Oscillatory shear alignment of a non-Brownian fiber in a weakly elastic fluid

Abstract

Observations of the orientation of single fibers suspended in a polybutene-based Boger fluid under oscillatory shear were made to test a recent theory [O.G. Harlen, D.L. Koch, J. Non-Newtonian Fluid Mech. 73 (1997) 81] that predicts a preferred alignment in the flow direction at strain amplitudes much less than unity. The steady state fiber orientation was found to be dependent on the amplitude of the fiber rotation during a cycle of the oscillation. For large fiber rotations, due to large strain amplitude ( ε ≥ 1.0) or large fiber projections in the velocity gradient direction, the majority of the fibers drifted slowly towards the vorticity axis, as would be expected for steady shear. A slow rotation towards the flow direction was observed for small strains ( ε = 0.05), as expected, and for moderate strains with smaller fiber projections in the velocity gradient direction. The theory predicted the correct order of magnitude of this orientational drift. The observations at moderate strains suggest the existence of multiple steady states with both the flow and vorticity directions being stable fixed points.