Journal of non-Newtonian fluid mechanics vol:155 issue:1-2 pages:39-50
The flow-induced orientation of spheroidal particles in viscoelastic fluids is studied by means of rheo-optical methods and flow microscopy. Using suitable model systems a wide range of rotational Péclet and Weissenberg numbers have been covered, providing a systematic and global picture of the various orientational transitions. The effects of particle size and aspect ratio have been considered separately. Increasing the shear rate gradually causes first the well-known change from a random orientation to spinning in Jeffery orbits. Upon further increasing both the Péclet and Weissenberg numbers, the period of rotation becomes larger. At this stage the orbits start to drift slowly toward a ‘log-rolling’ state, which leads to an orientation distribution function that is narrowly peaked around the vorticity axis. For the fluids tested here, the orbit drift rates are proportional to the shear rate. At still higher elasticities, the particles are observed to reorient again in the flow direction, with the notable exception of suspensions in Boger fluids. Some of the experimentally observed features are qualitatively in line with the results of a theory for slender bodies in second order fluids, despite both relatively small aspect ratios and a more complex rheological behavior of the suspending media. An effect of absolute particle size is also noted which might indicate interference from Brownian motion, as suggested earlier. With suitable flow histories bimodal flow-vorticity orientation distributions can be generated. Finally, specific flow-induced alignment and aggregation are observed, they are more difficult to generate than in suspensions with spheres.