Alignment of spherical particles in rheologically complex fluid under torsional flow

The microstructures of suspensions of spherical particles under the torsional flow between two parallel plates are studied by using the optical microscopy and the small angle light scattering technique (SALS) along the velocity gradient direction. The particle diameter is 2.2 micrometer and the gap distance between two parallel plates is 200 micrometer. The dispersing media are glycerin, aqueous solution of 4% xanthan gum and 1500 ppm polyacrylamide solution in glycerin. The xanthan gum solution is strongly shear thinning with the shear thinning index of 0.2. The polyacrylamide solution behaves as a Boger fluid rheologically. The result shows that the microstructure observed in the wide gap experiment is qualitatively different from the microstructure observed in the monolayer experiments reported in the literature. In glycerin, a random structure is observed. In the shear thinning fluid, particles appear to be weakly chained and aligned along the vorticity direction below 100 s-' and along the flow direction over 100 s'. In the Boger fluid, particles align along the vorticity direction and form short strings within the fluid. The alignment appears to be originated from the elastic effect rather than the shear thinning effect. A new flow pattern of the banded structure is occasionally observed for the Newtonian suspension which still shows a random microstructure of suspended particles. This flow pattern is always present in the shear thinning fluid while it is never present in the Boger fluid.