Computational Chemistry on Dynamic Properties of Ferromagnetic Colloidal Dispersions by Means of Stokesian Dynamics Method

(1)通过Stokesian动力学模拟研究了铁磁胶体分散体在正弦剪切流作用下的动力学特性。厚链状团簇的形成受到振荡剪切流的显著影响，即使振幅相对较小，因为厚链状团簇的内部结构对剪切流方向的变化高度敏感。厚链状团簇的运动与剪切速率呈正弦相反，相位差与黏度和正应力系数的相位差密切相关。随着振荡剪切流动频率的降低，粘弹性特性变得更加明显，因为这种特性与厚链状团簇的形成有很强的关系。(2)研究了由铁磁球形和多圆柱形颗粒组成的稀胶体分散体在简单剪切流作用下的流变性能和取向分布。磁场越强，定向分布函数的峰值越陡，当磁场开始控制剪切流时，峰值的位置由流动方向向磁场方向变化。由于当磁场变强时，颗粒的取向在磁场方向上受到高度限制，因此粘度显著增加。长径比越大的颗粒在流场中会产生更大的阻力，从而导致粘度的增加越大。(3)研究了Stokesian动力学方法所依据的力和速度可加性两种近似的特点和区别。本文对一个简单剪切流进行了铁磁胶体分散体的Stokesian动力学模拟，并对聚集体结构和平均粘度进行了评估。从平衡状态下聚集体结构的结果来看，力加和法得到的相关函数与忽略水动力相互作用近似得到的相关函数在数量上不同，但在质量上是一致的。速度可加性的结果给出了上述两种近似之间的介质特性。随着磁相互作用的影响减小，速度可加性的对相关函数的形状与力可加性的对相关函数的形状接近。从初始状态的瞬态特性计算结果来看，速度加和性与力加和性的结果吻合得很好。相反，没有流体动力相互作用的结果偏离了两个可加性近似的结果，在一个时间步长，当接近粒子-粒子接触的情况开始出现。少

(1) We have conducted Stokesian dynamics simulations to investigate the dynamic properties of ferromagnetic colloidal dispersions subjected to a sinusoidal shear flow. Thick chain-like cluster formation is significantly influenced by an oscillatory shear flow even if the amplitude is relatively small, since the internal structures of thick chain-like clusters are highly sensitive to the change in the direction of the shear flow. The motion of thick chain-like clusters is out of phase to a sinusoidal shear rate, and the phase difference is strongly correlated with that of the viscosity and normal stress coefficients. The viscoelastic properties become more apparent with decreasing frequency of the oscillatory shear flow, since such properties have a strong relationship with the thick chain-like cluster formation.(2) We have investigated the rheological properties and the orientational distributions of particles of a dilute colloidal dispersion, which is composed of ferromagnetic spheroc … More ylinder particles, subject to a simple shear flow. The orientational distribution function has a sharper peak for a stronger magnetic field, and the position of the peak changes from the flow direction to the magnetic field direction as the magnetic field comes to govern the shear flow. Since the orientation of the particle is highly restricted in the field direction as the magnetic field becomes strong, the viscosity increases significantly. The particles with a larger aspect ratio lead to the larger increment in the viscosity, since they induce a larger resistance in a flow field.(3) The characteristics and differences in the two approximations, I.e., the additivity of forces and the additivity of velocities, on which Stokesian dynamics methods are based, have been investigated. Stokesian dynamics simulations of a ferromagnetic colloidal dispersion have been carried out for a simple shear flow, and the aggregate structures and averaged viscosities have been evaluated. From the results of aggregate structures in equilibrium, the correlation functions obtained by the additivity of forces are quantitatively different from those by the approximation of ignoring hydrodynamic interactions, although a qualitative agreement is recognized. The results obtained by the additivity of velocities give medium characteristics between the two above approximations. The shape of the pair correlation function for the additivity of velocities approaches that for the additivity of forces as the influence of magnetic interactions decreases. From the results of transient characteristics from an initial state, the results by the additivity of velocities agree well with those by the additivity of forces. In contrast, the results without hydrodynamic interactions deviate from those of the two additivity approximations at a time step when a nearly particle-particle touching situation starts to appear. Less