DYNAMICS OF COLLOIDAL PARTICLES IN SHEARED, NON-NEWTONIAN FLUIDS

The dynamics of colloidal, axisymmetric particles suspended in non-Newtonian, polymeric liquids are examined by using optical techniques that are capable of following their average orientation. Polarimetry measurements of birefringence and dichroism are used on dispersions of colloidal iron oxide spheroids and small-angle light scattering is used on hardened, prolate human red blood cells. A variety of suspending liquids are used including Boger fluids designed to have some of the attributes of a second order fluid under steady state flow conditions, and concentrated polystyrene solutions with rheological properties similar to more commonly encountered polymer liquids. As a result of the non-Newtonian properties of the polymeric suspending fluids, and in particular the presence of normal stresses, the particles were observed to drift out of the orbits associated with their motion in Newtonian fluids. As predicted by Cohen and coworkers, in flows where the elastic forces on the particles dominate Brownian forces, the particles tend to drift away from orientations favoring the flow direction and towards the vorticity axis. When Brownian forces are large compared with the elastic forces, alignments in the flow direction are more probable. This trend, however, was only observed when the Boger fluids were used as suspending fluids. Although the polystyrene solutions possessed shear viscosities and first normal stress difference coefficients that were comparable in magnitude to those of the Boger fluids, only orientations along the flow direction were found. © 1990.