Shear Flow Properties of Semiconcentrated Fiber Suspensions

Measurements are reported for the shear stress of a semiconcentrated suspension of rigid fibers in a Newtonian solvent in the startup of steady shear flow. There are n fibers each of length L per unit volume. A variety of different volume fractions of fibers was used; the fiber aspect ratio was varied as well. It is found that the viscous resistance of a randomly oriented fiber suspension is roughly (nL3) times the resistance observed when the particles lie in the planes of shear. The transient shear stress measurements show the rheological properties depend on the total strain and not on shear rate or time separately. Experiments were done in torsional flow between parallel plates separated by distance H to determine the effect of H/L on the measured properties. It is found that there is no effect for H/L>1. Two different models are presented to account for wall effects for H/L<1. Good agreement is found between computed and measured values.