Orientation in simple shear flow of semi-dilute fiber suspensions .1. Weakly elastic fluids

The orientations of fibers suspended in dilute polymer solutions subject to simple shear flow were observed. A single fiber was observed to spiral toward the vorticity axis as has been predicted theoretically and observed in previous experiments. Orientation distributions were observed in semi-dilute fiber suspensions prepared using the same polymer solution. At the largest fiber concentration, most of the fibers were oriented near the flow axis of the simple shear flow and the orientation 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 distributions were calculated based on the assumption that the fibers follow Jeffery orbits to leading order and experience small perturbations from these orbits due to fiber-fiber interactions and viscoelastic stresses. The theory predicted the qualitative trend of increasing vorticity alignment at decreasing fiber concentration. However, the value of the non-Newtonian stress parameters that gave the best fit to the suspension observations was lower than that obtained by fitting the single fiber experiment.