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.

中文翻译：

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铁磁流体的屈服行为研究：非常剪切稀化现象。

铁磁流体的屈服行为对于许多应用至关重要。但是，以前的研究主要集中在较高剪切速率下的磁粘性效应，很少涉及在非常低剪切速率下铁磁流体的屈服过程。在这项研究中，制备了不同颗粒体积浓度的铁磁流体样品，并在各种磁场强度和温度下系统地研究了它们在宽剪切应力范围内的剪切稀化行为。首先观察到铁磁流体非常剪切稀化的现象，并分析了其微观机理。在铁磁流体的粘度曲线中，低剪切高原和高剪切高原之间出现了一个陡峭的脱落阶段，作为屈服的标志。随着磁场强度的增加或温度的降低，粘度曲线中的急剧下降阶段变得更加陡峭。对于具有较低颗粒体积浓度的铁磁流体，当温度超过一定值时，在低剪切区域下的高粘度极限消失，并且被解释为在高布朗热相互作用水平下平衡柱状结构的消失。事实证明，复合材料的Ellis模型适合于不同类型的屈服应力和结构数的拟合，并提出了Sn用于铁磁流体剪切稀化行为的无量纲分析。