An experimental investigation on the magnetoviscous effect and shear rate-dependent viscosity of a magnetic suspension under longitudinal and transverse magnetic fields

In the present investigation, we report experimental evidence that the magnetoviscous effect of a ferrofluid can be described by a suspension of ellipsoidal particles. The studies are carried out in shear and pipe flows, and comparison with theory for non-spherical particle is made. Flows of this type appear in many applications where magnetic fluids are used as lubricants or magnetic seals. The chain-like aggregates present in the magnetic fluid are modeled as equivalent prolate spheroids. A power-law correlation accounts for the breakup of aggregates due to the shear flow. The viscosity of a real ferrofluid is measured in a parallel disk rheometer with an applied magnetic field parallel to velocity gradient and in a capillary viscometer with a field in the velocity direction. An asymptotic solution for the low Péclet number limit presents an excellent agreement with the experimental results in this regime, while the numerical results provide a good agreement up to moderate values of Péclet. In addition, the numerical results for the non-dimensional viscosity of the fluid and the magnetic increment viscosity are verified by comparing results with the experimental measurements of the same quantities for moderate values of the Péclet number. The relative importance of the mean stresslet and rotlet contributions for the non-dimensional magnetic suspension viscosity is also examined in a regime of low Péclet number. The results also were important to verify our previous theoretical work presented recently in a companion paper Sinzato and Cunha [Phys. Fluids 33, 102006 (2021)] for the regime of Pe < 1.