(326b) Time-Varying Flows of Concentrated Suspensions of Rigid Fibers

The orientation distribution of non-Brownian fibers was measured within an oscillatory shearing flow as a function of strain amplitude for concentrated conditions and aspect ratios (fiber length L to diameter d). The optical measurements relied on fluorescence and were collected from a custom-built cell that generates a simple shear flow between two planar surfaces separated by a distance H [1]. The results show that fibers align in the flow direction at large strain amplitudes, but can align in the vorticity direction at intermediate strain amplitudes depending on the confinement (H/L). Simulations that account for hydrodynamic drag and excluded volume accurately predict the orientation distributions [1-2], and reveal that the vorticity alignment varies strongly with distance from the bounding walls. We show that maximum vorticity alignment occurs when the product of strain amplitude and volume fraction is approximately 0.4; this corresponds to the lowest value of strain amplitude and volume fraction at which the imaginary component of the complex viscosity is non-zero [3]. Additionally, simulations of the dynamic changes in the orientation distribution during shear reversal are used to estimate viscosity changes during the reversal. These are compared with published measurements of the orientation distribution and viscosity during shear reversal experiments.

[1] S. Strednak, J.E. Butler, L. Bergougnoux, and E. Guazzelli, Phys. Rev. Fluids 6, 014302, (2020).

[2] B. Snook, E. Guazzelli, and J.E. Butler, Phys. Fluids, 24, 121702, (2012).

[3] A. Franceschini, E. Filippidi, E. Guazzelli, and D.J. Pine, Soft Matter 10, 6722 (2014).