Shear-induced migration and orientation of rigid fibers in an oscillatory pipe flow

The distributions of fibers in suspension were measured during oscillatory flow within a circular pipe. The fibers were rigid and non-colloidal, and two aspect ratios (length L to diameter d ratios) were tested; the suspending fluid was viscous, Newtonian, and density matched to the particles. As with spheres in parabolic flows, fibers in the concentrated suspensions migrated toward the center of the pipe. The migration was similar for the fibers, irrespective of the aspect ratio, at the same dimensionless number density nL²d (n is the particle number density), rather than at the same volume fraction. The extent of migration was maximum at nL²d=0.84 for both aspect ratios. The orientation distribution of the fibers was spatially dependent. Fibers near the center of the channel aligned closely with the flow direction, while fibers near the pipe wall had an enhanced probability of aligning in the vorticity direction. Additionally, qualitative observations indicate that nematic crystallites of fibers form near the center of the tube for highly concentrated suspensions.