ORIENTATION IN SIMPLE SHEAR-FLOW OF SEMIDILUTE FIBER SUSPENSIONS .1. WEAKLY ELASTIC FLUIDS

The orientations of fibers suspended in dilute polymer solutions subje ct to simple shear flow were observed. A single fiber was observed to spiral toward the vorticity axis as has been predicted theoretically a nd observed in previous experiments. Orientation distributions were ob served in semi-dilute fiber suspensions prepared using the same polyme r solution. At the largest fiber concentration, most of the fibers wer e oriented near the flow axis of the simple shear flow and the orienta tion distribution was nearly the same as that observed in a Newtonian fluid. The orientation distribution shifted toward the vorticity axis with decreasing fiber concentration. The fiber orientation distributio ns were calculated based on the assumption that the fibers follow Jeff ery orbits to leading order and experience small perturbations from th ese orbits due to fiber-fiber interactions and viscoelastic stresses. The theory predicted the qualitative trend of increasing vorticity ali gnment at decreasing fiber concentration. However, the value of the no n-Newtonian stress parameters that gave the best fit to the suspension observations was lower than that obtained by fitting the single fiber experiment.