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

(1)采用Stokesian动力学模拟方法研究了正弦剪切流作用下铁磁胶体分散体的动力学特性。由于厚链状团簇的内部结构对剪切流方向的变化非常敏感，因此即使在振幅较小的情况下，厚链状团簇的形成也会受到振荡剪切流的显著影响。厚链状团簇的运动与正弦剪切率是异相的，其位相差与粘滞系数和法向应力系数的位相差密切相关。随着振荡剪切流动频率的降低，粘弹性质变得更加明显，这是因为这种性质与厚的链状团簇的形成有很大的关系。(2)我们研究了由铁磁性球形…组成的稀胶体分散体的流变性质和颗粒的取向分布更多的Ylinder颗粒，受制于简单的剪切流。磁场越强，取向分布函数的峰值越尖锐，当磁场控制剪切流时，峰值的位置由流动方向变为磁场方向。由于随着磁场的增强，颗粒在磁场方向上的取向受到很大的限制，粘度显著增加。长径比越大的颗粒，由于在流场中产生的阻力越大，粘度的增量越大。(3)研究了Stokesian动力学方法所基于的两种近似，即力的可加性和速度的可加性的特点和差异。对简单剪切流动进行了铁磁性胶体分散的Stokesian动力学模拟，并对聚集体结构和平均粘度进行了估算。从平衡聚集体结构的结果来看，力的可加性得到的关联函数与忽略水动力相互作用的近似得到的关联函数在数量上是不同的，尽管定性上是一致的。由速度的可加性得到的结果给出了介于上述两种近似之间的中等特征。随着磁相互作用的影响减小，速度可加性的对关联函数的形状与力的可加性的对关联函数的形状接近。从初始状态的暂态特性计算结果来看，速度可加性的计算结果与力的可加性计算结果吻合较好。相反，没有流体动力相互作用的结果在一个时间步长上偏离了两个加性近似的结果，当开始出现接近颗粒-颗粒接触的情况时。较少

(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