Different methods for determining the yield stress of ferrofluids

Abstract

The yield stress of ferrofluids has been a hot topic in their rheological studies, and it is still controversial which method could obtain the more accurate yield stress value. In this work, we used varieties of methods to obtain the yield stress of the ferrofluid, including steady shear and oscillatory shear. We obtained the static yield stress and dynamic yield stress of the ferrofluid by Herschel–Bulkley fitting of the flow curves measured by the controlled shear rate mode and the controlled shear stress mode, and found that at any magnetic field strength, the static yield stress was always significantly greater than the dynamic yield stress. We then performed oscillatory shear of the ferrofluid and determined the yield stress using three methods, where the yield stress value corresponding to the intersection of the storage modulus G′ and the loss modulus G″ is similar to the static yield stress value fitted by H-B at low magnetic field strength. The power law function and shear stress as a function of shear strain consistently gave the maximum value of the yield stress. Ultimately, we believe that which method is ultimately chosen depends on the practical application conditions of interest.

Study of Dynamic Viscoelasticity of a Mineral Oil-Based Magnetic Fluid

“…When resting und magnetic field, the magnetic moment of magnetic particles inside the magnetic fluid aligned along the magnetic field direction, and the particles form anisotropic chain Before the experiments, the magnetic fluid was pre-sheared and the magnetic particles were dispersed into a disordered distribution, which was free in the base carrier fluid and showed a Newtonian fluid characteristic with low viscosity. When resting under a magnetic field, the magnetic moment of magnetic particles inside the magnetic fluid was aligned along the magnetic field direction, and the particles form anisotropic chain and cluster structures under this interaction, and the chain and cluster structure [30] magnitudes grew rapidly with the magnetic field strength, and lateral agglomeration occurred in the non-magnetic action, which made the magnetic fluid system further undergo phase transformation from a stable colloidal system to a thixotropic system, and finally formed a stable hexagonal equilibrium structure, when the internal storage modulus of the magnetic fluid was greater than the loss modulus, the magnetic fluid elasticity dominated, and the complex fluid was solid-like, showing the Bingham characteristic of high viscosity and low mobility (Bingham Fluid) [31].…”