Rheology of dilute suspensions of charged fibers

The rheology of a dilute suspension of charged fibers with a large ratio of length L to diameter d is examined. The fibers may possess both a net charge and a charge dipole and it is assumed that the Hartmann number is small. The double layer thickness lambda is large compared with the rod diameter and may be comparable with the fiber length. Although the shear rate is sufficiently small so that the deformation of the double layer is small, no restriction is placed on the rotary Peclet number of the rods. The velocity disturbance caused by the rods is neglected when calculating the double layer deformation. This approximation is accurate when lambda/L = O(1) and Lid is very large, but leads to an overestimate of the ion cloud distortion for small values of lambda/L or moderate values of Lid, The counterion cloud affects the stress both directly through a primary electroviscous stress and indirectly by exerting an electric torque that changes the fiber orientation distribution. The additional stress caused by electrostatics shear thins for a fiber with a net charge. However, when the fiber has a charge dipole, the electrical stress may increase with increasing shear rate at sufficiently high Peclet numbers. At very high Peclet numbers: the electrical torque can lead to a fixed orientation of the fibers near the flow direction of the simple shear flow. (C) 1996 American Institute of Physics.