(299e) Shear Alignment of Fibers in Oscillatory Flows

Rigid fibers suspended in a viscous, Newtonian fluid can be aligned
perpendicular to the flow-gradient plane by applying an oscillatory
shear flow.  Direct comparisons with published experiments demonstrate
that a simple model, which considers only excluded volume and
self-mobilities, can accurately predict the orientation distributions.
Stresses calculated from the numerical simulations are also reported
and compared to the experimentally measured rheology.

These simulations reveal that the alignment occurs over a narrow range
of strain amplitudes and concentrations.  Furthermore the fibers must
be highly confined between the bounding walls.  For example
simulations of fibers of aspect ratio 11 sheared between planar walls
separated by a distance of 1.5 times the fiber length align strongly
in the vorticity direction at a volume fraction of 15% and strain
amplitude of 3, whereas simulations with fully periodic boundary
conditions under identical conditions indicate a preference to align
with the direction of flow.  For a volume fraction of 15%, the rods
exhibit no change in orientation distribution at strain amplitudes
less than one and the distribution is similar to that of steady
shearing flows, with alignment in the flow direction, for strain
amplitudes in excess of five.  At a strain amplitude of 3, the extent
of alignment of the orientations with the vorticity direction is a
non-monotonic function of the concentration.