Thixotropic yielding behaviors of ferrofluids

Abstract

The thixotropic yield behaviors of ferrofluids with different particle volume concentrations were studied through a series tests under constant stresses for sufficiently long times. A lubrication oil based ferrofluid was prepared and a diluted sample was acquired by mixing the initial ferrofluid sample with carrier fluid according to a ratio of 1:3. The particle volume concentration of the samples was estimated through the magnetization curves as 7.8% and 1.9%. A critical bifurcation stress can be observed through the time evolution of the initial ferrofluid sample under moderate magnetic fields, over which the viscosity curves tend to infinity. For the diluted ferrofluid sample, no viscosity bifurcation can be observed even under the minimum stress values. Similar viscosity bifurcation phenomenon for initial samples can be seen from the curves under different magnetic field strengths or temperatures. The experimental results indicate that the yield process of ferrofluids is thixotropic and significantly influenced by the shear history. The microscopic mechanism behind the thixotropic yield behaviors of ferrofluids is discussed and analyzed. A non-linear structural model was presented to describe the time evolution of viscosity in ferrofluids under different magnetic field strength. The comparison between the fitting results and experimental data indicates that the thixotropic yielding behaviors of ferrofluids can be predicted in some extent. These findings provide a new perspective on the yielding behaviors of ferrofluids which takes the thixotropic effect into consideration.