Study on the yielding behaviors of ferrofluids: a very shear thinning phenomenon

The yielding behaviors of the ferrofluids are vital for many applications. However, previous studies have mainly focused on the magnetoviscous effect under relatively high shear rates and rarely involved the yielding process of ferrofluids under very low shear rates. In this study, ferrofluid samples of different particle volume concentrations were prepared and their shear thinning behaviors within a wide shear stress range were systematically studied under various magnetic field strengths and temperatures. The very shear thinning phenomenon of ferrofluids was first observed and its microscopic mechanism was analyzed. A precipitous fall-off stage as the mark of yielding appeared between the low shear and high shear plateaus in the viscosity curves of ferrofluids. The precipitous fall-off stage in the viscosity curves became steeper with the increase of the magnetic field strength or decrease in the temperature. For ferrofluids with relatively low particle volume concentrations, the high viscosity limit under the low shear region disappeared when temperature exceeded a certain value and was interpreted as the disappearance of the equilibrium columnar structures under high Brownian thermal interaction level. A composite Ellis model was proved to be suitable for the fitting of different types of yield stresses and a structural number, Sn was proposed for the dimensionless analysis of the shear thinning behaviors of ferrofluids. The findings in this study contribute to a better understanding of the microscopic mechanism of yielding behaviors of ferrofluids and also provide guidance for many practical applications.