Migration Dynamics and Orientation Distributions of Rigid Fibers within Viscometric Flow Fields

Mechanisms leading to unexpectedly large shear thinning for
suspensions of rigid rods within viscometric devices are explored.  At
large, but finite values of the Peclet number, the competition between
the randomization caused by Brownian rotation and tendency of the
shearing flow to align the rods breaks the symmetry about the flow
direction of the orientation distribution.  The asymmetric
distribution of the rod's orientation can then couple with
inhomogeneous shear flows to generate a net migration.  Examples
include the net migration of rods toward the axis of rotation in
torsional flows, which directly impacts the rheological measurement.
For semidilute suspensions of rigid rods in oscillatory shear flow,
rods can align in the vorticity direction depending upon the strain
amplitude and concentration.  This alignment results in a substantial
decrease in the measured viscosity of the suspension.