by interpolation a typical constant pressure flow curve. Additional theoretical and experimental studies are discussed in the paper.