In fiber suspensions, fibers with three-dimensional orientation states are dramatically flow oriented in the flow-flow gradient plane when the flow starts. In contrast, large strains are required for flow orientation in the flow-vorticity plane. Under oscillatory shear flow, when the strain amplitude is small, the flow orientation in the flow-vorticity plane is weakly induced, unlike that in the flow-flow gradient plane. The orientation in the flow-vorticity plane increases with the strain amplitude. At large strain amplitudes, fibers are oriented in the flow-flow gradient plane; thus, the rotational motion of fibers in the flow-flow gradient plane is dominant, i.e., fibers are almost flow oriented in the flow-vorticity plane. However, contributions of the oscillatory behavior of fiber orientation in the flow-flow gradient and flow-vorticity planes to complex viscosity are unclear. Herein, we adjusted the two initial orientation states (random and flow-oriented states) for the strain sweep test to determine the contributions of fiber orientation in each plane to complex viscosity. Furthermore, phase transition behaviors from the random to flow-oriented state in the flow-vorticity plane with increasing strain amplitude were qualitatively estimated as the orientation angle via visualization. Consequently, when the initial orientation was random, the fibers gradually vibrated in a medium strain amplitude region and complex viscosity was higher than that of the initial flow-oriented state. We divided the complex viscosity behavior of the strain sweep test into five strain amplitude regions and clarified the dominant orientation state in each region.
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