Orientation distribution in a dilute suspension of fibers subject to simple shear flow

The orientation distribution in a dilute suspension of Brownian fibers subj ect to simple shear flow is investigated theoretically. The fiber has a lar ge aspect ratio r and may carry charge. The flow strength is characterized by the rotary Peclet number Pe=gamma/D, where gamma is the shear rate and D is the rotary diffusivity of the fiber. Emphasis is placed on the microstr ucture of the suspension in strong flows, in which the advection and diffus ion are of equal importance in a small region of angular space near the flo w direction. A new computational method based on a finite difference scheme is developed to calculate the orientation distribution function and then e valuate the orientation moments. For the case of uncharged rods and 1 much less than Pe(1/3)much less than r, the obtained orientation moments compare favorably with those deduced from the spherical harmonic method. The effec ts of the fiber aspect ratio and charge are also investigated when they bec ome crucial. It is found that at a given large Pe, the orientation distribu tion of uncharged fibers with r similar to O(Pe(1/3)) becomes broad and its peak shifts down towards the flow axis, compared with the case of thin rod s with r much greater than Pe(1/3). The microstructure of a suspension of c harged fibers depends on the charge distribution and the double layer thick ness relative to the fiber length. (C) 1999 American Institute of Physics. [S1070-6631(99)03210-9].