Eventos Anais de eventos
COBEM 2021
26th International Congress of Mechanical Engineering
Stress Relaxation Function Response of Ferrofluids in the Presence of a Magnetic Field to Transient Shear Flows: Step Strain and Oscillatory Shear.
Submission Author:
Igor Dal Osto Pereira , DF
Co-Authors:
Igor Dal Osto Pereira, Francisco Ricardo Cunha
Presenter: Igor Dal Osto Pereira
doi://10.26678/ABCM.COBEM2021.COB2021-1345
Abstract
In this article, the rheological response of two commercial ferrofluids to transient shearing flows is experimentally evaluated using a parallel disc rheometer device equipped with a magnetic cell. The basic difference between the ferrofluids is their volume fraction of magnetic particles. The ferrofluids are composed of magnetic magnetite nanoparticles with a high magnetization of saturation. Samples of both ferrofluids are tested according to two experimental protocols of the Laboratory of Microhydrodynamics and Rheology of the University of Brasília. The first transient shear flow explored is a step-strain under the influence of a magnetic field, from which the stress relaxation functions for both magnetic fluids tested are determined in terms of the magnetic field strength and the intensity of the step strain. The relaxation times of the fluids are calculated. For both ferrofluids, the main relaxation times present a nonlinear increase as the intensity of the magnetic field heightens. We also observed that the shear stress relaxes to a residual stress which is strongly dependent on both field strength and strain strength. This uncommon residual stress increases as the intensity of the magnetic field rises. In terms of strain strength, the residual stress is found to present two interesting behaviors. First, for small values of strain, the residual stress increases linearly until a maximum is reached. Further increases in the strain strength led to a nonlinear decrease in the residual stress. The linear regime was associated with a predominance of elastic deformation of the fluid microstructure while the nonlinear one to its plastic deformation or even to the break-up. The second procedure refers to tests under the condition of oscillatory shear in a linear viscoelastic regime and in the presence of an applied magnetic field. The main viscoelastic modulus of the ferrofluids as functions of the frequency and the magnetic field intensity are presented. Besides, it is also shown, for both ferrofluids, that viscous and elastic characteristics are severely increased when the applied magnetic field intensity is enhanced. Compatibility checks between the viscous modulus and the apparent shear viscosity under conditions of the same frequency and shear rate are performed and the first normal stress difference is calculated using Laun’s rule. The presence of normal stresses indicates that the microstructure of the fluid may drastically change to a configuration of strongly anisotropic structures (chain-like) in the presence of an external magnetic field and under the condition of weak flows.
Keywords
Ferrofluids, Stress Relaxation, residual stress, Linear Viscoelasticity, First Normal Stress Difference
