Electroviscous effect of rodlike polyelectrolytes in strong flow

Abstract

Primary electroviscous effect of a dilute solution of rodlike polyelectrolytes subject to a strong shear flow is investigated theoretically. A linear charge distribution along the macromolecule is assumed in this study. Also the Hartmann number and the ion Peclet number are assumed small so that the electrical body force gives small perturbation to the imposed flow, and the double layer deviates slightly from its equilibrium state. The distortion of the double layer not only gives rise to electrical stresses but also modifies the orientation distribution of the macromolecules, leading to an indirect contribution to the stress. This modification was treated as small perturbations to the orientation distribution of uncharged rods for weak and moderate flows in the previous work. The perturbation analysis, however, becomes invalid for strong flows, in which the rotational flux of the rods due to the charge effect is comparable to their Brownian diffusion and the advection by the fluid in a boundary layer region near the flow direction. In this study, a spherical harmonic method is used to solve for the orientation distribution of the rods by specifying the parameters associated with the charge effect. The results show that the moments of the orientation distribution and the stress stemming from the electrical effect do change in strong flow, in comparison with those obtained from a regular perturbation method.