Time evolution of rigid fiber orientation in highly viscoelastic fluids under circular Couette flow

Time evolutions of short rigid fiber orientation in viscoelastic suspensions under highly elastic shear flows are examined experimentally. We measure the orientation dynamics of fibers suspended in polyacrylamide solutions spanning moderately to strongly viscoelastic regimes using a transparent concentric-cylinder Couette device, covering a wide range of shear rates and fiber loadings. In viscoelastic media, fibers rapidly reorient from an initially isotropic state and exhibit clear exponential alignment. The characteristic timescale decreases with increasing shear rate, whereas the associated strain scale grows with fiber volume fraction and fluid relaxation time, indicating slower alignment under stronger elasticity or denser suspensions. For comparison, fibers in a Newtonian silicone oil follow nearly periodic Jeffery-like orbits and show no long-term alignment. By contrasting these distinct behaviors, we provide a simple theoretical interpretation showing that fiber alignment in viscoelastic flows is governed by stress relaxation within the fluid. These results establish a unified experimental picture of fiber orientation in highly viscoelastic shear flows and clarify the elasticity-dominated mechanism driving their exponential alignment.