An experimental investigation is undertaken into the shear-thinning behavio
r of suspensions of non-Brownian rigid fibers in Newtonian fluids. In parti
cular, the influence of the shear stress and the fiber concentration is inv
estigated. The shear stress is adjusted by varying both the shear rate and
the solvent viscosity. In the semidilute concentration regime, where direct
mechanical contacts between fibers are rare, the suspension is found to be
nearly Newtonian over the stress range investigated. In the concentrated r
egime, the suspension becomes shear thinning below a certain shear rate. Th
e shear thinning increases with concentration and decreases with solvent vi
scosity. Although shear-thinning behavior of fiber suspensions has often be
en reported in the literature, its physical origins are not well understood
. Our experiments are interpreted in terms of the formation and breakage of
fiber flocs due to the competition between hydrodynamic and colloidal forc
es. Our interpretation is confirmed by measurements of the adhesive forces
between two individual fibers. (C) 2001 The Society of Rheology.