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We numerically investigate the effects of fiber aspect ratio, roughness,flexibility, and flow inertia on the rheology of concentrated suspensions. Weperform direct numerical simulations modeling the fibers, suspended in anincompressible Newtonian fluid, as continuous flexible slender bodies obeyingthe Euler-Bernoulli beam equation. An immersed Boundary Method (IBM) isemployed to solve for the motion of fibers. In concentrated suspensions, fiberscome into contact due to the presence of asperities on their surface. We assumea normal load-dependent friction coefficient to model contact dynamics andfriction, which successfully recovers the shear-thinning behaviour observed inexperiments. First, we report the shear rate dependent behavior and theincrease in the suspension viscosity with the increasing volume fraction offibers, fiber roughness, and rigidity. The increase in the viscosity isstronger at a lower shear rate and finite inertia. Simulation results indicatethat for the concentrated suspensions, contact stresses between fibers form thedominant contribution to the viscosity. Moreover, we find the first normalstress difference to be positive and to increase with the Reynolds number forflexible fibers. Lastly, we explore the divergence of viscosity for differentaspect ratios and roughness of the fibers and predict the jamming volumefraction by fitting the data to the Maron-Pierce law. The jamming volumefraction decreases with increasing the aspect ratio and roughness. We concludethat the contact forces and interparticle friction become one of the crucialfactors governing the rheology of flexible fiber suspensions at highconcentrations.

Rheology of concentrated suspension of fibers with load dependent friction coefficient