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

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 p redicts a preferred alignment in the flow direction at strain amplitudes mu ch less than unity. The steady state fiber orientation was found to be depe ndent on the amplitude of the fiber rotation during a cycle of the oscillat ion. For large fiber rotations, due to large strain amplitude (epsilon grea ter than or equal to 1.0) or large fiber projections in the velocity gradie nt direction, the majority of the fibers drifted slowly towards the vortici ty axis, as would be expected for steady shear. A slow rotation towards the flow direction was observed for small strains (epsilon = 0.05), as expecte d, 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 stares with both the flow and vorticity direct ions being stable fixed points. (C) 2000 Elsevier Science B.V. All rights r eserved.